Isotonic length/force models of nine different skeletal muscles

The isotonic length/force relationships of nine skeletal muscles in the cat's hindlimb were determined using electrical stimulation of the sciatic nerve branches. Large variability in the active, passive, total force patterns and elongation ranges was found. The lateral gastrocnemius (LG), medial gastrocnemius (MG), peroneus longus (PL), flexor digitorum longus (FDL), tibialis posterior (TP) and soleus (Sol) showed symmetric active force curves, whereas those of the extensor digitorum longus (EDL), tibialis anterior (TA) and peroneus brevis (PB) were asymmetric. The total force curves of the EDL, LG, MG, FDL, TP and Sol increased quasilinearly through the elongation range, whereas the PL and PB increased in a nonlinear fashion. The TA had an intermediate plateau. The ranges were generally asymmetric, with a longer shortening range than lengthening past the optimum length. A simple model of the active force was fitted to all except the MG, EDL and TA, which are complex, with at least two compartments. These were successfully fitted with a two-compartment model. The variabilities encountered in the various isotonic length/force curves confirm the need to represent muscles according to their architecture to account for the variety of properties exhibited, which reflect their adaptations to their functions.