Behavior of fascicles and tendinous structures of human gastrocnemius medialis (MG) was determined by use of ultrasonography in vivo during jumping. Eight male subjects jumped vertically without countermovement (squat jump, SQJ). Simultaneously, kinematics, kinetics, and electromyography from lower leg muscles were recorded during SQJ. During phase I (-350 to -100 ms before toe-off), muscle-tendon complex (MTC) length was almost constant. Fascicles, 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 to toe-off), although fascicles generated force quasi-isometrically, MTC shortened rapidly by 5.3%, releasing prestored elastic energy with a higher peak positive power than that of fascicles. Also, the compliance of tendinous structures in vivo was somewhat higher than that of external tendon used in the simulation studies. The results demonstrate that the compliance of tendinous structures, together with no yielding of muscle fibers, allows MTC to effectively generate relatively large power at a high joint angular velocity region during the last part of push-off.

Behavior of fascicles and tendinous structures of the m. gastrocnemius medialis (MG) was quantitatively examined during human jumping in vivo. Eight male subjects performed maximal-effort counter movement jumping (CMJ) motions. Kinematic and kinetic data were obtained using a high-speed camera and a force platform. Behavior of fascicles and tendinous structures was determined using ultrasonography and electromyography. Although the muscle-tendon complex (MTC) shortened by only 1.6% during the downward phase of the counter movement, fascicles shortened as much as 10.4%. This shortening of fascicles caused elongation of tendinous structures by 2.2%. Although the MTC remained at almost constant length during the upward-I phase (-250 to -100 ms before toe-off), fascicles shortened by 19.2% of the initial length with an elongation of tendinous structures by 4.4%. The MTC shortened rapidly by 5.3% of the initial length during the upward-II phase (-100 to 0 ms), whereas fascicles shortened slightly during the first half of this phase and contracted in a quasi-isometric manner during the latter half of this phase. These findings implied that elastic energy was stored in tendinous structures throughout the latter half of the downward phase (1.0 J) and upward-I phase (5.6 J), which was thereafter rapidly released during the upward-II phase (3.8 J). It was found that muscle fibers of the MG were not stretched during counter movement; therefore, stretch reflex and potentiation of the contractile component of the MG might not contribute to the work enhancement in CMJ. It was suggested that the interaction between fascicles and tendinous structures was essential in a generation of higher joint power during the late push-off phase. This behavior of the MTC of the MG in CMJ was quite similar to what was observed in squat jumping performed without counter movement.

The fascicular and tendinous behavior of the human m. gastrocnemius medialis (MG) was measured using ultrasonography between vertical ankle-and drop-jumping in order to better understand how force is developed under dynamic conditions. These jumps employed movement at the ankle joint only, and the MG was defined as the agonist muscle of the jumps. Eight male subjects performed 1: Ankle plantarflexion jumping (AJ) from the standing position without countermovement, and 2: straight legged Drop jumping (DJ) from a height of 0.20 m; both with maximal effort. Length, force and power output of the muscle-tendon complex (MTC), fascicles and tendinous structures were estimated from a video tape recorder (VTR)/force-plate system and ultrasonography. EMG of the triceps surae was simultaneously measured. Vertical displacements of the body center of gravity were 0.16m in AJ and 0.33m in DJ. There was no stretching phase of the fascicles during the take-off phase in both AJ and DJ. Physiological effects such as stretch reflex and potentiation did not appear to be present. Even in AJ, the fascicles of the MG shortened in the earlier phase of take-off compared to the MTC of MG. As a result, the elastic energy was stored (2.2 J) and reutilized (2.1 J: 47% of total MTC work in latter phase) during the push-off phase. In DJ, on the other hand, much elastic energy was stored in the tendinous structures during dorsiflexion (7.6 J) and plantarflexion (5.8 J: 75%) phases. It was quantitatively indicated that there was a large advantage from elastic energy use in DJ compared to AJ. The MG fascicles operated within the optimal phase of the sarcomere force-length relationship in AJ and DJ. Also, the fascicles worked in the relatively low shortening velocity region of the instantaneous force-velocity relationship in AJ and DJ.

Athletes who aim to improve both muscular endurance and power often perform exercises that involve similar joint actions under different lifting conditions, such as changes in the load or speed, which are implemented at different times during a periodized exercise program or simultaneously. The prescribed exercises are considered to recruit the same muscles even if the lifting conditions differ to each other. The present study aimed to clarify this by examining whether the recruitment of individual hip and knee muscles during the squat exercise differs between lifting conditions adopted for muscular endurance and power training regimens. Moderately trained men performed back squats (BS), with a load of approximately 60% of one repetition maximum, as a muscular endurance training exercise, and they performed plyometric squat jumping (PSJ) for power training. During each exercise, the lower limb joint torques and the recruitment of five hip and knee muscles were determined with inverse-dynamics and T2-weighted magnetic resonance imaging, respectively. While the maximal and mean knee joint torques were greater during PSJ than during BS (p<0.01), the T2 values for the quadriceps femoris muscle did not differ between the exercises. In contrast, the T2 values of the gluteus maximus and hip adductor muscles were higher during PSJ (p<0.05) than during BS, although there was no significant difference in the mean hip extension torque between the two exercises. The current results indicate that the individual use of the agonist muscles differs between BS and PSJ, and it does not always correspond with the joint kinetics during the exercises. Therefore, in addition to the exercise type, the lifting condition should also be taken into consideration as a determinant of the major muscles trained during a resistance exercise.