Moment arm length variations of selected muscles acting on talocrural and subtalar joints during movement: an in vitro study.

The tendon excursion method was applied to estimate the moment arm length of triceps surae, flexor hallucis longus, tibialis posterior, tibialis anterior, peroneus brevis and peroneus longus at talocrural and subtalar joints of 10 anatomical preparations. Orthogonal polynomials were fitted to the tendon excursion data and by derivation with respect to the joint angle, the instantaneous moment arm length for any joint position was evaluated. Both joints were investigated separately. Individual data, the mean moment arm together with the significant moment arm coefficients of the polynomials are presented. Considering their moment arm, the most important plantar flexor were triceps surae, flexor hallucis longus and to a much lesser extent peroneus longus, peroneus brevis and tibialis posterior. Results suggest a partially new insight into the muscle action at the subtalar joint. Although displaying a mean inversion moment arm of about -5.3 mm over the whole range of motion, triceps surae had an inversion moment arm in eversion and an eversion one in inversion. Tibialis posterior exhibited the most important inversion moment arm (-19.1 mm), whereas tibialis anterior had a very low one (-3.8 mm). For some specimens this muscle was found to have an eversion moment arm. Peronei longus and brevis had a nearly identical moment arm at the subtalar joint (21.8 and 20.5 mm).