Tibialis posterior: plantarflexion, inversion and the medial arch
Tibialis posterior is a deep muscle that plantarflexes the foot (points it downwards), inverts it (turns the sole inwards) and helps support the medial longitudinal arch (the inner arch).
Remember
Tibialis posterior points the foot downwards, turns it inwards and helps hold up the inner arch, so when its tendon fails inversion weakens, pain develops on the inner side of the ankle and the arch gradually sags, leaving an acquired flat foot.
Memory aid
the 'turn in and tiptoe' muscle
Tibialis posterior turns the foot in (inversion), helps with tiptoe (plantarflexion) and props up the inner arch.
Tibialis posterior is the deepest muscle of the posterior compartment, the calf-side group of leg muscles. Its nerve supply is the tibial nerve (the nerve that supplies that group). The posterior compartment typically acts to plantarflex the foot, and tibialis posterior does so too.
Its tendon runs behind the medial malleolus (the bony bump on the inner side of the ankle) and attaches under the medial tarsal bones (the small bones on the inner side of the midfoot). Plantarflexion points the foot downwards, as in standing on tiptoe, and inversion turns the sole inwards. The same pull helps hold up the medial longitudinal arch, the high arch along the inner border of the foot.
The neighbouring compartments (groups of muscles wrapped together by connective tissue, with a shared nerve supply and job) do other things. The anterior compartment dorsiflexes the foot (lifts it up) and the lateral compartment everts it (turns the sole outwards). Both tibialis muscles invert the foot, so the compartment decides the second movement: posterior plantarflexes, anterior dorsiflexes.
When the tendon is damaged, as in posterior tibial tendon dysfunction, it stops working properly. Inversion weakens, pain develops along the inner side of the ankle, and the arch gradually sags into an acquired flat foot (pes planus).
A heel rise, which means standing on tiptoe, needs plantarflexion. In a healthy foot the heel turns inwards as it lifts. If tibialis posterior is deficient, the heel inversion is lost, so the foot stays everted and the rise is weak.
Key facts
- Actions
- plantarflexion and inversion of the foot, with a contribution to the medial longitudinal arch.
- Position and nerve supply
- the deepest muscle of the posterior compartment, supplied by the tibial nerve.
- Tendon course
- behind the medial malleolus, then under the medial tarsal bones.
- Clinical clue
- medial ankle pain and swelling behind the medial malleolus point to the tibialis posterior tendon.
- If it fails
- medial ankle pain, weak inversion and a gradually collapsing arch (acquired flatfoot).
- Single heel rise
- rising onto the toes while standing on one leg is weak or not possible when tibialis posterior fails.
- Tibialis muscles
- both invert the foot; the compartment decides whether the foot moves down (posterior) or up (anterior).
Exam traps
- Inversion and dorsiflexion of the foot is the pattern of tibialis anterior, not tibialis posterior. Both tibialis muscles invert the foot, but tibialis anterior is the strongest dorsiflexor and lies in the anterior compartment, while tibialis posterior lies in the posterior compartment and plantarflexes.
- Plantarflexion paired with eversion describes fibularis longus of the lateral compartment. Tibialis posterior shares plantarflexion with it, but eversion is the opposite of inversion, and losing eversion would not collapse the medial arch.
- Gastrocnemius, the bulky superficial calf muscle, plantarflexes the ankle and flexes the knee but does not invert the foot, so its dysfunction would not explain weak inversion with a flattened medial longitudinal arch.
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