The Effect of Excessive Subtalar Inversion/Eversion on the Dynamic Function of the Soleus and Gastrocnemius During the Stance Phase

Gastrocnemius and soleus are often considered as ankle plantarflexors. Their dynamic functions in normal and pathological gait have been well-studied. However, in a neutral position, the tendon passes medial to the subtalar joint axis and therefore produces an inversion moment in addition to the plantar-flexor moment [1]. It was believed that gastrocnemius and soleus are the major dynamic stabilizers preventing excess foot pronation. During normal gait, the subtalar joint experiences rapid eversion following heel strike and subsequent inversion during terminal stance [2]. Varus and valgus foot positions caused by excessive subtalar inversion/eversion can be found in spastic and flaccid paralysis [3]. Although it is widely understood that muscle forces can have important local and remote effects on joints and segments [4], the interrelations between dynamic gastrocnemius and soleus functions and excessive subtalar inversion/eversion remain unclear.