Peroneocalcaneus Internus
Leg & Ankle
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The peroneocalcaneus internus is a rare accessory muscle that can pass through the tarsal tunnel. Because of its unusual location it may be mistaken for a soft-tissue mass on imaging, or contribute to tarsal tunnel compression.
Function
Likely assists weakly with plantarflexion of the ankle and may contribute to inversion and stabilization of the hindfoot. Its exact functional role is not well established because of its rarity.
Origin
Medial or posteromedial surface of the distal fibula, often near or partially associated with the origin of the flexor hallucis longus.
Insertion
Medial surface of the calcaneus, commonly near the sustentaculum tali.
Innervation
Most likely a branch of the tibial nerve, consistent with its location in the deep posterior compartment of the leg.