Relation between fascicle length–to–moment arm ratio and joint torque during isokinetic contraction

in vivo observations in human soleus muscle and ankle plantarflexion

Abstract

The purpose of this study was to investigate the relationship between fascicle length (Lf) –to–moment arm (MA) ratio (Lf/MA) of soleus muscle and isokinetic plantarflexion torque in vivo. Twenty–two healthy men performed isokinetic (concentric) plantarflexions at 30°/s (Slow) and 150°/s (Fast) in a 90° flexed knee position. The Lf of the soleus and its shortening velocity were determined with ultrasonography at rest and during the plantarflexions, respectively. The MA of the Achilles tendon was measured with magnetic resonance imaging at rest. The results showed that Lf/MA was significantly correlated with the fascicle shortening velocity normalized by Lf at rest (Slow: r = ‒0.600; Fast: r=‒0.583). The normalized fascicle shortening velocity was significantly correlated with the muscle force normalized by physiological cross–sectional area (PCSA) of the soleus (Slow: r = ‒0.553; Fast: r = ‒0.788). The normalized muscle force was significantly correlated with the plantarflexion torque normalized by PCSA (Slow: r = 0.947; Fast: r = 0.957). Furthermore, there were significant correlations between Lf/MA and the normalized plantarflexion torque (Slow: r = 0.631; Fast: r = 0.600). These results suggest that large Lf/MA of soleus muscle is beneficial for attaining low fascicle shortening velocity and thus exerting large muscle force and joint torque during the isokinetic plantarflexion.