The peroneus group of muscles exert so little influence on flexion and extension that in many cases they may be ignored. The peroneus tertius flexes the ankle, while the longus and brevis extend it. The common movements of the foot when great strength is not required are performed by the flexor and extensor groups of muscles; the muscles of the calf are not so much for adding to the kind of movements as to the amount. The powerful calf muscles have the function of aiding the body in maintaining the upright posture and especially in lifting and propelling it forward in locomotion. When most of the flexors and extensors are paralyzed the foot hangs loose from the leg, the so-called flail-foot. Weakness of the flexor group (tibialis posterior, flexor longus digitorum, and flexor longus hallucis) tends to favor a descent of the arch with consequent pronation or eversion. Weakness of the extensors causes toe-drop and inversion or supination.

Paralysis of the calf muscles deprives the posterior pillar of the arch of its support and the action of the flexors and extensors elevates the arch while the heel descends, so that a condition of hollow foot is produced.

Fig. 587. - The foot in an adducted or supinated position.

Paralysis of the calf muscles is not rare, while that of the deep flexors is less common. The question of paralysis must be studied with reference to each individual case, because the affected muscles are not always completely paralyzed, neither are all the muscles of a group.