Modeling the Aging Achilles Tendon: Inter-fascicle Adhesions and Tendon Compliance Affect Plantarflexor Behavior during Walking

The Achilles tendon (AT) consists of distinct fascicle bundles arising from the gastrocnemius and soleus muscles that intertwine and twist before inserting onto the calcaneus [1]. Current musculoskeletal models simplify this architecture by representing the gastrocnemius and soleus as independent muscle-tendon actuators [2], which may be an appropriate assumption if the fascicles freely slide relative to one another. However, this assumption may not hold with aging, as inter-fascicle adhesions form and reduce sliding between adjacent tendon fascicles [3]. This adaptation could explain our recent finding that depth-dependent variations in AT deformations during walking are much smaller in old vs. young adults (Fig. 1A) [4]. A reduced capacity for sliding in old tendons may couple gastrocnemius and soleus muscle-tendon behavior, which could in turn alter muscle force production.