Differential contributions of lower leg muscle activation to plantar flexor function during single-leg hopping one year after Achilles tendon rupture: A shift from monoarticular to biarticular muscle work

Loss of plantarflexion function persists indefinitely after an Achilles tendon rupture, detrimentally affecting quality of life. This study aimed to quantify lower leg muscle activation and its contribution to side differences in plantarflexion work during single-leg hopping one year after Achilles tendon rupture. Thirty-seven participants of mean (SD) age 48 (8.9) years participated in the study. Ankle kinetics and electromyograms of the soleus, tibialis anterior, lateral and medial gastrocnemius muscles were recorded during single-leg hopping. Side-to-side differences in plantarflexion power and muscle activation were evaluated using one dimensional t-tests while the effect of muscle activation on plantarflexion work was examined using a linear mixed model. Plantarflexion power was reduced by 20-25 % (p < 0.001) and work by 42 % (p < 0.001) in the affected vs. unaffected leg. In contrast, no differences in muscle activation were detected. However, during stance phase the contribution of soleus activation to plantarflexion work was lower per percent of maximum voluntary isometric contraction (p < 0.001) in the affected (0.6mJ/kg) vs. unaffected leg (3.2mJ/kg). Conversely, the contribution of lateral gastrocnemius activation was higher (p < 0.001) in the affected (4.8mJ/kg) vs. unaffected (2.3mJ/kg) leg. In conclusion, plantarflexion work and power were reduced during single-leg hopping one year following Achilles tendon rupture, while muscle activation did not differ between the affected vs. unaffected leg. However, a marked increase in the contribution of biarticular vs monoarticular shank muscle activation to plantarflexion work was observed, suggesting an increased reliance on energy transport from the knee to the ankle joint following Achilles tendon rupture.