The series elastic stiffness (SES) of the human dorsiflexors was investigated in vivo with the fast controlled release method in 8 subjects. The maximum moment of a voluntary contraction (66 ± 17 Nm) was significantly higher than the maximum moment with electrical stimulation of tibialis anterior (34 ± 16 Nm). At an ankle moment of 34 Nm produced with either voluntary or electrical stimulation, we found a significantly different SES of 219 ± 54 and 149 ± 54 Nm · rad¹, respectively. It is proposed that this is due to the fact that, during voluntary contraction, more elastic tissue parallel with each other is involved, because of coactivation of the extensor hallucis longus, extensor digitorum longus, and tibialis anterior. This shows that, for a functional assessment of the SES of the dorsiflexors, one has to include the toe extensors, which is possible with the fast controlled release method. Additionally, our results demonstrated that the SES of the human dorsiflexors at moment levels up to about isometric maximum did not reach an asymptote at which the stiffness is independent of moment, i.e., the series elastic component of the dorsiflexors is during daily activities loaded for the greatest part in the nonlinear part of the stress-strain function.