Objective: The peroneocalcaneus internus muscle is a rare muscle of the posterior calf that to our knowledge has not been reported in the radiology literature. The purpose of our study was to describe the normal anatomy and MR characteristics of this muscle in eight patients in whom the muscle was identified.
Conclusion: The peroneocalcaneus internus muscle is a rare muscle that originates from the inner aspect of the distal fibula, descends within the tarsal tunnel, and inserts on a small tubercle in the calcaneus, just distal to the sustentaculum tali. The muscle may displace the flexor hallucis longus muscle medially and thus indirectly encroach on the neurovascular bundle.