The muscles of the leg are composed of four distinct groups of three each. They are extensors, flexors, abductors, and muscles of the calf.

The extensor group comprises the tibialis anterior, extensor longus digi-torum, and extensor longus hallucis.

The flexor group comprises the tibialis posterior, flexor longus digitorum, and flexor longus hallucis.

The abductor group comprises the peroncus longus, peroneus brevis, and peroneus tertius.

The calf muscles comprise the gastrocnemius, soleus, and plantaris.

It will thus be seen that the extensor and flexor groups are composed of precisely similar muscles only on opposite sides of the leg. They tend to move the foot and toes forward and backward and balance each other. The abductors form a group around the fibula on the outer side of the leg and they abduct the foot. They tend to pronate it. The most active agents in adduction are the tibialis anterior and tibialis posterior. The muscles of the calf form a separate posterior group designed for use in walking and to compensate for the greater length of the foot anterior to the centre of motion at the ankle and its shortness posteriorly.

Fig. 559.   Extensor and abductor muscles of the leg.

Fig. 559. - Extensor and abductor muscles of the leg.

Fig. 560.   Flexors and muscles of the calf of the leg.

Fig. 560. - Flexors and muscles of the calf of the leg.

The extensor group lies between the tibia and fibula anterior to the interosseous membrane. The abductor group forms a mass over the fibula, and the flexor group lies between the tibia and fibula on the posterior surface of the interosseous membrane. The muscles of the calf constitute a superficial layer of muscles which end below in the tendo calcaneus (Achillis). The soleus, with the two heads of the gastrocnemius, is known as the triceps surae muscle. It is absolutely essential to understand the grouping of these muscles of the leg because thereby its construction is rendered evident and their influence on distortions of the foot can be appreciated.