Why does man have a quadratus plantae? A review of its comparative anatomy.

Quadratus plantae is a muscle in the sole of the foot, typically originating from the calcaneus and inserting into the posterolateral surface of the tendons of flexor digitorum longus. It is implicated in heel pain, claw toe deformity and diabetic polyneuropathy. Phylogenetic considerations suggest that quadratus plantae is getting bulkier, implying its significance in human locomotion. Is it simply an accessory flexor that brings the line of pull of flexor digitorum longus in line with the long axis of the foot, as its name would suggest? We cite evidence from electromyographic studies that suggest it actually acts as a primary toe flexor in voluntary movements, being preferentially recruited over flexor digitorum longus. From comparative anatomical considerations it also seems likely that quadratus plantae is an intrinsic evertor of the foot. Eversion is an important evolutionary asset, especially in erect bipedalism. Human electromyographic experiments have yet to confirm this. However, they do suggest that quadratus plantae functions to resist extension of the toes during the stance phase of locomotion, which serves to increase the stability of the foot. Future electromyographic experiments may provide more information on the role of quadratus plantae in human locomotor evolution and in foot eversion in particular.