Muscular Anatomy of the Foot and Ankle
Khushdeep S. Vig
Andrew J. Rosenbaum
The muscular anatomy of the foot and ankle and really the entire lower extremity represent an intricate balance between agonistic and antagonistic musculotendinous groups that are constantly altering their action and function to facilitate the human gait cycle. Because structures in the foot and ankle are rarely a freely moving appendage and are typically fixed to the floor, the concept of a muscle exerting a force upon its insertion and the distally attached free-moving appendage is not easily applicable. Standing as well as the “stance” phase of the gait cycle fixes the foot and ankle to the floor. This converts the foot into the origin acting upon the insertions in the leg, which highlights the complex, balanced, muscular anatomy of the lower extremity.
Muscles can be divided into the “extrinsic” and “intrinsic” foot musculature. The extrinsic muscles originate proximally and insert/act upon the foot, whereas the intrinsic foot musculature originate and insert within the foot itself. The extrinsic muscles acting on the foot and ankle can be divided into their respective compartments: anterior, lateral, superficial posterior, and deep posterior.1
The superficial posterior compartment is responsible for the majority of plantar flexion of the ankle which is driven by the gastrocnemius and soleus muscles. The gastrocnemius originates from the posterior aspect of the nonarticular medial femoral condyle and lateral aspect of the lateral femoral condyle via two heads. It then crosses the knee joint posteriorly and converges into the thick, flat Achilles tendon that crosses the ankle joint posteriorly and attaches to the posterior aspect of the calcaneus to plantar flex the ankle or elevate the heel in a weight-bearing stance. The soleus is deep and anterior to the gastrocnemius originating from the upper tibia and fibula distal to the knee joint and travels distally within the deep posterior compartment joining the achilles tendon as the triceps surae attaching to the posterior calcaneus. Because the soleus does not cross the knee joint proximally, it cannot flex the knee, but when the knee is flexed, the gastrocnemius is out of phase allowing for the soleus to plantarflex the ankle, which is the basis of the Silfverskiöld knee flexion test differentiating gastrocnemius versus soleus contracture. Because of the oblique orientation of the subtalar joint, the triceps surae functions as a strong subtalar supinator when the foot is fixed to the ground. The gastroc-soleus muscle group function by pulling the leg posteriorly while both feet are fixed to the ground. This tonic muscle action pulls the knee into extension, the hips rest anteriorly on the iliofemoral ligament, and spine leans on the anterior-longitudinal ligament; this creates the relaxed erect posture which is supported principally through ligamentous structures except for tonic gastroc-soleal contracture.2 The plantaris muscle lies between the gastrocnemius and soleus muscles and has a long tendon that arises near the lateral head of the gastrocnemius and inserts on the medial aspect of the Achilles tendon.