Length-tension curves were constructed by stimulating tetanically single motor units belonging to the peroneus longus muscle. The lengths at which the motor units showed the maximal tension appeared to be shorter than the length at which the whole muscle developed the maximal twitch tension. Moreover, the optimal lengths appeared to be different for the different motor unit types, corresponding to very short muscle lengths for Slow units and to slightly higher lengths for the Fast Fatiguable and the Fatigue Resistant units.