Muscle synergism during isometric plantarflexion in achilles tendon rupture patients and in normal subjects revealed by velocity-encoded cine phase-contrast MRI

Background The triceps surae muscle is often considered to be the sole contributor to the plantarflexion torque although the deeper plantarflexor muscles may also be activated in the plantarflexion task. Methods We measured coordinative strategies and muscle synergism during isometric plantarflexion task from 11 volunteers. Velocities from the distal end of the medial gastrocnemius, soleus, and flexor hallucis longus muscles were encoded in the superior–inferior direction using cine phase-contrast magnetic resonance imaging. Eight of the subjects were healthy and served as controls for subject to subject variability. Three of the subjects had experienced an Achilles tendon rupture followed by surgical repair and they were measured up to 8 weeks of rehabilitation. Results There were marked individual differences in the use of the different muscles during plantarflexion task in control subjects. Soleus to flexor hallucis longus displacement ratio during contraction varied from 0.4 to 9.6 while the moment arm ratio between Achilles tendon and flexor hallucis longus tendons was 1.9 (standard deviation 0.2). In Achilles tendon rupture patients the relative contribution of flexor hallucis longus was very high both in their injured and uninjured leg. This coordinative strategy remained throughout rehabilitation. Interpretation The findings suggest that early recovery of plantarflexion torque after Achilles tendon rupture may be due to compensation by flexor hallucis longus as well as to normalization of the triceps surae muscle function. Also, this study suggests that the individual differences in coordinative strategies in addition to moment arms and muscle–tendon properties can influence the calculation of forces produced by individual muscles.