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Throwing injuries: Biomechanics and mechanism of injury

Throwing injuries: Biomechanics and mechanism of injury
Literature review current through: Sep 2023.
This topic last updated: Dec 14, 2021.

INTRODUCTION — Millions of people throughout the world participate in sports that involve throwing or throwing-like movements. These movements range from classic ball throwing, as performed by baseball pitchers or cricket bowlers, to throwing implements other than balls, such as a javelin, to throwing-like actions that do not involve a ball directly, such as a tennis serve. All such movements involve complex biomechanics and great stresses being placed on the musculoskeletal system. Improper biomechanics, excessive stress beyond the capacity of an individual's musculoskeletal system, or cumulative trauma from throwing too frequently can cause injury.

This topic will review the biomechanics of throwing and the relationship of these biomechanics to common throwing-related injuries. The clinical presentation and diagnosis of specific injuries are discussed separately. (See "Throwing injuries of the upper extremity: Clinical presentation and diagnostic approach".)

EPIDEMIOLOGY — While the prototypical throwing athlete in the United States is a baseball player, many athletes use similar motions, including softball players, American football quarterbacks, javelin throwers, cricket bowlers, and water polo players. In addition, athletes involved in sports with repetitive overhead motions such as swimming, tennis, and volleyball often suffer similar injuries.

Based on data from 2017 to 2019, it is estimated that there are approximately 9.7 million softball players, 15.65 million baseball players, 21.6 million tennis players, 6.5 million volleyball players, and 27.6 million swimmers in the United States alone. At the high school level, 2018 to 2019 data from the United States National Federation of High School Sports report that there are more than 480,000 baseball players, 360,000 softball players, 348,000 tennis players, 516,000 volleyball players, and 44,000 water polo players [1].

Upper extremity injuries are common in the overhead athlete and account for 75 percent of the time missed from sport among collegiate baseball players [2]. A cohort study with 10-year follow-up of 481 youth pitchers revealed that 5 percent experienced serious injury (defined as injury requiring elbow or shoulder surgery or retirement from pitching) over that time [3]. Most of these upper extremity injuries are caused by overuse. Risk factors among baseball players include position (pitchers and catchers are at greatest risk), high number of pitches per game, which should include warm-up pitches (ie, high pitch counts), high number of innings pitched, higher number of pitches in bullpen sessions, higher pitch velocity (pitchers who throw harder are at greater risk of injury), year-round play, early sports specialization, and inadequate recovery time between throwing sessions (games or practice) [2-15]. Year-round play in particular is thought to the increase the injury rate among youth baseball pitchers [7,8,16]. (See 'Risk factors' below.)

Injury rates among fast bowlers in cricket are relatively high. During the 2011 Cricket World Cup, the rate of injury among fast bowlers was approximately 231 per 1000 bowling days [17]. Reports of injuries to cricket fast bowlers suggest that the stress of throwing affects all areas of the kinetic chain. A 10-year prospective cohort study showed that injuries to 209 fast bowlers affected multiple anatomical areas with the most common being hamstring, abdominal muscles, calf, quadriceps and groin, all of which were more common than shoulder injury [18]. During one West Indian tournament, fast bowlers sustained eight low back injuries, three abdominal injuries, and one shoulder injury [19]. In a review of cricket injuries at first class level competitions over five years in Australia, injuries to the shoulder tendons and shoulder dislocations/subluxations totaled 33 of 527 injuries documented overall [20].

Many throwing-related injuries involve structures in and around the shoulder, but other parts of the upper extremity may be affected, particularly the elbow [21]. As an example, a prospective cohort study of 298 youth pitchers found that 26 percent developed elbow pain and 32 percent shoulder pain [6]. In baseball, the frequency of elbow pain appears to increase with the age of the competitor, ranging from approximately 30 percent among little league players and 40 percent among adolescents to over 50 percent among older high school and professional players [22].

Throwing-related injuries are not limited to the upper extremity. As examples, back injuries are common among cricket bowlers, tennis players, volleyball players, swimmers, and baseball players [9,23-26]. The odds ratio for developing degenerative disk disease of the lumbar spine was approximately 3.0 among a cohort of 308 college baseball and swimming athletes, although the injuries could not be linked definitively to throwing and overhead motions [27]. In a retrospective series of 127 cases of lumbar spondylolysis, high risk sports included baseball and tennis [25]. Strains of the rectus abdominis in particular, but also other abdominal muscles, are associated with tennis service and are relatively common [28].

Throwing-type injuries are not limited to classic throwers. Studies estimate that 4 to 17 percent of tennis players have suffered a shoulder injury [23]. Shoulder injuries are common among water polo players [29]. In volleyball, about 10 to 20 percent of all injuries involve the shoulder and a similar number the low back and trunk. The mechanism for these is thought to involve spiking, as they generally occur in front line players [24].

RISK FACTORS

Anatomic — Anatomic risk factors include joint motion restrictions and weakness. However, some of the anatomic findings described here (eg, increased shoulder external rotation), while increasing the stress placed on particular structures and thereby increasing injury risk, enable increased throwing velocity and pitching effectiveness. A clinical approach seeking to balance injury risk and athletic performance is often needed, particularly for elite performers.

Lower extremity and trunk weakness force the thrower to rely more on the shoulder girdle to generate power and thus increase the risk of upper extremity injury. Poor lumbopelvic control (ie, inability to stabilize the lumbar spine and pelvis in a single-leg stance) is associated with an increased risk for upper extremity injury [30].

Weakness and strength imbalances in the dynamic elbow and shoulder stabilizers (eg, scapular stabilizers, rotator cuff muscles, forearm muscles) lead to increased loads on the static stabilizers (eg, ligaments, joint capsules) with commensurate increase in injury to these structures [31]. Other anatomic factors, while not directly correlated with injury rates, are associated with increased stress on the elbow and shoulder. These factors include glenohumeral internal rotation deficiency (GIRD) [32], increased shoulder external rotation, increased total shoulder rotation [33], decreased hip range of motion [34,35], and diminished ankle dorsiflexion [36].

Biomechanical — Flaws in throwing motion can place increased stress on particular body parts. Common biomechanical errors may involve positioning of the shoulder, elbow, wrist, trunk, or foot. These flaws and possible resulting injuries are discussed separately. (See 'Biomechanics of throwing and related risk of injury' below and "Throwing injuries of the upper extremity: Clinical presentation and diagnostic approach".)

Some biomechanical flaws may enable greater ball motion or velocity, and pitching effectiveness may be compromised by proposed corrections [37,38]. A clinical approach seeking to balance injury risk and athletic performance is often needed, particularly for elite performers.

Overuse — Overuse, as determined by high pitch counts or year-round play, is associated with an increased risk of injury [7,8,16]. Pitch counts may be useful for monitoring general overuse but must include warmup and practice pitches in addition to in-game pitches [39,40]. Note that counting pitches and innings provide only a partial measurement of total load. Pitching frequency (ie, rest or lack thereof) and time off from sport are additional important factors [41,42]. A history of pitching with arm fatigue or shoulder pain associated with heavy use are additional risk factors for shoulder injury [31,43].

Early sports specialization — Increasingly, young athletes are foregoing participation in several sports to specialize in one. In part, this is due to the popularization and misinterpretation of the "10,000 hours" theory of skill mastery and the success of several athletes who specialized at a very young age. While many believe that parents are the primary drivers of early sports specialization, child athletes may also be responsible for choosing early sports specialization [44].

Evidence is mixed as to whether early sports specialization is helpful for achieving elite athletic skill. While early specialization may be helpful for individual sports (eg, figure skating, golf), it may hinder development for team sports [45,46]. Evidence is growing that early sports specialization increases the risk of injury and burnout [47-49]. Among early specializers, youth baseball players in particular appear to be at increased risk of injury [50-52].

CLINICAL ANATOMY — A complex network of anatomic structures endows the human shoulder with tremendous mobility, greater than any other joint in the body (figure 1 and figure 2 and figure 3 and figure 4 and figure 5 and figure 6 and figure 7 and figure 8 and figure 9). While such mobility enables athletes to throw, the shallow depth of the glenoid and limited contact between the glenoid and humeral head that allows for this mobility play a role in many throwing-related injuries. Key structures that help to maintain the stability of the relatively shallow glenohumeral joint (figure 10) include the labrum, glenohumeral ligaments, and rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) (table 1). Extra-articular stabilizers that play a key role include the scapula and periscapular muscles.

The prime movers of the shoulder are the deltoid, pectoralis major, latissimus dorsi, and teres major (picture 1 and picture 2 and figure 11 and figure 12). The upper extremity muscles for the primary motions during the throwing motion include the deltoid and supraspinatus for shoulder abduction; the superior trapezius and serratus anterior for upward rotation of the scapula; the pectoralis major, latissimus dorsi, anterior deltoid, teres major and subscapularis for internal rotation; the pectoralis major, anterior deltoid and subscapularis for horizontal adduction (table 2 and figure 13 and table 3 and figure 14) [53]. A description of the basic anatomy and biomechanics of the shoulder joint is provided separately. Structures of particular relevance to throwing-related injuries are discussed below. (See "Evaluation of the adult with shoulder complaints", section on 'Anatomy and biomechanics'.)

Throwing is a complex movement that requires contributions from both static shoulder stabilizers (glenohumeral ligaments and shoulder capsule) (figure 15 and figure 5) and dynamic stabilizers, including the scapula and periscapular muscles (figure 16 and figure 14 and figure 12) as well as the rotator cuff (figure 17). The static stabilizers and the labrum are at greater risk of injury if the dynamic stabilizers fail to modulate the degree and velocity of humeral head translation that occurs during throwing. If the periscapular muscles are weak or substantially weaker than their antagonists, protraction (internal rotation) of the humeral head in the glenohumeral joint can occur, increasing the risk for impingement of the supraspinatus tendon between the humeral head and acromion. (See "Subacromial (shoulder) impingement syndrome".)

Throwing athletes often develop increased external rotation of the shoulder, which allows for increased throwing velocity [54-56]. Some of this increased external rotation is functional and due to soft tissue stretching, which is thought to occur primarily in the rotator cuff and the capsular and ligamentous tissues [57,58]. The posterior shoulder capsule appears to thicken in response to the repetitive stress of overhead throwing, and maintaining elasticity in the capsule may be important for preventing injury [59]. Among athletes who began throwing extensively during childhood or adolescence, changes in bone formation (increased proximal humeral retrotorsion and glenoid retroversion) appear to account for part of this increased mobility [60,61]. Similar changes may occur in cricket bowlers [62].

Os acromiale is an anatomic variant in which the anterior acromial apophysis fuses to the scapular spine with a fibrocartilaginous rather than a bony connection. It is found in approximately 8 percent of the population worldwide and is more common in Black individuals and males [63]. Although associated with shoulder impingement and rotator cuff injury in some studies, os acromiale is usually an incidental finding [63,64].

The elbow is a complex joint that must control significant deforming forces during throwing (figure 18). It is composed of the humeroulnar, humeroradial, and proximal radioulnar joints. The humeroulnar and humeroradial joints act together in flexion and extension. The proximal radioulnar joint allows for pronation and supination of the forearm. The growth plates of the elbow are usually closed by the mid-teens.

In elbow flexion, the ulnar (or medial) collateral ligament (UCL) complex provides the primary static restraint against valgus stress (55 percent of the stabilizing force versus 33 percent in elbow extension) [65,66]. The UCL complex is composed of three ligaments: anterior oblique, posterior oblique, and transverse (figure 19 and figure 20). The anterior oblique ligament originates at the medial epicondyle and is composed of two functional bands, the anterior and the posterior. The anterior bundle inserts along the medial aspect of the coronoid process near the sublime tubercle, while the posterior bundle inserts on the medial olecranon. The anterior bundle provides valgus stabilization from full elbow extension to 85 degrees of flexion, whereas the posterior bundle provides stabilization after 55 degrees of flexion. The anterior bundle is the most important stabilizer against valgus forces exerted on the elbow during throwing [67].

The resting (or "open-packed") position of the elbow consists of 70 degrees of flexion and 10 degrees of forearm supination. In this position, the elbow possesses the least bony stability and thus depends primarily upon the soft tissues for stability. The UCL hypertrophies over time in response to the repetitive loading from pitching [68,69]. Although the UCL is the most important static stabilizer, when the elbow is in a valgus position during throwing, elbow stability is provided dynamically by the flexor-pronator muscles. Of these muscles, the flexor carpi ulnaris serves as the primary stabilizer and the flexor digitorum superficialis serves as the secondary stabilizer [70].

The kinetic chain for throwing begins in the lower extremities. The force generated by the lower extremities is then transmitted rotationally through the trunk muscles to the throwing arm. The abdominal and pelvic muscles (transverse abdominus, internal oblique, external oblique, rectus abdominus, quadratus lumborum, psoas, and iliacus) (figure 21 and figure 22 and figure 23 and figure 24 and figure 25 and figure 26) and back muscles (shoulder girdle muscles, serratus posterior, splenius group, erector spinae group, and transversospinalis group) (figure 27 and figure 28 and figure 29) are critical to maintaining posture, stabilizing the spine, rotating the torso as the thrower turns to face the target, and dissipating energy during follow-through. Contraction of the diaphragm, abdominal muscles, and pelvic floor muscles provides trunk stability, and increases intraabdominal pressure which further contributes to stability [71].

The muscles that rotate and stabilize the scapulothoracic joint also help to stabilize the glenohumeral joint and provide a base for the efficient transfer of energy to the throwing arm (figure 30 and figure 12) [72,73]. The trapezius, levator scapulae, and rhomboids elevate the scapula; the pectoralis minor, subclavius, latissimus dorsi, trapezius, serratus anterior, and pectoralis major depress the scapula. The trapezius and serratus anterior rotate the scapula upwards; the rhomboids, levator scapulae, pectoralis minor and major, and latissimus dorsi rotate the scapula downwards. The serratus anterior and pectoralis major and minor protract the scapula; the trapezius, often in conjunction with the rhomboids and latissimus dorsi, retract the scapula [74].

BIOMECHANICS OF THROWING AND RELATED RISK OF INJURY

Overview and images — Throwing involves the transfer of energy generated by the body to an object [53,75-77]. The largest muscles with the greatest potential to generate force are found in the lower extremity and trunk. During throwing, the upper extremity muscles act primarily as a means to transfer energy generated by the lower extremity and trunk to the thrown object, and to guide that object (or projectile) along the desired flight path. While there are subtle differences in the kinematics and kinetics of various pitches (eg, fastball, curveball), the overall biomechanics are relatively similar [78]. However, object weight, size, and shape, as well as the presence of a pitching mound, all can affect the biomechanics of throwing [79,80].

The biomechanics of throwing are often separated into the following six phases (figure 31 and movie 1):

Windup (picture 3)

Stride (sometimes called early cocking) (picture 4)

Cocking (sometimes called late cocking) (picture 5)

Acceleration (picture 6)

Deceleration

Follow-through (picture 7)

A smooth transition from one phase to the next maximizes the velocity of the object thrown, while reducing the risk of injury. As structures are placed under varying compressive and distractive loads during each phase of throwing, determining during which phase the injury or pain occurs can help the clinician in making a diagnosis. Youth pitchers typically demonstrate similar biomechanics to adults, albeit with lower forces [81,82].

Clinicians should consider the kinetic chain when evaluating the cause of injury, as abnormalities in one part of the chain often lead to compensation and excessive stress being placed on another area.

Windup phase — The purpose of the windup (picture 3) is to move from the initial position of play (eg, pitcher standing on mound, quarterback receiving the ball from the center) to the position from which the athlete will begin throwing. The windup is the phase with the greatest variability. During this movement, the pitcher rotates their torso until it is positioned approximately perpendicular to the target, which enables subsequent hip rotation and force generation. In baseball pitching, the knee of the lead leg is typically raised to its highest point when the body weight is fully shifted. For most throws, the ball is held near the chest with the hands close together at the end of this phase. The windup ends when the athlete has shifted their weight onto the stance (ie, rear) foot.

Overall forces and velocities are low during the windup phase and there is minimal risk for injury. However, without adequate hip and sacroiliac joint motion, the thrower will have difficulty achieving a full windup, which may result in increased shoulder torque during the acceleration phase [83].

Stride (early cocking) phase — The purpose of the stride phase (picture 4) is to initiate motion towards the target and generate momentum. This phase begins with hip abduction of the stance leg, which drives the lead leg toward the target. The hands separate from one another as the upper extremities are abducted. These abduction movements generate potential energy by stretching the muscles, tendons, and connective tissues.

Next, as the lead hip continues to abduct and extend, the hip of the rear leg internally rotates, while the knee extends. Both shoulders are abducted further by the actions of the deltoid and supraspinatus, and stabilized by the other rotator cuff muscles. The scapula is rotated superiorly by the trapezius and serratus anterior. The elbows extend as the shoulders abduct, with the elbow position controlled by eccentric contraction of the flexors, primarily the biceps [84]. The wrist is moved from slight flexion to hyperextension by the concentric contractions of the wrist and finger extensors. The stride phase ends when the lead foot contacts the ground.

For most baseball pitchers, stride length averages 75 percent of body height with a stride offset of approximately 0.4 cm. In other words, the lead foot lands almost directly in front of the rear foot. If the athlete over-strides, he or she will be unable to rotate their hips, resulting in a loss of velocity. The athlete may try to compensate by overloading the shoulder, which may lead to rotator cuff injury. As examples, if the pitcher "opens up too soon," in other words strides off-line to the side away from the throwing arm (eg, first base side for a right-handed pitcher), the torso will be ahead of the shoulder and the anterior capsule will be over-stretched, which may result in shoulder instability. If the pitcher throws "across their body," or strides off-line towards the side of the throwing arm (eg, third base side for a right-handed pitcher), the labrum can be subjected to larger shear forces.

Cocking phase — The main purpose of the cocking phase (picture 5) is to place the shoulder in maximum external rotation. The cocking phase begins when the lead foot makes contact with the ground and ends when the arm is maximally externally rotated, which may approach 180 degrees in a professional baseball pitcher. The lead leg decelerates through eccentric contraction of the quadriceps muscles, which then stabilize the leg through isometric contraction once it contacts the ground. The pelvis begins rotating forward toward the target, with the trunk following. During the time between pelvic and trunk rotation, the abdominal and oblique muscles are stretched, which creates potential energy. The speed of hip rotation correlates closely with ball velocity.

During this phase, the rotator cuff and scapular muscles stabilize the arm at approximately 90 degrees of abduction while the shoulder externally rotates. The elbow reaches approximately 100 degrees of flexion and is held in this position by eccentric contractions of the triceps and anconeus muscles. Elbow extension begins just prior to the end of the cocking phase. During the cocking phase, the wrist and hand continue to be held in a hyperextended position.

When the shoulder is maximally externally rotated and abducted to 90 degrees, internal impingement may occur, with the undersurface of the superior rotator cuff being pinched against the labrum. This may cause tearing of the undersurface of the rotator cuff and fraying of the labrum. In addition, repetitive stretching of the anterior capsule may cause increased laxity anteriorly or inferiorly, and possibly glenohumeral instability.

Acceleration phase — The main purpose of the acceleration phase (picture 6) is to impart maximal velocity to the thrown object along the desired trajectory. This phase is explosive. Acceleration begins with initiation of shoulder internal rotation and ends with release of the thrown object. The trunk moves from hyperextension to forward flexion over the lead leg as the rectus abdominus and oblique muscles concentrically contract. The shoulder internal rotators concentrically contract to accelerate the arm, which internally rotates with great acceleration (from 0 to 7000 degrees/second in less than one-tenth of a second). During this acceleration, the rotator cuff and peri-scapular muscles stabilize the shoulder and scapula. The acceleration phase is completed when the thrown object is released.

Although the arm-ground angle differs in sidearm pitchers compared with overhead pitchers because of trunk tilt, typically the arm remains abducted to approximately 90 degrees throughout the acceleration phase for both types of throwers. The elbow is stabilized by its flexors (biceps, brachialis, and brachioradialis). The forearm is pronated by the pronator teres. The wrist is brought from a hyperextended to a neutral position by the flexor digitorum and wrist flexors (flexor carpi radialis and flexor carpi ulnaris).

The explosive movement of the acceleration phase is the result of strong contractions by the internal rotators, including the subscapularis, pectoralis major, and latissimus dorsi, and abduction by the deltoid. In response, the stabilizers of the shoulder, including the supraspinatus, infraspinatus, teres minor, and scapular stabilizers (trapezius, serratus anterior, rhomboids and levator scapula), contract eccentrically to maintain the humeral head within the glenoid [53,85].

During this phase, maximal valgus stress is exerted on the elbow, placing the ulnar collateral ligament (UCL) under tension (and increasing the risk for tears) and compressing the radiocapitellar articulation (increasing the risk for osteochondral injury) (picture 8) [84]. In addition, as the elbow extends while under a valgus load, the posteromedial aspect of the olecranon may impinge against the olecranon fossa, causing posteromedial elbow pain. Over time, spurring and degenerative changes may occur at the posteromedial aspect of the ulnohumeral articulation, causing valgus extension overload (VEO) syndrome or an olecranon osteochondral defect (OCD).

Deceleration phase — The main purpose of the deceleration phase is to dissipate energy after the thrown object is released. This phase lasts from object release to maximum shoulder internal rotation and terminal elbow extension. A longer deceleration arc allows forces to dissipate over a greater distance, thus reducing the risk of injury. Therefore, most throwing athletes horizontally adduct the arm across their trunk to lengthen the deceleration arc.

During deceleration, distractive forces at the shoulder can reach the equivalent of the thrower's body weight, requiring large eccentric contractions by the rotator cuff, posterior deltoid, and latissimus dorsi muscles to maintain proper glenohumeral position. The elbow flexors eccentrically contract to decelerate the elbow, which usually stops moving just short of full extension. Simultaneously, the biceps and supinators contract to decelerate forearm pronation, while the wrist and finger extensors contract to decelerate the flexing wrist and fingers.

During deceleration, traction on the superior labrum at the insertion of the long head of the biceps tendon may lead to a superior labral anterior to posterior (SLAP) lesion. (See "Superior labrum anterior to posterior (SLAP) tears".)

Follow-through phase — The main purpose of the follow-through phase (picture 7) is to complete the dissipation of energy begun in the deceleration phase. A secondary purpose in some sports (eg, baseball, softball, and tennis) is to position the body to prepare to field or return the ball. The follow-through phase begins at the end of shoulder internal rotation and ends when the trailing leg touches the ground. Flexion of the trunk and extension of the lead knee increases the deceleration arc and allows increased amounts kinetic energy to be dissipated within the larger muscles of the trunk and legs. Overall, forces and velocities are relatively low during this phase and there is minimal risk for injury.

BIOMECHANICAL VARIATIONS BY SPORT — The biomechanics of throwing differ from the basic pattern described above to varying degrees depending upon the activity [53]. Below, we describe some important differences.

Baseball pitchers — Baseball pitchers typically throw from a windup when there are no runners on base. They start with their feet pointing toward home plate with the stance (rear) foot alongside or on the pitching rubber. The throwing motion starts with the pitcher pivoting the back foot until it is parallel and against the rubber. With runners on base, pitchers often throw from a set position, commonly referred to as "the stretch," since starting from this position shortens the length of the pitching motion and decreases the risk of a stolen base. Many relief pitchers routinely pitch from the set position even if there are no runners on base in order to have a single routine when throwing. In the set position, the pitcher starts with his back foot parallel and against the rubber, the front foot closer to home plate, and the trunk already perpendicular to the direction of the throw. Despite the difference in starting positions, biomechanically there is no significant difference between pitching from the windup or stretch positions [86]. Similarly, although differences exist in the throwing motions of fastballs and breaking pitches, the overall forces placed on the body are similar [87]. The presence and height of the mound also affect throwing biomechanics [79].

Baseball fielders — Baseball infielders and catchers generally throw using mechanics similar to pitchers, albeit with varying arm angles and less of a windup. However, baseball outfielders trying to throw out a runner use a running approach rather than a stationary windup. This allows the linear momentum of the body to be transferred to the ball, thereby increasing velocity and distance. Infielders sometimes need to throw quickly and across their body without time to set their feet. This requires that the shoulder generate most of the force, significantly decreasing velocity and distance.

Softball pitchers — Softball pitchers typically throw the ball underhanded with a windmill windup. Overall, the forces exerted on the shoulder and elbow are similar to those found in overhead pitching, although the timing of these force differs [88]. With underhand pitching, the distraction forces at the shoulder and elbow are greatest during acceleration. One exception to the similarity of forces is the valgus torque exerted on the elbow, which appears to be decreased in underhand pitching, leading to a lower incidence of ulnar collateral ligament (UCL) injuries. Softball pitchers typically pitch more frequently than baseball pitchers, often twice in one day or on consecutive days, thereby increasing the risk of injury [89,90].

American football — Football players make several adjustments to compensate for the greater weight and different shape of the football. The quarterback externally rotates his shoulder sooner than the baseball pitcher, starting during the stride (early cocking) phase. This allows the quarterback to have greater external rotation at foot contact and a longer arc of acceleration. In addition, the quarterback's throwing motion typically involves greater elbow flexion and less shoulder abduction, typically 45 degrees or less, compared to the typical baseball pitcher [53].

Javelin — Throwing a javelin is similar an outfielder trying to throw out a runner, as both use a running approach rather than the stationary windup of a pitcher. The running approach allows the linear momentum of the body to be transferred to the javelin, increasing velocity and distance.

Cricket — Technically, a cricket bowler does not throw a ball. A bowler is only permitted to use shoulder rotation to impart velocity to the ball and is not permitted to bend the elbow. Overall, the overhand motion of fast bowling is similar to that of a baseball outfielder or javelin thrower in that it involves a running approach. However, cricket bowling is more variable since the object is not to throw for long distance but instead to strike a certain spot with a particular speed and spin [91,92].

The phases of cricket bowling include the following:

Run-up phase (approach)

Pre-delivery stride

Delivery stride phase

Back foot strike

Mid-delivery

Front foot strike

Ball release phase

Deceleration phase

Follow-through phase

Run-up phase — The run-up phase ends after the pre-delivery stride. The bowler takes a variable number of steps before leaping off the front (takeoff) foot into the air at the start of the pre-delivery stride. During the leap, the back foot passes the front (takeoff foot) and in the case of a side-on bowler rotates such that the back (landing) foot and body land at a right angle to the batsman. In contrast, a front-on bowler lands with their body facing the batsman.

Delivery stride phase — Delivery begins with the back foot strike and the body leaning away from the batsman. This lean is particularly pronounced in side-on bowlers. During the mid-delivery, the arm and shoulder swing down and back. During the front foot strike, ground reaction forces reach four to nine times body weight [93]. Many bowlers keep their knees as straight as possible to increase velocity. However, landing with the knee in complete extension may increase the risk for trunk and lower extremity injuries. The arm and shoulder continue in counter-rotation and usually reach a position of maximal distance behind the body at or near the front foot strike, as the hips and shoulder start to rotate toward the batsman. At this point, the non-bowling arm is rapidly adducted and extended and the trunk starts flexing forward. With mixed bowling (a combination of front-on and side-on techniques), there is a higher degree of hyperextension, lateral flexion, and twisting of the spine that increases the risk of back and spine injury [93].

Ball release phase — The release point and degree of arm circumduction that occurs between the front foot strike and ball release is variable and related to the bowler's technique.

Follow-through phase — The bowler must veer to the side to avoid stepping on the pitch, which can damage the playing surface and may result in disqualification of the bowler if done repeatedly. Repeated performance of these rotational movements and changes of direction theoretically increase the risk of overuse injuries, such as labral tears of the hip and meniscal tears of the knee, as well as acute injuries, such as ankle sprains.

Cricket fielders use throwing mechanics and sustain injuries that are similar to baseball fielders.

Water polo — Water polo players have a relatively high incidence of shoulder injuries [29]. While some of these injuries can be attributed to the repetitive overhead motion of swimming, others are related to throwing. A biomechanical study of seven elite water polo players reported a correlation between shot volume and shoulder pain [94].

Since water polo players are not permitted to touch the bottom of the pool during play, the shoulder must generate a much larger portion of the forces needed for throwing than is necessary for land-based throwing sports [29]. In addition, because the opponent is often obstructing the shot, water polo players, usually are not able to use the ideal throwing motion, often using an abbreviated cocking phase. This results in velocity being generated primarily by the shoulder, which may result in overload of the anterior capsule and posterosuperior labrum and impingement of the glenoid [95].

Tennis — The tennis serve and overhead stroke involve biomechanical actions similar but not identical to those found in throwing. The shoulders of elite tennis players can reach rotational velocities of over 2400 degrees/second during a serve. Compared with the throwing athlete, the tennis player's body undergoes greater trunk hyperextension and lateral flexion during serves, and the pelvis does not rotate prior to the upper body [96].

In addition to the throwing phases described above, the tennis serve can be divided into biomechanical nodes composed of specific positions and motions, which can vary somewhat depending upon the type of serve performed. The basic nodes of a tennis serve generally include [97]:

Knee flexion in cocking that progresses to knee extension at ball impact

Hip and trunk counter rotation away from the court in cocking

Coupled scapular retraction/arm rotation to achieve cocking in the scapular plane

Back leg to front leg motion to create a ''shoulder over shoulder'' position as the ball is struck

Long-axis rotation into ball impact and follow-through

YOUTH THROWING ATHLETES — The epidemiology, anatomy, and biomechanics of youth and adult throwing athletes differ in a number of important ways. In adults, motion limitations, core muscle weakness, and asymmetries in strength and mobility are as common as biomechanical issues for many throwers. Among youth throwers, faulty biomechanics and excessive throwing beyond the physiologic tolerance of growing bone are much more likely to be the cause of injury. Year-round play in particular is thought to increase the injury rate among youth baseball pitchers [7,8,16], as is early sports specialization [12].

Additional risk factors of importance include inadequate recovery time between games pitched and a high number of pitches per game (generally >80 pitches). Guidance about pitch limits is provided at the website of Major League Baseball [98]. Mound height affects shoulder and elbow kinetics, and pitching on flat ground rather than from a mound may reduce injury risk. Therefore, youth throwers should not be rushed into throwing off of a mound [79].

Young pitchers who add different types of pitches to their repertoire before their peers may be doing so to stay competitive because their basic pitches are less effective. This approach often bodes poorly for long-term success. Many experts in youth pitching believe that young pitchers should concentrate on mastering basic pitches, believing that this approach results in better mechanics and a lower risk of injury.

Other major differences in youth pitchers include less muscle mass, open skeletal growth plates, and increased plasticity of bone. The physis of the proximal humerus usually closes at 14 to 17 years in girls and 16 to 18 years in boys. Until the physis closes, youth throwers are at risk for humeral apophysitis. Two other injuries common in youth throwers are medial epicondylar injuries of the elbow and osteochondritis dissecans of the capitellum, both of which are related to bone immaturity and the varus and valgus stresses placed on the elbow during throwing.

Increased bone plasticity poses a unique risk among youth throwers. Immature bones have less mineralization and undergo greater deformation when exposed to stress, which allows the bone to absorb greater force without fracturing. Excessive or incorrect throwing can lead to structural changes in the upper extremity as bones remodel. Examples include increased retroversion of the humeral head or scapular glenoid and the increased carrying angle of the elbow seen after repetitive bouts of "little league elbow." These changes may increase the risk of subsequent injury [99,100].

Ligamentous laxity allows greater motion at both the shoulder and elbow, particularly in throwers at the Tanner 3 stage of sexual maturity. The ratio of Type 1 to Type 3 collagen determines whether a joint possesses the increased mobility typical of adolescents or the more restricted motion seen in adults. Increased ligamentous laxity can lead to joint instability and impingement. Persistence of laxity as the adolescent grows and gains greater strength can produce increased stress on the elbow and shoulder, including the glenoid labrum, a structure more commonly injured in mature throwers [101].

A key concept when dealing with youth and adolescent throwers is that growth and development and biomechanical efficiency can progress at markedly different rates in individuals. Thus, no single group of guidelines about specific pitches or pitch counts applies uniformly to all pitchers of a given age or size. Clinicians must err on the side of caution regarding pitching volume and pitch type when advising young throwers who may be maturing late.

SUMMARY

Epidemiology – Throwing injuries, often involving the shoulder or elbow, are relatively common and are sustained by both traditional throwers (eg, baseball pitchers, Cricket bowlers) and by athletes who engage repeatedly in overhead motions similar to throwing (eg, swimmers, tennis players, volleyball players). (See 'Epidemiology' above and 'Biomechanical variations by sport' above.)

Anatomy – Throwing involves complex interactions among many musculoskeletal structures, and places great stress on the shoulder and elbow joints. The clinical anatomy related to throwing is discussed in the text above and related topics. (See 'Clinical anatomy' above.)

Biomechanics – Throwing involves the transfer of energy generated by the body to an object. During throwing, the upper extremity muscles act primarily as a means to transfer energy generated by the lower extremity and trunk to the thrown object, and to guide that object (or projectile) along the desired flight path. The basic phases of throwing include (figure 31 and movie 1):

Windup

Stride (sometimes called early cocking)

Cocking (sometimes called late cocking)

Acceleration

Deceleration

Follow-through

The mechanics of throwing, including important variations with different types of throwing, are described in the text. (See 'Biomechanics of throwing and related risk of injury' above and 'Biomechanical variations by sport' above.)

Risk factors – In adults, motion limitations, core muscle weakness, and asymmetries in strength and mobility are common biomechanical issues for many throwers. Among youth throwers, faulty biomechanics and excessive throwing beyond the physiologic tolerance of growing bone are much more likely to be the source of injury. Other major differences in youth pitchers include less muscle mass, open skeletal growth plates, and increased plasticity of bone, all of which may play a role in injury. For both age groups, overuse is a major risk factor for injury. (See 'Youth throwing athletes' above and 'Risk factors' above.)

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Topic 95726 Version 20.0

References

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