Ultrasound biceps contraction and relaxation

This is an axial real-time image of the biceps (B) over the mid humerus (H); note a thin layer of subcutaneous tissue (S). The elbow is initially extended, and then gently flexed to 90 degrees; a slow full isometric contraction of the biceps follows, with repeated cycles of isometric relaxation and contraction. Note the shifting of fascicles as the muscle contracts and bulges, significantly increasing the diameter of the muscle belly.