The effects of movement amplitude and contraction intensity on triceps surae and quadriceps femoris muscle function were studied during repetitive hopping. In vivo forces from Achilles and patellar tendons were recorded with the optic fibre technique from eight volunteers. The performances were filmed (200 Hz) to determine changes in muscle-tendon unit length and velocity. When hopping with a small amplitude (23° knee flexion during the ground contact phase), the Achilles tendon was primarily loaded whereas patellar tendon forces were greater in large-amplitude hopping (56° knee flexion). In spite of the different magnitudes of stretch in the quadriceps femoris muscle, the stretching velocity and activity patterns of the quadriceps muscle were similar in both conditions. Simultaneously performed electromyographic (EMG) recordings revealed that preferential preactivation of the gastrocnemius muscle was evident in both jumping conditions. The triceps surae muscle was strongly active in the eccentric phase of small-amplitude hopping. Results from hopping with small knee-joint displacement suggest that there may be a particular frequency and jumping height at which the elastic bouncing is best utilized and at the same time the concentric phase is most economical. Results also support earlier observations that the economy of the shortening phase must be compromised at some point in order to produce more power and improve the jumping height.