Eccentric loading of triceps surae modulates stretch shortening cycle behaviour - a possible therapeutic mechanism

Context: Eccentric exercises are increasingly being used to treat lower limb musculoskeletal conditions such as Achilles tendinopathy. Despite widespread clinical application and documented efficacy, mechanisms underpinning clinical benefit remain unclear. Positive adaptations in motor performance are one potential mechanism. Objective: To investigate how an eccentric loading intervention influences measures of stretch-shortening cycle (SSC) behaviour during a hopping task. Design: Within subjects repeated measures observational study. Setting: University motion analysis laboratory. Participants: Healthy adults. Interventions: A single intervention of 5 sets of 10 eccentric plantarflexion contractions at 6 RM using a commercial seated calf raise machine. Main outcome measures: Lower limb stiffness, sagittal plane ankle kinematics, and temporal muscle activity of the agonist (soleus) and antagonist (tibialis anterior) muscles, measured during sub-maximal hopping on a custom-built sledge-jump system. Results: Eccentric loading altered ankle kinematics during sub-maximal hopping; peak ankle angle shifted to a less dorsiflexed position by 2.9° and ankle angle pre-contact shifted by 4.4° (p<0.001). Lower limb stiffness increased from 5.9 to 6.8 Nm (p<0.001), whilst surface EMG measures of soleus occurred 14 to 44% earlier (p<0.001) following the loading intervention. Conclusions: These findings suggest that eccentric loading alters SSC behaviour in a manner reflective of improved motor performance. Decreased ankle excursion, increased lower limb stiffness and alterations in motor control may represent a positive adaptive response to eccentric loading. These findings support the theory that mechanisms underpinning eccentric loading for tendinopathy may in part be due to improved ‘buffering’ of the tendon by the neuromuscular system.