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.