The series elastic stiffness (SES) of the human dorsi flexors 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 m. tibialis anterior (34 ± 16 Nm). At an ankle moment of 34 Nm produced with either voluntary or electrical stimulation we found a significant difference SES of 219 ± 54 Nm rad^-1 and 149 ± 54 Nm rad^-1 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 co-activation of the m. extensor hallucis longus, m. extensor digitorum longus, and m. tibialis anterior. This shows that for a functional assessment of the SES of the dorsi flexors 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 dorsi flexors at moments levels up to about isometric maximum did not reach an asymptote where the stiffness is independent of moment i.e. the series elastic component of the dorsi flexors is during daily activities loaded for the greatest part in the non-linear part of the stress-strain function.