Ten active males performed reactive drop jumps from a height of 40 cm in six experimental conditions: jumps with additional loads of 100 N (BW + 100 N) and 200 N (BW + 200 N), an ordinary jump with body weight (BW) and three jumps in which the body weight was artificially reduced (BW-172 N, BW-337 N and BW-495 N). The vertical ground reaction forces, the angular displacement in the knee and ankle joints as well as the surface electromyogram (EMGs) of the triceps surae muscles and tibialis ant. muscle were recorded. When compared to the control condition (BW) in the jumps with extra load and in the jumps with reduced body weight, both the take-off velocity as well as the mean vertical ground reaction force were decreased during the push-off phase. The integrated EMG before ground contact as well as the duration of the preactivation phase was significantly reduced as a function of the load condition. Upon the touchdown, the coactivation of the muscles acting around the ankle joint was greatest in the control jump. Through all experimental conditions, the mean activation amplitude remained rather constant both for the impact as well as for the push-off phase of the contact. It is concluded that the centrally programmed activity prior to the contact can be seen as the decisive mechanism in the regulation of the stiffness behavior of the tendomuscular system. The extent of the preprogrammed activity determines mainly the physical output of the entire jump exercise.