The force-extension characteristic of the series-elastic component of the human triceps surae muscle has been measured in vivo by means of a hydraulic controlled-release ergometer in 12 subjects. The SEC characteristic can be described by a linear relation between muscle moment and extension, with a stiffness K1, preceded by a quadratic 'toe' region at low moments. Stiffness K1 increases with the level of activation of the muscle. At an ankle moment of 100 N m, values range between 250 and 400 N m rad(-1). The elastic stretch corresponds then to about 30, which is considerable compared with the total ankle movement range. At moments of 150 and 180 N m, which correspond to the peak moments in walking and running, respectively, the elastic energy stored in the SEC was 23-37 and 31-57 J. These energy values are amply sufficient to enable the conservation of negative (eccentric) work into a subsequent phase of positive (concentric) work in both walking and running.