In vivo measurement of the series elasticity release curve of human triceps surae muscle

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.