Behavior of fascicles and tendinous structures of human gastrocnemius during vertical jumping

Behavior of fascicles and tendinous structures of human gastrocnemius media lis (MG) was determined by use of ultrasonography in vivo during jumping. E ight male subjects jumped vertically without countermovement (squat jump, S QJ). Simultaneously, kinematics, kinetics, and electromyography from lower leg muscles were recorded during SQJ. During phase 1 (-350 to -100 ms befor e toe-off, muscle-tendon complex (MTC) length was almost constant. Fascicle s, however, shortened by 26%, and tendinous structures were stretched by 6% , storing elastic energy of 4.9 J during phase I. During phase II(-100 ms t o toe-off), although fascicles generated force quasi-isometrically, MTC sho rtened rapidly by 5.3%, releasing prestored elastic energy with a higher pe ak positive power than that of fascicles. Also, the compliance of tendinous structures in vivo was somewhat higher than that of external tendon used i n the simulation studies. The results demonstrate that the compliance of te ndinous structures, together with no yielding of muscle fibers, allows MTC to effectively generate relatively large power at a high joint angular velo city region during the last part of push-off.