Hamstring muscle and tendon injuries

INTRODUCTION — Hamstring muscle injuries occur frequently among recreational and elite athletes. Several terms, including posterior thigh injury, hamstring strain, hamstring tendinopathy, and hamstring tear, are used to describe such injuries, but they are not always synonymous. In most cases, the severity of the injury determines treatment and the amount of time the athlete must take off from sport [1-9].

For the purpose of this review, we will define hamstring injury as any strain or tear, including avulsion, of any of the muscles or tendons within the hamstring group, including the biceps femoris, semitendinosus, and semimembranosus muscles.

The diagnosis, management, risk factors, and prevention of hamstring muscle and tendon injuries are reviewed here. Other musculoskeletal injuries of the lower extremity are discussed separately. (See "Anterior cruciate ligament injury" and "Ankle sprain in adults: Evaluation and diagnosis".)

EPIDEMIOLOGY — The incidence of hamstring injuries among athletes varies based upon the definition used. Nevertheless, most researchers agree that hamstring injuries comprise a substantial percentage of acute, sports-related musculoskeletal injuries and are likely the most common injury among participants in sports involving sprinting and cutting [10-12]. In a meta-analysis of 63 prospective studies encompassing 5952 injuries sustained over more than seven million  hours of exposure to field-based team sports (eg, soccer, rugby), hamstring injury incidence was 0.81 per 1000 hours, comprising 10 percent of all injuries sustained [11]. The incidence has not changed over the past 30 years, according to the review.

Prevalence ranges from 8 to 25 percent, depending upon the sport. The timing for return to sport ranges from as early as two weeks to never, depending upon injury severity and the sport in question. One observational study reports a single-season prevalence rate greater than 50 percent among elite soccer players [13]. Recurrent hamstring injuries develop in more than 30 percent of athletes, with most occurring during the ensuing sporting season [4,5,7,8].

Data collected from the United States’ National Collegiate Athletic Association indicates that the three highest risk sports are American football and men’s and women’s soccer [14]. The overall rate of hamstring injury was 3.05 per 10,000 sport exposures. The risk was about twice as high during competition and preseason training compared to general practice. In this database, 12.6 percent of injuries were recurrent strains and 6.3 percent were severe enough to cause the athlete to miss three or more weeks of sport. In a meta-analysis of 12 prospective studies, the incidence of hamstring injury among female field sport athletes was 0.6 per 1000 exposure hours [15].

A 13-year longitudinal analysis of elite soccer athletes in Europe clarifies the injury burden in that sport. The overall rate of injury was 1.2 per 1000 hours. Injuries in games occurred at nine times the frequency of practice. On average 21.8 percent of players experienced an injury during a season. The average time loss was 19.7 days per 1000 hours with a mean loss of 17 days per injury. Hamstring remained the most common injury in elite soccer and in spite of known preventive measures appeared to be increasing in frequency over the study period [16].

The risk of hamstring injury increases with age [3,11]; injuries are significantly less common among younger athletes. In a three-year observational study of 1157 youth players (average age 13.5 years) at a Barcelona football (soccer) club, only 50 hamstring injuries occurred [17]. These included:

●14 injuries to biceps femoris

●17 injuries to semimembranosus/tendinosus

●9 avulsions from ischial tuberosity

●10 cases of muscle soreness

The average time away from sport was 21 days, similar to that for older athletes.

ANATOMY AND PHYSIOLOGY — The hamstring is composed of three muscles in the posterior thigh: the biceps femoris, semitendinosus, and semimembranosus (picture 1 and figure 1). The long head of the biceps femoris originates at the ischial tuberosity, while the short head originates from the femoral shaft; both insert at the fibular head (figure 2). The semitendinosus and semimembranosus both originate from the ischial tuberosity and insert at the medial tibia in the pes anserine region (figure 3 and figure 4).

An understanding of the anatomy of the posterior hip region, thigh muscles, and adjacent nerves and vascular structures is important when considering the differential diagnosis of posterior thigh pain [18]. As an example, the sciatic nerve's location adjacent to the hamstrings can lead to radicular symptoms following some hamstring muscle injuries [19-24]. (See 'Differential diagnosis' below.)

Contributions from the lower lumbar plexus and upper sacral plexus give rise to the sciatic nerve. This nerve passes through the grater sciatic foramen of the pelvis and descends along the posterior aspect of the thigh (figure 5 and figure 6). Terminal branches of the sciatic nerve innervate the three hamstring muscles (figure 7). The primary actions of the hamstring muscles are knee flexion and hip extension. In addition, the biceps femoris contributes to external rotation of the hip, while the semitendinosus and semimembranosus contribute to internal rotation.

From a functional standpoint, the hamstring's key role is in deceleration during walking, running, and cutting (ie, making acute changes of direction at high speed). This deceleration is accomplished by eccentric muscle contraction (ie, contraction while the muscle lengthens). The hamstring muscles decelerate the lower limb during the forward swing phase of running, before the hip is extended to achieve forward motion. This braking function is also critical in kicking movements [6,25,26].

During locomotion, the hamstring muscle group must act in coordinated fashion with the other muscles involved in extension, flexion, and stabilization of the knee and hip. Injury to any of the muscles involved in locomotion is likely to have ramifications elsewhere.

DEFINITION OF HAMSTRING INJURY AND GRADING SYSTEM — Hamstring injuries traditionally have been classified as mild (grade I), moderate (grade II), or severe (grade III):

●Grade I injuries signify a small disruption of the structural integrity of the musculotendinous unit with minor swelling and pain and no, or minimal, strength loss.

●Grade II strains are partial tears with some intact musculotendinous fibers, pain is present, and there is a definite loss of strength.

●Grade III tears are complete tears of the musculotendinous unit with complete loss of muscle function and are typically accompanied by a large hematoma.

While attractive for its clarity, this traditional grading system based on clinical assessment does not provide clear guidance for treatment and prognosis. Some clinical experts prefer the BAMIC (British Athletics Muscle Injury Classification) scheme, which uses four grades of injury based on magnetic resonance imaging (MRI) findings, ranging from a small tissue tear (grade 1) to extensive or complete tear (grade 4) [27]. The BAMIC also includes a grade 0 injury with normal imaging but clinical findings of injury.

An international committee of experts was convened in 2020 to review evidence related to hamstring injury, including classification [28]. Multiple classification schemes incorporating factors including anatomy (eg, site of injury), mechanism, clinical findings, and diagnostic imaging have been described. However, studies demonstrating clear correlations between classification schemes and prognosis or the effectiveness of targeted rehabilitation programs are lacking.

RISK FACTORS

Overview — A wide range of potential risk factors, both modifiable and non-modifiable, contribute to hamstring injury [1,2,5,29-31]. Sport-specific risk factors are discussed separately. (See 'Sport-specific risks' below.)

Few large prospective studies have addressed potential risk factors for hamstring injury or the best methods for reducing risk. According to one statistical model, approximately 20 to 50 injuries are needed to assess risk factors strongly associated with hamstring injury, while 200 or more injuries are needed to assess risk factors with small to moderate associations [32]. In a systematic review of 8319 hamstring injuries, older age and previous hamstring injury were the risk factors most strongly associated with hamstring strain [33]. Other risk factors whose association reached statistical significance were previous anterior cruciate ligament (ACL) injury and previous calf strain.

Modifiable risk factors may include:

●Inadequate warm-up

●Increased training volume

●Muscle fatigue

●Hamstring and possibly ankle inflexibility [34]

●Hamstring weakness (may be weakness relative to ipsilateral quadriceps or contralateral hamstring)

●Cross-pelvic posture (ie, lumbar lordosis with anterior pelvic tilt) (figure 8 and picture 2)

●Lumbar-pelvic weakness

●Poor biomechanics (eg, running or change of direction)

●For high-level athletes, coaching change [35]

Non-modifiable risk factors include:

●Previous hamstring or lower extremity muscle injury [33]

●Previous ankle sprain [36]

●Older age

●Genetic variants

Previous injury — A previous hamstring injury is the most frequently noted risk factor for subsequent injury. Most studies showing this association involve Australian Rules football and report that athletes with a previous hamstring injury were two to six times more likely to suffer recurrent strains [30,33,37-41]. While most subsequent injuries occur within the first two months after return to sport, increased risk persists thereafter [37,42,43]. In some studies, the risk was three times that of an uninjured athlete up to one year or longer following the initial injury [38].

The increased risk associated with previous injury is due in part to the weakness of the muscle during healing which, according to some laboratory and clinical studies, requires up to nine months [1]. However, debate persists about the relative importance of inadequate rehabilitation and premature return to sport and the role of permanent tissue damage created by the initial injury [6,37,39,44,45]. Some researchers believe that skeletal muscles are at risk for reinjury because of scar tissue formation and changes in the muscular architecture [37,39], but others dispute this contention [45].

The association between the severity of the initial injury (defined in some studies by the number of days missed from competition) and recurrence during the same season is unclear. Some studies report no relationship [46], while others found a high risk of recurrence within two seasons in athletes with severe strains (>18 days missed) [25]. According to three prospective cohort studies, injury size (as determined by magnetic resonance imaging) did not clearly identify individuals at risk for reinjury, but did correlate with rehabilitation time [47-49]. This suggests that premature return to sport increases the risk of reinjury.

Muscle inflexibility — Limited hamstring flexibility may increase the risk of injuring these muscles. However, studies both supporting and contradicting this assertion have been published [7,50]. Adding to the confusion are the problems inherent in interpreting studies of muscle flexibility. Measuring hamstring flexibility can be difficult, since motion occurs at both the hamstring and the lumbo-pelvic junction, and many athletes participate in regular stretching routines and demonstrate good flexibility at baseline [51].

Hamstring flexibility may be less important than the flexibility of the quadriceps muscles (the antagonists of the hamstrings) and hip flexor muscles in determining the risk of hamstring injury. One prospective study reported an inverse relationship between increased quadriceps flexibility and the incidence of hamstring strain [3]. Athletes able to achieve greater than approximately 50 degrees of knee flexion (using a modified Thomas test) were less likely to suffer a hamstring strain. In the same study, hip flexor tightness posed a significant risk for hamstring injury, although the older-aged athletes in this subgroup may have confounded the results.

One reason why tight hip flexor muscles may predispose athletes to hamstring injury is the higher potential energy this tightness creates when the hip is extended and the knee flexed during the pre-swing phase of running [26]. This energy generates greater forward propulsion of the leg from the passive recoil of these muscles during the subsequent swing phase. The more forceful propulsion increases the load on the hamstrings as they decelerate the leg.

Muscle weakness — Substantial hamstring weakness relative to the contralateral hamstring or to the ipsilateral quadriceps, as well as absolute hamstring weakness, may increase the risk of hamstring injury [30]. While not every study concurs, larger prospective studies support the role of relative hamstring weakness in injury susceptibility. However, although muscle weakness and imbalance have been linked to hamstring injury risk, muscle strength testing protocols, even when extensive and carefully performed, have not proven to be effective prediction tools for hamstring injury.

The findings of several observational studies suggest that substantial discrepancies in strength between hamstrings predispose to injury [5,40,52,53]. As an example, a prospective cohort study of 178 rugby athletes found that limb to limb differences in eccentric hamstring strength (measured during performance of eccentric (Nordic) hamstring exercise) of greater than 15 percent was associated with a statistically significant increase in the risk of hamstring injury [40]. A 15 percent difference led to a 2.4-fold increase in risk (95% CI 1.1-5.5), while a 20 percent difference led to a 3.4-fold increase (95% CI 1.5-7.6). Among athletes who remained uninjured, a 10 percent strength discrepancy was noted, suggesting that athletes have a dominant leg and some baseline differences in hamstring strength are common but inconsequential if not too great. In another prospective observational study involving 100 professional soccer (football) athletes, asymmetries in hamstring strength of greater than 15 percent were associated with an increased risk of non-contact hamstring muscle strain [53].

Strength imbalances between the quadriceps and hamstring muscle groups may play an important role in hamstring injury. Relative weakness of the hamstrings compared with the quadriceps in the injured leg has been noted in several observational studies [2,30,52,54]. As described above, the hamstring muscles decelerate the leg during the swing phase of running and during kicking movements. The force of the leg swing may be greater in individuals with relatively stronger quadriceps, placing greater demands on the hamstrings during deceleration. Some researchers speculate that the quadriceps strength of many elite athletes may be too great, predisposing them to hamstring injury [7].

Strength imbalance or abnormal recruitment patterns among hamstring muscle may play a role in injury. In a functional MRI study involving 54 football (soccer) players, altered muscle contraction patterns were seen more often in players with a history of hamstring injury compared with players without such history, and the presence of such patterns was associated with an increased risk of subsequent injury [55].

Absolute weakness of the hamstrings too may predispose to injury, although there are conflicting data. In a prospective study of high-level soccer players, those with eccentric knee flexor strength below 337 Newtons, as determined during preseason testing, were more likely to sustain a hamstring strain (n = 27) [56]. However, a meta-analysis of 12 prospective studies involving 508 hamstring strains in 2912 athletes found that only slow-speed knee flexion eccentric strength measurements demonstrated a statistically significant association with subsequent hamstring strain [57]. In a well-designed prospective study, 413 soccer athletes were assessed with a comprehensive battery of lower-extremity strength tests, but only the isokinetic quadriceps concentric strength measurement was associated with a risk of hamstring injury (n = 66) over the following two seasons [58].

Problems with related structures (kinetic chain) — Injuries to other muscles in the affected leg may predispose to hamstring strain. This is consistent with the concept of the kinetic chain, whereby muscles act as one component in a series of interdependent musculoskeletal structures that work together to coordinate and control movement. The hamstring is an important structure in the posterior kinetic chain, a term often used to describe the inter-related structures in the back of the body involved in extending the hip and spine, among other functions.

Weakness (eg, muscle injury) or tightness in any part of a kinetic chain increases the strain placed on other components, which must compensate, thereby increasing their risk of injury. For instance, a non-hamstring injury may alter an athlete's running biomechanics, placing added stress on the hamstring [5]. As one example, the risk of subsequent hamstring injury is increased in individuals with a history of osteitis pubis or previous injury to the calf or quadriceps muscles or knee ligaments, according to observational studies [5,39].

Multiple abnormalities of the posterior kinetic chain occur with a cross pelvic posture (picture 2), and preliminary research suggests that this posture increases the risk for hamstring injury [59]. The pelvic cross syndrome (also known as lower cross syndrome or cross pelvic posture) is defined by an increased lumbar lordosis and anterior tilt of the pelvis (figure 8) and is associated with tight hip flexor (iliopsoas and rectus femoris) and lumbar extensor (lumbar erector spinae) muscles, weak hip extensor (gluteus maximus) and lumbar flexor (rectus abdominus) muscles, and sacroiliac joint dysfunction. These musculoskeletal abnormalities place added stress on the hamstring muscles during activity. Further studies are needed to determine more precisely the role of cross pelvic posture and sacroiliac joint dysfunction in hamstring strain [2].

Lumbar-pelvic weakness is another abnormality of the posterior kinetic chain that may increase the risk for hamstring muscle injury. Weakness in this region results in abnormal posture, joint malalignment, and inefficient movement patterns [60].

Age and other factors — Although the age ranges for athletes included in studies of hamstring injury are relatively narrow, increasing age appears to be another risk factor for hamstring injury, according to several studies of Australian footballers [3,5,11,26,30,33,38,39,43]. Specifically, athletes older than 23 years were 1.3 to 3.9 times more likely to suffer a hamstring injury than younger players [5,26], while athletes older than 25 years were 2.8 to 4.4 times more likely [3,43]. Data suggest that the overall risk of injury increases by 30 percent annually after the start of a professional playing career [39].

Different theories have been proposed to explain why hamstring injury risk increases with age. One theory suggests that aging correlates with a reduction in the hamstring muscles' cross-sectional area, which reduces the muscles capacity to generate tension and resist loads before failure [7]. However, the studies supporting this theory involved younger athletes, possibly with less developed musculature, and older athletes, who may already have sustained subclinical hamstring injuries, and these factors may have confounded the results. A second theory is that age-related lumbar degeneration leads to L5 and S1 nerve impingement and subsequent hamstring muscle fiber degeneration, and ultimately, hamstring injury [5].

Genetic factors may predispose to injury. Athletes of African or Aboriginal ethnic origin appear significantly more likely to suffer hamstring strains [25,39]. In Australian Rules football, professional Aboriginal footballers were 11.2 times more likely to suffer hamstring strains than non-Aboriginal footballers [39]. Genetic studies have identified five single nucleotide polymorphisms significantly associated with hamstring injury. However, studies have yet to demonstrate that these variants predict injury [61].

Studies of the relationship between increased weight or body mass index (BMI) and hamstring injury report inconsistent results [3,5,37,39,43,54].

Training-induced physical fatigue appears to increase the risk of injury. According to a prospective observational study of more than 130,000 soccer matches, players with six or more days of rest between matches had approximately a 20 percent reduction in the risk of muscular injury [62]. A study of Australian rules football used monitoring to assess distance covered and relative amount of high-speed running for matches and training sessions. The study reported that a greater volume of high-speed running (above the two-year average for a given player) in the week prior to a match increased hamstring injury risk, while a lower than average volume reduced injury risk [35,63]. Multiple studies of several sports report a greater injury frequency in the second half of matches.

Coaching change poses a significant short-term risk of muscular injury to professional soccer athletes [35]. In a study of two professional teams, the risk of muscular injury increased from 2.3/1000 to 5.3/1000 hours in the two weeks after a coach's dismissal. The rate dropped to 4.5/1000 hours at one month post-dismissal. The authors postulate that new training regimens likely raise the risk of injury, but whether psychological factors (eg, trying to impress a new coach) play a role is unknown.

Sport-specific risks — Hamstring injuries are common in sports that involve high-speed running. Track and field runners and hurdlers, who combine forceful hip flexion with full knee extension during their events, are prone to hamstring injury [52,64,65]. Participants in sports that involve rapid deceleration and acceleration, such as football (soccer), American football, and Australian Rules football, are also prone to hamstring strain [11].

With each of these sports, there are specific circumstances when injuries are more prevalent. As examples, playing at a higher level of competition increases the risk factor for hamstring injury, and match play poses greater risk than team practice. One observational study of English Premier League (EPL) football (soccer) reported a significantly higher prevalence of hamstring strain in the first division versus the second [25]. A similar study of Australian Rules football found a significantly higher prevalence (>20 percent) of hamstring injuries among players in the Australian Football League (AFL) compared with the less competitive South Australian National Football League (SANFL) [39]. These differences may reflect the increased physical challenges of the higher leagues, where the tempo of games may be faster and training more demanding.

Positions that require more running are associated with higher hamstring injury rates. As an example, outfield players exhibit a higher incidence (22 to 37 percent) of hamstring strain compared with goalkeepers in English football and Australian Rules football leagues [25,54]. A 10-year study of hamstring injuries (n = 1716) in the National Football League of the United States reported that 43.9 percent were sustained by wide receivers or defensive secondary players [66].

Exercises involving movements of the knee and hip joints at the outer limits of their range of motion, as with ballet dancers, represent another risk factor for hamstring injuries. Sports with physical demands similar to dance demonstrate similar patterns of symptoms, hamstring injury location, and recovery time. Such hamstring injuries usually occur during movements that combine extreme hip flexion and knee extension.

A 13-year longitudinal study of over 1600 hamstring injuries among elite soccer players showed a 4 percent annual increase in hamstring injuries. Even though hamstring injuries in matches are still higher than in practice, the increased rate was due more to higher practice injuries [16]. Some experts speculate that a shift toward higher intensity training sessions that increase the amount of sprinting and acceleration-deceleration movements account for the higher rate.

CLINICAL PRESENTATION AND EXAMINATION

History and mechanism of injury — Most individuals with hamstring injuries present acutely complaining of the sudden onset of posterior thigh pain while performing a high-risk activity. The individual may report a pop or feel localized warmth at the site of injury. (See 'Risk factors' above.)

Some researchers hypothesize that there are two primary mechanisms for hamstring injury [64,67-69]. These can be referred to as a running or high-speed mechanism and a stretching or low-speed mechanism; although not all stretching movement (eg, high kick) injuries occur at truly low speeds.

Classically, hamstring injury occurs during high-speed running. As noted above, the hamstrings contract eccentrically to decelerate the leg during the end of the swing phase of running (when the lower extremity is fully extended in front of the runner's body). Injury occurs when a sudden forceful eccentric contraction of the hamstring occurs against resistance (eg, concentric quadriceps contraction), overloading the muscle [20,24,31,64]. (See 'Anatomy and physiology' above.)

With the low-speed mechanism, hamstring strain occurs during stretching movements with the hip and knee joints at their extreme limits of motion, forcing the hamstring beyond its elastic capacities [10,64]. Such injuries can occur in dancers performing movements involving forced hip flexion with simultaneous full knee extension (eg, high kick).

Injury location — In most studies, the biceps femoris is the most commonly injured hamstring muscle, while the semimembranosus and semitendinosus are strained less often [39,46,67,70]. As an example, a prospective observational study of 51 soccer-related hamstring injuries using ultrasound assessment reported that approximately two-thirds of injuries involved the biceps femoris while the other one-third involved the semitendinosus [71]. However, numbers vary among studies, and in approximately 30 percent of cases, more than one hamstring muscle is injured [64,70].

Proximal injuries, including tendon avulsion (which occurs proximal to the origin of the short head of the biceps femoris), are more common than distal injuries regardless of the muscle involved. Most injuries occur in the musculotendinous junction, a 10 to 12 cm transition zone in which myofibrils merge with tendon. Even injuries that occur along the length of the hamstring commonly occur at an intramuscular musculotendinous junction. Avulsion fractures of the ischium involving a hamstring muscle origin rarely occur in adults but may occur in skeletally immature athletes (image 1 and image 2 and image 3 and image 4) [24,46,64,67,70].

Sprinters tend to injure the long head of the biceps femoris, including the intramuscular tendon (or aponeurosis) and adjacent muscle fibers proximally [47,67]. This appears to be true of professional soccer players as well, according to a study of 27 hamstring strains of which 24 involved the long head of the biceps femoris [56]. According to one study, hamstring injuries among sprinters are often disabling initially but tend to require shorter rehabilitation compared with those sustained by dancers. The closer the injury is to the ischial tuberosity, the longer the recovery time required [67].

Dancers and other athletes who sustain hamstring injuries via a low-speed mechanism typically strain the semimembranosus muscle and its proximal tendon. They require a longer convalescent period, likely due to the involvement of free tendon (ie, tendon without presence of myofibrils) or a bone fragment. However, within this subset of proximal injuries there appears to be no correlation between the distance of the injury from the ischial tuberosity and recovery time [64,68,72]. (See 'History and mechanism of injury' above.)

Physical examination — Examination of a potential hamstring injury follows the standard approach used for all musculoskeletal injuries: inspection, palpation, range of motion assessment, strength testing, and special testing. Gait abnormalities should be noted. These may include limited hip flexion on the affected side, lack of full hip extension, decreased lift in the float (ie, swing) phase of gait, or an antalgic gait (ie, limp). Inspect the leg and hip region for swelling, ecchymosis, muscular defects (picture 3), and atrophy. Each of the three hamstring muscles should be palpated from origin to insertion, paying particular attention to the origin at the ischial tuberosity and to the popliteal fossa. Focal tenderness is common. (See 'Anatomy and physiology' above.)

Range of motion can be assessed clinically with the patient prone and is easily quantified with a goniometer. We advocate assessing concentric hamstring strength starting with the knee at 90 degrees of flexion (picture 4) and assessing eccentric strength with the knee extended between 15 and 30 degrees (full knee extension [ie, leg straight] equals 0 degrees) (picture 5). During strength testing, greater emphasis can be placed on the biceps femoris by externally rotating the leg (picture 6), while internal rotation accentuates the role of the semimembranosus and semitendinosus (picture 7). With the patient supine, assess hip range of motion and the strength of the hip flexors, extensors, abductors, and adductors.

Knee range of motion, hamstring strength, the degree of ecchymosis, and the presence or absence of palpable muscle defects all assist the physician in grading the severity of injury and creating an appropriate treatment plan. Severe injury is suggested by pain accompanied by diffuse ecchymosis, extensive swelling, palpable defects in the muscle, and/or notable strength deficits compared with the uninjured hamstrings. Advanced imaging and referral to an orthopedic surgeon is needed in such cases. (See 'Diagnostic imaging' below and 'Indications for orthopedic consultation' below.)

When a patient presents with vague, proximal, posterior thigh pain, several tests may be used to help identify a proximal hamstring injury [73]. Of these tests, the modified bent-knee stretch test seems to have the greatest clinical utility and is reasonable to include in the examination. In this test, the patient lies supine with the hip and knee maximally flexed. The examiner passively extends the knee and hip quickly. If the patient experiences significant pain at the hamstring origin near the ischial tuberosity, the test is positive, suggesting proximal hamstring injury.

If radiculopathy is present, neurologic testing and a straight leg raise test should be performed; if the injury is not clearly a hamstring tear, a complete back and hip examination is indicated [21-23,74]. (See "Acute lumbosacral radiculopathy: Etiology, clinical features, and diagnosis" and "Evaluation of low back pain in adults".)

DIAGNOSTIC IMAGING — Musculoskeletal ultrasonography (US) and magnetic resonance imaging (MRI) are the best methods for assessing hamstring injuries. Both provide detailed information about the location and extent of injury [31].

The advantages of US include low cost and portability. In addition, US allows for dynamic assessment of the hamstring tendon, which provides information about tendon and muscle integrity at varying degrees of resisted contraction. US has greater sensitivity during the acute phase of injury when inflammatory fluid is found in the soft tissue (image 5 and image 6). In addition, US is sensitive for avulsion injuries (image 2) and can reveal abnormalities in damaged muscle tissue (image 5). However, the sensitivity of US declines when scanning is delayed. As the fluid resolves (usually within two weeks), US becomes less accurate at depicting myofibrillar abnormalities [46-48]. Further study to determine the sensitivity and specificity of US more precisely are needed. One limitation of US is that accuracy depends upon the skill of the ultrasonographer. US is not a useful tool for determining the time required for an athlete to return to sport, even when it accurately portrays the extent of muscular injury [71].

MRI is more reliable than US at depicting hamstring tendon and osseous avulsion injuries and injuries at the deeper musculotendinous junction (image 7 and image 3). MRI allows accurate assessment of the degree of tendon retraction and of tendon edge morphology (image 8 and image 9) [46]. This gives the surgeon important information, as tendon avulsion may require surgical repair (image 3) [7,20-22,75,76]. In addition, MRI sensitivity does not wane over time. Some research suggests that MRI is useful for estimating time away from sport since the length and cross sectional area of the injury can be measured [47,48,77], but other studies suggest that baseline MRI findings add little and should not be used to determine when an athlete is ready to resume full athletic activity [78-80]. While MRI grading criteria have evolved, they have not consistently correlated with the time required for return to play [81,82]. (See 'Return to work or sport' below.)

When experienced musculoskeletal ultrasonographers are available, initial imaging should be performed with US because of its low cost and high sensitivity. However, in current clinical practice in the United States, MRI remains the imaging modality of choice when severe injuries (eg, hamstring rupture) are suspected. In studies that compare the ability of MRI and US to assess hamstring muscle volume as an indirect marker of recovery from injury, US measurement of hamstring muscle thickness appears to correlate well with MRI findings of muscle volume [83]. Imaging markers on both MRI and US can suggest a more difficult time with return to play [84]. However, neither US nor MRI has shown strong efficacy for estimating recovery time [10,83,85].

Standard radiographs may be useful in skeletally immature athletes, in whom bony avulsion injuries are more common (image 1), although even in these circumstances US may be preferable. In one study of 243 young athletes, US demonstrated greater sensitivity for avulsion injury compared with plain radiography [86].

DIFFERENTIAL DIAGNOSIS — In most cases, the mechanism of injury and localization of symptoms at the time of injury makes the diagnosis of acute hamstring strain or rupture straightforward. However, in athletes with long standing pain, recurrent symptoms, or an unclear mechanism of injury, diagnosis can be more difficult. The differential diagnosis for posterior thigh pain includes:

●Proximal thigh:

•Lumbar radiculopathy – Lumbar radiculopathy may occur concurrently with a hamstring strain, complicating diagnosis, but most often develops independently. Both conditions can cause pain in the posterior thigh, but pain from lumbar radiculopathy involves the low back and often radiates below the knee. History may reveal worsening of symptoms with sitting, coughing, bending forward, or other factors that would not typically aggravate a hamstring injury but can increase neural tension. Physical examination reveals no hamstring tenderness or weakness in most cases, but may demonstrate other focal weakness related to the level of the affected nerve root, and ultrasound examination of the hamstrings is unremarkable, unless the two conditions occur together, which is uncommon. MRI of the lumbar spine reveals the cause of lumbar radiculopathy in most cases. (See "Acute lumbosacral radiculopathy: Etiology, clinical features, and diagnosis".)

•Hamstring syndrome – Hamstring syndrome is a chronic gluteal sciatic pain in which post-traumatic scars (typically associated with recurrent hamstring injury) or congenital fibrotic bands irritate the sciatic nerve at the region of the hamstring muscles' origins around the ischial tuberosity. The condition is a potential complication of hamstring injury and is discussed below. (See 'Hamstring syndrome' below.)

•Adductor (groin) muscle strain – Adductor strains typically cause focal pain in the groin area or the medial thigh, either in the muscle belly of the injured adductor or proximally at its origin. Such injury typically involves an acute groin strain sustained while sprinting or performing some other explosive movement during sport involving a stretched adductor. Examination reveals pain with resisted hip adduction; hamstring examination is generally unremarkable. Ultrasound examination confirms the diagnosis in most instances. (See "Adductor muscle and tendon injury".)

•Femoral neck or shaft stress fracture – Femoral stress fractures are overuse injuries typically caused by repetitive tensile or compressive stresses, as may occur in distance runners. The onset of pain is insidious. Pain initially develops only with activity and increases with impact. Eventually, pain persists with less activity or even at rest. Clinical diagnosis is suggested by a positive fulcrum test and painful hop test on the affected lower extremity; careful physical examination of the hamstring muscle is typically unremarkable, as is ultrasound examination. Plain radiographs can be diagnostic of stress fracture, but no abnormalities may be visible during the first few weeks of injury and advanced imaging (bone scan, MRI) is often needed for definitive diagnosis. (See "Femoral stress fractures in adults" and "Overview of stress fractures".)

•Posterior thigh compartment syndrome – Chronic exertional compartment syndrome (CECS) is an overuse injury that usually occurs in endurance athletes. Similar to acute compartment syndrome, CECS is due to increased pressure within a muscle compartment that exceeds perfusion pressure and results in muscle and nerve ischemia. The primary symptom is pain that is described as aching, cramping, squeezing and tightness, and begins within minutes of starting the inciting activity. Pain is not sharp or focal over one area of the hamstring muscle, as occurs from an acute muscle strain, but is diffuse and cramping. Complete resolution of symptoms occurs after stopping the activity, usually within 10 to 20 minutes, whereas pain following a hamstring strain persists for hours to days. Diagnosis involves measurement of compartment pressures. (See "Chronic exertional compartment syndrome".)

•Sacroiliac joint dysfunction – Sacroiliac dysfunction refers to pain caused by dysfunction or abnormal motion at the sacroiliac joint. Diagnosis is made clinically, as imaging tests are usually normal. Physical exam maneuvers such as the FABER test (picture 8) can usually distinguish pain from the sacroiliac joint. While pain may radiate to the posterior thigh, hamstring strength is typically unaffected and ultrasound examination is unremarkable.

•Piriformis syndrome – Piriformis syndrome remains a controversial condition that is diagnosed clinically. Symptoms involve a cramping or aching pain in the buttock and/or proximal hamstring area. Pain is exacerbated by hip flexion combined with active hip external rotation or passive internal rotation. Symptoms of sciatic nerve involvement may be present (see immediately below). Focal piriformis muscle spasm may be palpable. Biomechanical assessment often reveals restricted hip external rotation and lumbosacral muscle tightness. Often there is tenderness over the sciatic notch that is aggravated by flexion, adduction, and internal rotation of the hip (Lasègue sign). Weakness in hip rotation is common, but hamstring strength remains unaffected, even when symptoms radiate to the proximal posterior thigh.

•Sciatic nerve irritation or compression – Sciatic nerve irritation most commonly occurs from a herniated lumbar disc (discussed above) but may be caused by compression from bone spurs or other abnormal tissue or bleeding, causing pain that radiates along its path into the posterior thigh, and possibly more distally. Radiation of pain to the low back may occur and lumbar radiculopathy should be ruled out. Local compression of the sciatic nerve can occur at the region of the piriformis or proximal hamstrings. In contrast, pain from hamstring injuries is typically focal at the site of injury without radiation. MRI reveals the cause of sciatic nerve compression in most cases. (See "Overview of lower extremity peripheral nerve syndromes", section on 'Sciatic nerve'.)

•Pelvic bone tumor – Rarely, benign or malignant tumors develop in the pelvic bones in the region where the hamstring muscles originate [87-90]. Pain may initially mimic that of a hamstring strain, but unlike such strains persists despite rest and at night. Radiographs or CT reveals such tumors. (See "Bone tumors: Diagnosis and biopsy techniques", section on 'Clinical presentation'.)

●Distal thigh:

•Deep venous thrombosis (DVT) – DVT can cause pain in the area of the hamstring, but pain is more diffuse than is typical for a hamstring muscle or tendon injury and nonfocal. Clots this proximal in the lower extremity are typically associated with significant swelling and thigh circumference is greater than the unaffected side. Doppler ultrasound studies can identify clot within the vein and demonstrate the normal appearance of the hamstrings. (See "Clinical presentation and diagnosis of the nonpregnant adult with suspected deep vein thrombosis of the lower extremity".)

•Meniscal injury – Pain from an acute injury of the posterior portion of a meniscus may be confused with that of a distal hamstring injury. The two can usually be distinguished by the history and examination findings. The mechanism for meniscal injury typically involves rotation or twisting of the knee. Provocative tests of the meniscus (McMurray, Thessaly, Apley) are usually positive, while the hamstrings themselves are nontender and provocative tests (eg, knee flexion against resistance) normal. If the meniscal injury has been present for a few days or longer, the hamstrings may be tender from overuse, as the patient may favor walking with a bent knee to minimize discomfort from the injured meniscus. Ultrasound examination of the hamstrings shows a normal appearance. (See "Meniscal injury of the knee".)

•Gastrocnemius injury – Gastrocnemius injuries can cause extensive swelling, and since the hamstring tendons cross those of the gastrocnemius the swelling may extend to both tendon sheaths. Gastrocnemius-semimembranosus bursitis causes the tenosynovium to become extensively swollen and appear like a bursa wrapping around the tendons. All these injuries can cause pain when the hamstrings are tested, but the area of maximal tenderness typically involves the musculotendinous junction of the gastrocnemius and proximal Achilles tendon, rather than the distal hamstring muscles and tendons. Ultrasound effectively demonstrates gastrocnemius injuries, and normal hamstring appearance. (See "Calf injuries not involving the Achilles tendon", section on 'Gastrocnemius strain'.)

•Popliteus injury – The popliteus muscle crosses under the biceps femoris before attaching to the lateral femoral condyle. Thus, pain caused by a popliteus injury occurs close to the insertion of the distal biceps femoris. Palpation of the popliteus and provocative examination maneuvers (eg, resisted internal knee rotation) elicit pain from a popliteus injury, but not from a hamstring injury. The popliteus is easily visualized with ultrasound, which can be used to distinguish between popliteus and hamstring injury. (See "Calf injuries not involving the Achilles tendon", section on 'Popliteus tendinopathy'.)

•Popliteal artery entrapment syndrome – Popliteal artery entrapment is an uncommon cause of peripheral vascular disease in young fit individuals that presents with symptoms and signs of progressive claudication or sudden limb ischemia. Repeated popliteal artery compression causes trauma to the arterial wall, leading to premature localized atherosclerosis. Arterial Doppler ultrasound or magnetic resonance angiography confirms the diagnosis. The patient typically feels a cramping type of pain in mid popliteal space radiating into the calf and not directly over the hamstring insertions, which appear normal on ultrasound. (See "Calf injuries not involving the Achilles tendon", section on 'Popliteal artery entrapment' and "Popliteal artery aneurysm", section on 'Symptomatic patients'.)

•Popliteal cyst – A popliteal cyst presents as posterior knee pain caused by pressure in the popliteal space from a synovial cyst. Diagnosis is made clinically and is usually obvious based on appearance. The cyst is most apparent with the patient standing and the knee in full extension. Pressure from the cyst can be relieved with knee flexion to approximately 45 degrees. If the diagnosis remains uncertain, ultrasound can both identify the cyst and demonstrate normal appearance of the hamstrings. (See "Popliteal (Baker's) cyst".)

INDICATIONS FOR ORTHOPEDIC CONSULTATION — The great majority of hamstring injuries can be managed successfully with rest and physical therapy. However, in some circumstances, operative repair may yield favorable results. Such circumstances include:

●Complete (grade III) proximal hamstring rupture

●High-grade (grade II or III) distal hamstring tears

We suggest orthopedic consultation in these cases. In addition, all hamstring avulsion injuries should be evaluated by an orthopedic surgeon or sports medicine specialist.

There is no clear consensus about the indications for operative repair of hamstring injury. An international committee of experts was convened in 2020 to review evidence related to hamstring injury, including indications for surgery [91]. The group reached high levels of agreement (>70 percent) that four clinical scenarios warranted operative intervention:

●Gaps at the site of tendon injury. A gap on magnetic resonance imaging (MRI) was defined as a clear area of separation of either hamstring muscle or tendon. Following acute injury, retraction of muscle or tendon of 2 cm or more is an indication for surgical referral, even if only one tendon is affected [92].

●Loss of tension. The British Athletics Muscle Injury Classification study defines this as a palpable loss of tension in the muscle tissue during resisted strength testing compared with the contralateral hamstring [93].

●Symptomatic, displaced bony avulsion injuries.

●Proximal tendon injuries associated with refractory functional loss or performance deficits. Injury is generally considered refractory if the patient is unable to return to full activity after six months of rehabilitation.

Other factors when considering operative intervention include whether the athlete competes at an elite level (eg, professional, Olympic), whether significant anatomic disruption is present, and the surgeon’s perception about whether function can be restored. Participating surgeons felt more confident about the benefits of surgical intervention; no consensus was reached about whether surgery can help to prevent recurrent injury.

No randomized trials have been performed to assess the effectiveness of surgery to treat hamstring injuries. Any recommendation for surgical treatment is based upon expert opinion, case series, and cohort studies. Magnetic resonance imaging (MRI) helps to determine the need for orthopedic referral by identifying complete versus partial ruptures and quantifying the degree of muscular retraction (see 'Diagnostic imaging' above).

While bedside ultrasound has greater sensitivity in the acute phase (within two weeks), ultrasound becomes less sensitive thereafter [46-48]. Therefore, MRI remains the study of choice to determine the extent of injury and the need for orthopedic surgery referral. The physical findings that typically accompany more severe hamstring injuries are described above. (See 'Physical examination' above.)

Typically, the initial treatment for pediatric patients with bony apophyseal injuries is conservative. However, avulsions should be referred to an orthopedist or sports medicine specialist to determine whether surgical management may be needed. While opinions vary and the number of clinicians using more conservative approaches is substantial, classic references recommend surgery for larger bony avulsions with hamstring retraction greater than 2 cm [22,24].

Complete proximal hamstring ruptures (grade III) should be referred to an orthopedist for evaluation and possible repair (image 8). In limited case series, complete proximal ruptures managed nonoperatively did poorly, and most patients ultimately underwent operative repair with improved outcomes [20-22,24,94-96]. In addition, surgical repair of these injuries decreased the incidence of post-injury sciatica. A meta-analysis of twenty-four studies involving 795 proximal hamstring avulsion injuries reported greater hamstring strength and function and higher patient satisfaction among those treated with operative repair [97]. However, surgery was associated with a higher complication rate. Complications included rerupture, need for reoperation, infection, neurologic complications including peri-incisional numbness, and deep vein thrombosis.

Not all study results support surgical management of complete proximal ruptures. The authors of a prospective observational study, limited by small numbers and discrepancies in study groups, reported comparable outcomes at one year between patients with grade III injuries treated surgically and those managed conservatively, and emphasized the importance of shared decision-making [98]. For low-demand athletes and others able to perform activities of daily living (ADLs), conservative management may be appropriate. We define a low-demand athlete as an athlete in a sport that does not require excessive contraction or stretching of the hamstring beyond regular walking.

Distal hamstring ruptures (image 7) are rare and there are few published studies on which to base management decisions. Functional outcomes may be better with surgery [75].

In addition to the indications described above, we believe it is reasonable to obtain orthopedic consultation if there is no improvement of the patient's symptoms and function despite four to six months of appropriate rehabilitation and conservative care.

INITIAL TREATMENT — Many interventions are used to treat hamstring injuries, but few are supported by evidence from randomized controlled trials or high quality prospective studies [9,99]. The great majority of relevant studies involve small numbers of patients, raising the possibility of Type 2 error. Therefore, our treatment suggestions below are based primarily upon small case series and clinical experience. (See "Hypothesis testing in clinical research: Proof, p-values, and confidence intervals", section on 'Explanation for the results of a study'.)

Acute management — Protection, rest, ice, compression, and elevation (PRICE) therapy is traditionally used to limit the extent of localized bleeding, swelling, and pain from acute muscle strains [100]. Elastic wraps (eg, ACE bandage (picture 9)) or taping are frequently used by trainers acutely. During treatment, many athletes continue to use materials to protect and compress the muscles, such as wraps (picture 9), fabric sleeves (picture 10) or compression pants. Cryotherapy appears to be a safe and effective means of reducing pain and likely plays a role in lowering tissue metabolism and reducing secondary hypoxic injury [101]. Observational data suggest that two to six days of restricted motion reduces scar formation and reinjury rates [4,8,10,102-104]. Therefore, it seems reasonable to delay the start of stretching and rehabilitation for two to six days, depending upon injury severity.

Over the counter analgesics such as acetaminophen or ibuprofen are generally adequate for pain relief. In rare instances, a short course of opioids may be needed to treat severe injuries. Nonsteroidal antiinflammatory drugs (NSAIDs) have been used for many years to reduce pain and inflammation. However, the only published study to assess the effectiveness of NSAID therapy for hamstring injury is a double-blinded, randomized trial of 75 injuries that reported no overall difference in pain, swelling, strength, or endurance between the NSAID and placebo groups [105]. A nonsignificant trend towards reduction in pain was found in patients with more severe muscle strains.

Concern has been raised that NSAID therapy for muscle injury may delay healing, weaken tissue, and lead to impaired function [10,100,106]. However, there is no clinical evidence to suggest that short-term use of NSAIDs in the treatment of hamstring injury has important deleterious effects. If NSAIDs are used, the duration of treatment should be limited (generally no longer than five to seven days) and based upon patient response. The effect of NSAIDs on tendon healing is discussed separately. (See "Nonselective NSAIDs: Overview of adverse effects", section on 'Healing of musculoskeletal injury'.)

Glucocorticoid injection — We suggest not performing local glucocorticoid injections for the treatment of acute hamstring injuries because of the catabolic effects of glucocorticoids upon skeletal muscle and the absence of strong evidence supporting the benefits of this therapy. In addition, glucocorticoids may cause tendon degradation and increase the risk of tendon rupture [107,108]. However, our suggestion is based upon scant evidence and glucocorticoid injection may be beneficial in some cases. (See "Glucocorticoid-induced myopathy".)

One such exception might be elite or professional athletes in important competitions treated under the guidance of experienced sports medicine physicians. As an example, a retrospective case series of 58 players in the United States National Football League treated over a 13-year period with intramuscular glucocorticoid injections following a hamstring injury reported no negative outcomes and all the athletes were able to return to play [109]. Glucocorticoids reduce pain, inhibit cytokine production and the inflammatory cascade, and they may reduce fibroblast proliferation, collagen synthesis, and scar formation [101,110].

PRP and other biologic injections — A number of injectable therapies, including platelet-rich plasma (PRP) [111,112], have been used or proposed for the treatment of acute muscle injury, but evidence supporting their use in humans is limited and conflicting. Other injectable substances that have been used for acute muscle injury include autologous blood, autologous conditioned serum [113], prolotherapy, and isolated growth factors such as IGF-1. Pending further research, we do not suggest using any of these substances for the treatment of acute hamstring injury. The use of PRP and other biologic injections for muscle and tendon injuries is reviewed in detail separately. (See "Biologic therapies for tendon and muscle injury".)

PRP is anecdotally being used in elite athletes, but there is little high-quality scientific data to support its use for hamstring injuries [80,114-118]. In addition, current preparations are expensive [112]. A systematic review of 10 studies of PRP to treat hamstring muscle injury noted many methodologic limitations among studies that prevented the authors from drawing clear conclusions [119]. Noted issues included the following:

●Six studies were rated as fair quality; only two studies were rated as excellent quality

●PRP injections varied widely in content (eg, leukocyte rich versus leukocyte poor, varied platelet concentration, with or without lidocaine) and volume

●Physical therapy program content varied; four studies did not describe the program

●Reporting of functional outcomes was poor: six studies reported functional outcomes using five different scoring systems; only two studies reported on functional outcomes at baseline and final follow-up

●Mean time for return to play was not reported in four studies, accounting for about one-fourth of all injuries

●No study defined "reinjury"

The two studies rated as excellent in the systematic review included the following:

●A well-blinded randomized trial of PRP therapy for acute hamstring strains sustained by 80 recreational and competitive athletes found no significant difference in the time until patients could return to sport (median 42 days in each group) or injury recurrence (16 percent in PRP group versus 14 percent in placebo group) [115,116].

●A randomized trial of 90 professional athletes with acute hamstring injury confirmed by MRI reported no benefit (ie, return to play was not faster; reinjury rates were no different) from a single PRP injection compared to intensive physical therapy [117].

Although PRP is no longer prohibited by the World Anti-Doping Agency, purified or recombinant growth factors (ie, IGF-1, VEGF, PDGF) remain banned substances. (See "Prohibited non-hormonal performance-enhancing drugs in sport" and "Prescription and non-prescription medications permitted for performance enhancement" and "Nutritional and non-medication supplements permitted for performance enhancement".)

Unproven treatments — There is little evidence to support other proposed treatments for hamstring injuries, including low-intensity pulsed ultrasound, therapeutic laser, electrical stimulation, massage, and extracorporeal shockwave treatment. According to several systematic reviews, therapeutic ultrasound is no better than placebo and the data regarding electrical stimulation and therapeutic laser is conflicting [4,74,101,102]. There is some evidence that application of heat within the first three days may prolong rehabilitation [102].

FOLLOW-UP CARE AND REHABILITATION — Physical therapy has been used for many years to treat acute muscle injuries, and the experience of many clinicians is that patients do better when they are compliant with a well-designed rehabilitation program. Nevertheless, the published evidence supporting the majority of these treatments is limited. Pending further study, several reasonable approaches to rehabilitation may be used. We suggest using the functional rehabilitation program described in the attached table and reference (table 1) [10]. This program incorporates the following elements:

●Multiplanar movements

●Exercises to enhance trunk stability and proprioception

●Agility training

●Eccentric strength exercises for the hamstring

For each of the three phases of this program, it is important that the intensity of the exercises is increased gradually. Video demonstrations of several rehabilitative techniques used in this program are available to those with access to the Journal of Orthopaedic and Sports Physical Therapy [10].

Once the injured hamstring has healed sufficiently to allow patients to begin performing strengthening exercises, we suggest incorporating three specific exercises that involve eccentric contraction into any rehabilitation program [114,118,120,121]. In two randomized trials (described below) [114,120,121], these exercises led to a significantly shorter time until athletes could return to full sport. The exercises are described below, and video demonstrations are available in the on-line supplement to the trials published on the website of the British Journal of Sports Medicine. A comprehensive hamstring rehabilitation program from the Aspetar sports medicine group with video demonstrations of the exercises can be found in the following reference [122].

●The Extender – The patient lies supine with their uninjured leg flat on the ground. Next, the patient flexes the hip of their injured leg to approximately 90 degrees, and stabilizes the thigh of that injured leg by grasping behind the knee with both hands. Maintaining this position, the patient then slowly performs knee extensions to the point just before pain is felt (movie 1). The patient performs three sets of 12 repetitions, twice daily if possible.

●The Diver – As the name suggests, this exercise simulates a diving motion to create an eccentric stress on the hamstring. Beginning from an upright position, the patient bears weight only on the injured leg with the knee bent slightly. While maintaining the torso and thigh of the uninjured leg in a straight line, the patient slowly flexes at the hip of the injured leg, thereby bending forward, while extending both arms straight ahead (movie 2). The knee of the supporting leg remains slightly bent during the maneuver while the knee of the leg being lifted (uninjured leg) is bent to approximately 90 degrees. The patient slowly returns to the starting position, and repeats the movement for a total of six repetitions. The patient performs three sets every other day.

●The Glider – This exercise is essentially a slow, controlled front split (not a slide split). It requires a low friction sock to be worn on the foot of the uninjured leg. The patient begins with the trunk upright, one hand holding a firm support (eg, securely attached railing) at their side, and the legs slightly split. The patient's body weight is placed entirely on the heel of the injured leg with the knee of that leg flexed to approximately 10 to 20 degrees. Slowly, the patient allows the uninjured leg to slide backward to a point just before pain develops in the hamstring (movie 3). As the leg slides back, the patient inclines their torso forward over the injured leg to maintain weight on it. Using both arms for assistance, the patient returns to the starting position. Every third day, the patient performs three sets of four repetitions.

An international committee of experts was convened in 2020 to review evidence related to hamstring injury, including rehabilitation [12,28]. Consensus on the best approach could not be reached. Nevertheless, over 70 percent of participants agreed with certain principles for rehabilitation and return to sport, including the following:

●Early during rehabilitation, loads should be kept low. Many experts prefer isometric exercises initially, although research suggests lower muscle tension during concentric exercises [12]. It is best to avoid dynamic loads or high strain (eccentric stress) early during rehabilitation.

●Rehabilitation should address kinetic chain deficits (eg, hip abduction weakness, quadriceps-hamstring strength imbalance, gait abnormalities, additional ipsilateral injuries).

●Hamstrings should have regained full eccentric strength before athletes can return to full sport.

●For those in running sports, progressive running and sprinting should be part of rehabilitation. Athletes should sprint without pain before return to competition. The appropriate role for flexibility exercises remains unclear.

●Some therapists use blood flow restriction and other adjunctive treatments purportedly to increase the rate of recovery, but studies specifically of hamstring injury rehabilitation are lacking [123].

Controlled studies of hamstring injury rehabilitation are scant [4,124]. Prospective studies include the following:

●A randomized trial of 80 athletes with grade II hamstring tears found that static stretching started 48 hours after injury and performed four times a day shortened the time needed to regain normal range of motion (5.6 versus 7.3 days) and unrestricted activity (13.3 versus 15 days) when compared with patients performing stretches once a day [104].

●Treatment with isolated static stretching and progressive resistance exercises (STST group) or with progressive agility and trunk stabilization exercises (PATS group) was compared in a randomized trial involving 24 athletes with hamstring muscle strains [8]. The PATS group demonstrated lower reinjury rates two weeks after returning to sport (0 out of 13 athletes versus 6 out of 11) and at one year (1 out of 13 athletes versus 7 out of 10).

●A randomized trial of 20 patients found that those with hamstring strain and sacroiliac (SI) joint pain who were treated with SI joint manipulation showed increased hamstring peak torque compared with those who did not receive such treatment [59]. This study did not evaluate patient-oriented outcomes such as reinjury rates or the time required to return to activity.

●A randomized controlled trial of 75 elite football (soccer) athletes with hamstring injuries compared two rehabilitation protocols [120]: the L-protocol, which emphasized loading the hamstrings during extended lengthening (eccentric exercises), and the C-protocol, which consisted of conventional exercises for rehabilitation. The time to return to activity was significantly shorter for athletes treated with the L-protocol regardless of whether they had sustained a sprinting-type or stretching-type of hamstring injury. A similar trial by the same research team performed in elite sprinters and jumpers reported similar reductions in the time needed to return to sport [121].

These results support incorporating frequent static stretching early during rehabilitation, depending upon the severity of injury, eccentric strength exercises once the hamstring is sufficiently healed, and agility and trunk stabilization exercises as part of a comprehensive rehabilitation program. During treatment, many athletes continue to use materials to protect and compress the muscles, such as wraps (picture 9), fabric sleeves (picture 10), or compression pants.

RETURN TO WORK OR SPORT

Time required — Hamstring injuries vary widely in severity and therefore general recommendations about returning to work and sport are unhelpful. Furthermore, sport and work conditions vary. Participants in less demanding sports or sedentary occupations often can resume activity within a few days of injury. For patients with severe hamstring injuries who engage in high-risk sports, months may be needed before they can return to unrestricted play [84]. According to observational studies, the time needed to return to preinjury fitness is longer in athletes with strains sustained from stretching type injuries compared with those sustained from high-speed running (mean 31 versus 16 weeks), although these differences did not reach statistical significance [64,67,68,72].

According to a systematic review of studies of return to play following a hamstring injury, other factors associated with a prolonged recovery include the following [84]:

●Injuries requiring >one day before normal ambulation

●Substantial deficits in range of motion

●Extensive or severe injury based on MRI or ultrasound findings

●Stretching- (as opposed to sprinting) type injury (eg, dance)

With aggressive rehabilitation programs, some athletes with significant hamstring injuries may return to sport in approximately two weeks, but timing ranges from weeks to months, according to limited data [8,104]. For patients with physically demanding work, clinicians must consider the functional requirements of the job. In many such cases, a rehabilitation protocol like that used for participants in high-risk sports is appropriate, and the time needed to return to activity similar. Nevertheless, data for specific occupations does not exist and all recommendations about returning to work must be tailored to the individual's job and clinical status.

Determination of readiness — Given the variability in healing rates following hamstring injuries, clinical assessment must guide decisions about returning to activity. Before returning to full sport or physically demanding work, individuals need near-normal knee range of motion and normal bilateral concentric and eccentric strength, tested at 90 degrees and at 15 to 30 degrees of knee flexion with the patient prone [78,84]. Functional testing, including running, cutting, and any sport-specific or work-specific motions, should be evaluated, and there should be no sign of functional deficits, gait abnormalities, or pain. Focal muscle tenderness is associated with a high risk of reinjury. Diagnostic imaging with MRI or ultrasound either at baseline or near the completion of rehabilitation does not predict reinjury as effectively as clinical evaluation just prior to the athlete's return to play [71,78,81,125]. (See 'Physical examination' above.)

In a study of rugby athletes, those with prior hamstring injuries who returned to play only after eliminating significant limb to limb differences in hamstring strength had a far lower risk of injury recurrence than those who returned to play while strength discrepancies persisted, suggesting that rehabilitation to both strengthen the hamstring and reduce asymmetries reduces the risk of recurrence [40]. While evidence is limited, other studies support this concept of minimizing strength discrepancies before returning to play [54,126,127].

The Askling H test incorporates a ballistic movement to assess hamstring flexibility and can be used as a complement to other clinical assessments [128]. We use the H test to help determine when an athlete is ready to begin easy running as part of their rehabilitation. Although not well studied, our clinical experience suggests the H test is a useful guide. The test should not be performed unless all other basic physical assessments (eg, muscle palpation, strength testing) suggest the hamstring is healed.

To perform the H test, the patient lies supine and performs a straight leg raise with their knee in full extension as fast as possible. The patient is told to elevate the leg to the highest point that does not cause significant pain. After one practice trial with submaximal effort, the patient performs three consecutive attempts. If the subject experiences any pain or hesitancy during the test, they are not ready to begin running and are not allowed to return to play. After approximately two weeks, the test may be repeated if the initial attempt was unsuccessful.

PREVENTION

Overview of interventions — Few adequately powered, prospective studies evaluating the effectiveness of hamstring injury prevention programs have been performed. Therefore, recommendations for prevention programs are necessarily based upon a small number of primarily nonrandomized studies and clinical experience. The accompanying table outlines a prevention plan for athletes at high risk for recurrent hamstring injury (table 2).

A systematic review of seven studies concluded that there was insufficient evidence from randomized trials to draw conclusions about the effectiveness of interventions for preventing hamstring injuries among athletes involved in high-risk activities [129]. An underpowered study of manual therapy showed some benefit, but adequately powered follow-up studies are needed.

Interventions evaluated in the systematic review, as well as those from other studies, include strengthening exercises [13,130,131], stretching [132], mobilization of the sacroiliac joint and pelvis, sport-specific training and correction of dysfunctional movement, manual therapies [133,134], proprioceptive neuromuscular training [135,136], and generalized warm-up and conditioning [132].

Additional interventions that have been used to prevent hamstring injury include exercises to alter pelvic angulation, which may increase peak torque [59], improve core strength [8], and improve neuromuscular control and proprioception.

Eccentric hamstring strengthening — Properly performed eccentric strength training of the hamstrings substantially reduces the risk of injury [9,31,130,137,138]. The performance of the Nordic hamstring exercise, a widely used example of such training, is included in the accompanying prevention plan (table 2), while proper positioning for the exercise is demonstrated in the following photographs (picture 11A-B).

Multiple studies, including several randomized trials, support the use of eccentric hamstring strengthening exercises (eg, Nordic hamstring exercise) for reducing the risk of hamstring injuries, including the following [138-140]:

●In a randomized trial involving 50 professional and amateur Danish soccer teams, players (n = 461) who participated in a progressive eccentric hamstring strengthening program sustained hamstring injuries at a significantly lower rate than players (n = 481) who engaged in standard training (15 versus 67 injuries) [141]. Overall, acute hamstring injury rates per 100 player seasons were 3.8 versus 13.1 (RR 0.29; 95% CI 0.15-0.57); new injury rates per 100 player seasons were 3.1 versus 8.1 (RR 0.41; 95% CI 0.18-0.93); and the number needed to treat (NNT) to prevent 1 recurrent hamstring injury was 3 (95% CI 2-6).

●A systematic review and meta-analysis of 15 studies (7 randomized trials), including 8459 athletes participating in a range of sports who sustained 525 hamstring injuries, found that performing the Nordic hamstring exercise, either alone or as part of a more comprehensive program, reduced the risk of injury by 51 percent (RR 0.49; 95% CI 0.32-0.74) [142]. Four studies reported compliance rates below 50 percent, suggesting that injury rates may be reduced further with better compliance.

Compliance can be improved by having athletes concentrate on performing the Nordic hamstring exercise with proper technique and greater intensity (ie, maintaining postural control for as long as possible before collapsing to the ground), rather than by increasing the number of sets and repetitions (ie, training volume) per training session [143]. This approach is supported by a systematic review of 13 studies involving several hundred athletes that found significant improvements in hamstring strength and muscle architecture regardless of whether a low-volume or high-volume training regimen was used, provided the program duration was at least six weeks. Both low- and high-volume programs produced comparable gains in eccentric strength, increases in fascicle length, and reductions in pennation angle.

●The results of a meta-analysis of five studies involving 4455 participants suggest that prevention programs that include the Nordic hamstring exercise reduce the risk of a hamstring injury by 51 percent (of note, three studies included the Fédération Internationale de Football Association [FIFA] 11+ program) [144]. (See "Anterior cruciate ligament (ACL) injury prevention", section on 'General ACL injury prevention programs'.)

COMPLICATIONS

Reinjury — The primary complication after a hamstring injury is the increased risk of reinjury to the same muscle. The rate of reinjury is at least 30 percent, and some studies report rates as high as 60 to 70 percent [4,5,7,8]. Lifetime reinjury rates vary considerably by activity.

A systematic review of recurrent hamstring injury reported that increased severity of the initial injury and ipsilateral anterior cruciate ligament reconstruction are associated with an increased risk of reinjury, while rehabilitation consisting primarily of functional stability and agility training is associated with a decreased risk. However, the authors of the review emphasized the low quality of the available evidence [145]. Developing and maintaining muscle strength, and limiting discrepancies in strength relative to the ipsilateral quadriceps and contralateral hamstring may also reduce the risk of reinjury. (See 'Risk factors' above and 'Sport-specific risks' above and 'Follow-up care and rehabilitation' above.)

Studies to identify important risk factors for reinjury are ongoing. Magnetic resonance imaging (MRI) findings have not been found to be useful for determining reinjury risk [146].

Persistent mobility and strength deficits — A number of observational studies report that, following significant hamstring strains, deficits in mobility, strength, and motor function may persist in some individuals for many months, increasing the risk for reinjury. This may be due to inadequate or incomplete rehabilitation programs in some cases. Suggestive studies include the following:

●A study using video analysis of 45 professional football (soccer) players' kicking biomechanics found that players with a history of prior hamstring injury (n=14) altered their kicking form, using a shorter backswing and different knee flexion angles, among other variables [147]. This suggests that some rehabilitation programs may not effectively restore normal kicking.

●A study of 20 professional football players with a history of unilateral hamstring strain found a significant reduction in the horizontal force generated by the previously injured leg during sprint testing on a treadmill dynamometer [148].

●In a study of 37 Australian rules football players, the ten with a history of hamstring within 3 years (n=10) demonstrated significantly weaker knee flexion strength compared with players without a history of hamstring injury [149].

Hamstring syndrome — Another potential complication of hamstring injury is chronic, radiating, sciatica-type pain in the region of the gluteal muscles and ischial tuberosity where the hamstring muscles originate. Different terms have been used to describe this syndrome, which is associated with prior hamstring tears, including hamstring syndrome [19] and proximal hamstring syndrome [23].

The diagnostic triad for the condition is pain induced by sitting, stretching the affected posterior thigh (ie, hip flexion plus knee extension), and running fast. Direct pressure at the site of pain may reproduce sciatic symptoms. Cyclic stress involving repeated hip flexion and knee extension causes persistent pain near the ischial tuberosity that radiates down the posterior thigh. While Ultrasound may demonstrate fibrosis around the sciatic nerve and chronic changes at the ischial tuberosity, MRI is considered the best imaging study to detect sciatic compression and delineate pathological changes at or near the ischial tuberosity.

A few explanations for this syndrome have been proposed. There is operative and radiological evidence that scar tissue forms around the sciatic nerve following some hamstring injuries, and scar tissue may play a role in this syndrome [20-22,24,94,95]. Alternatively, congenital fibrotic bands that irritate the sciatic nerve may be the cause. These scars or fibrotic bands compress the sciatic nerve, causing the symptoms described above.

Some researchers believe the syndrome may be due to a proximal hamstring tendinopathy. One study of the histopathology from 15 cases of persistent ischial tuberosity and gluteal pain following hamstring injury reported that all specimens exhibited classic findings of tendinosis [76]. Chronic pain is associated with tendinopathy of numerous tendons, including the Achilles, extensor carpi radialis brevis (tennis elbow), and patellar, and this study suggests that the proximal hamstring syndrome may result from tendon degradation, release of pain producing substances, and diminished tendon heterogeneity, similar to other chronic tendinopathies. (See "Overview of overuse (persistent) tendinopathy".)

The proximal hamstring syndrome is not well understood and there is insufficient data to provide clear guidance about treatment. Surgery to decompress the nerve may be needed for cases in which a well-conceived and rigorously followed physical therapy program is unsuccessful at improving symptoms.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: General issues in muscle and tendon injury diagnosis and management" and "Society guideline links: Muscle and tendon injuries of the lower extremity (excluding Achilles)".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topics (see "Patient education: Muscle strain (The Basics)" and "Patient education: Hamstring injury (The Basics)")

SUMMARY AND RECOMMENDATIONS

●Epidemiology and definition – Hamstring injuries are common in sports, particularly those involving high-speed running. They can be defined as any strain or tear of any of the muscles or tendons within the hamstring group of the posterior thigh, including the biceps femoris, semitendinosus, and semimembranosus muscles. (See 'Introduction' above and 'Epidemiology' above.)

●Hamstring function – The hamstring muscles' primary function is to decelerate the lower limb during the forward swing phase of running or walking; the hamstrings also contribute to the stability of the hip and knee during movement. Secondary functions include knee flexion and hip extension. (See 'Anatomy and physiology' above.)

●Risk factors – Modifiable risk factors for hamstring injury may include:

•Inadequate warm-up

•Increased training volume

•Muscle fatigue

•Hamstring inflexibility

•Hamstring weakness (may be weakness relative to quadriceps or to contralateral hamstring)

•Cross-pelvic posture (ie, lumbar lordosis with anterior pelvic tilt) (figure 8 and picture 2)

•Lumbar-pelvic weakness

•Poor biomechanics (eg, running or change of direction) (see 'Risk factors' above)

●Mechanism and clinical presentation – Hamstring injury occurs when a sudden, forceful, eccentric contraction of the hamstring occurs against resistance. Most individuals with hamstring injuries present acutely complaining of the sudden onset of posterior thigh pain while performing a high-risk activity, such as sprinting. Focal warmth and tenderness at the site are common. Injury can usually be diagnosed on the basis of the history and physical examination. (See 'History and mechanism of injury' above and 'Physical examination' above.)

●Diagnostic imaging – When the diagnosis is unclear or a more detailed assessment of the hamstrings is needed, musculoskeletal ultrasonography (US) and magnetic resonance imaging (MRI) are the best methods for assessing hamstring injuries. US allows for bedside evaluation of the muscles and tendons while they move. However, neither US nor MRI is accurate for estimating recovery time. (See 'Diagnostic imaging' above.)

●Differential diagnosis – The cause of posterior thigh pain may be more difficult to determine in athletes with long standing pain, recurrent symptoms, or an unclear mechanism of injury. A differential diagnosis for posterior thigh pain is provided in the text. (See 'Differential diagnosis' above.)

●Indications for orthopedic referral – The great majority of hamstring injuries can be managed successfully with rest and physical therapy. However, in some circumstances, operative repair may yield favorable results. Orthopedic consultation should be obtained in such circumstances, which include:

•Complete (grade III) proximal hamstring rupture.

•High-grade (grade II or III) distal hamstring tears.

•In addition, hamstring avulsion injuries should be evaluated by an orthopedic surgeon or sports medicine specialist. (See 'Indications for orthopedic consultation' above.)

●Treatment and prevention – Standard treatment for acute hamstring injuries includes protection, rest, ice, compression, and elevation (PRICE). Rehabilitation involves a gradual progression of exercises designed to improve the function of the hamstring muscles and the entire posterior kinetic chain. Successful hamstring rehabilitation programs emphasize eccentric strengthening exercises. Programs for rehabilitation and prevention have much in common; sample programs are provided. (See 'Follow-up care and rehabilitation' above and 'Prevention' above.)