The tibialis posterior (french : muscle tibial postérieur) - named from its position in the back part of the leg and its origin from the tibia - is a thick fusiform sheet.

Origin

• The whole of the back of the interosseous ligament with the exception of the lowest portion ;
• the posterior surface of the tibia close to the interosseous ; line, from the upper end of the oblique line to the junction of the middle and lower thirds of the shaft ;
• the antero-internal (or inner part of the flexor) surface of the fibula at the back of the interosseous ridge to within an inch or two of the ankle ;
• the intermuscular septa which intervene between it and the muscles of the third layer, viz. the flexor hallucis longus and the flexor digitorum longus ;
• and a small portion of the deep fascia covering the back of the third layer.

Insertion

• The tuberosity of the scaphoid bone ;
• by several smaller offsets into the front of the lower surface of the sustentaculum tali and the under surface of all the other tarsal bones with the exception of the astragalus ;
• and the under surface of the bases of the second, third, and fourth metatarsal bones.

Structure

A strong bipenniform muscle, the central tendon of which, lying upon the middle of the back of the muscle, begins about the middle of the leg, and passes downwards and inwards upon the back of the muscle, receiving its last fleshy fibres about an inch above the ankle. Having passed inwards beneath the tendon of the flexor longus digitorum, it enters the innermost groove on the back of the internal malleolus, and is contained in a synovial sheath which accompanies it to its insertion upon the scaphoid bone. From this insertion strong fibrous bands radiate backwards, outwards, and forwards to the tarsal and metatarsal bones, being initially blended with the ligaments by which these bones are held together. This muscle, which is very strong, is contained, so to speak, in a four-sided prismatic case formed in the front by the interosseous membrane ; at the sides, by the opposing surfaces of the tibia and fibula ; and behind, at a distance of nearly half an inch from the interosseous membrane, by the intermuscular septa which separate the muscle from the flexor longus hallucis and the flexor longus digitorum.

Just above its insertion into the tuberosity of the scaphoid bone, the tendon often contains a sesamoid bone.

Nerve-supply

From the posterior tibial, which sends branches forwards to the back of the muscle in the upper third of the leg.

Action

• To adduct the front of the foot ;
• to invert the sole ;
• to extend the ankle - the last of these movements is somewhat limited ;
• to support the longitudinal arch of the foot - firstly, by drawing backwards the lower art of the scaphoid, and so preventing the descent of the head of the astragalus between the scaphoid and calcaneum, and secondly by its traction upon the other tarsal bones upon which the secondary offsets of its tendon are inserted.

Relations

Superficially, the soleus and third layer of muscles of the leg, the posterior tibial and peroneal vessels and the posterior tibial nerve above ; the tendon of the flexor longus digitorum and the abductor hallucis below ; deeply, he ankle joint and inferior calcaneo- scaphoid ligament. The anterior tibial vessels "ass through a notch at the upper extremity of the muscle, between its tibial and fibular origins.