This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
Lateral movements of the foot are comparatively weak when compared with those of flexion and extension. They are intended largely to maintain the balance or equilibrium and to adapt the position of the foot to uneven surfaces, etc. Three muscles act very distinctly as abductors; they are the peroneus longus, brevis, and tertius. Two act as distinct adductors, viz.: the tibialis anterior and the tibialis posterior.
The muscles of the calf act more as abductors than adductors, because the insertion of the tendo calcaneus (Achillis) is not directly behind the ankle-joint but more to its outer side.
When the foot is deformed in the position of inversion, as in club-foot, the tibialis anterior and posterior are usually contracted, but when in the position of eversion, as in flat-foot, then spasm of the peronei or calf muscles is frequent.
Plantar flexion of the foot is a far more powerful movement than extension - flexion is associated with adduction or inversion and extension with abduction or eversion; hence it is that inversion is the position of strength and eversion of weakness. Feats of strength and agility cannot be performed by those who have markedly everted feet.