Flexor digitorum accessorius longus is a variant accessory muscle of the deep posterior compartment, present in approximately 6-8% of individuals. It originates in the lower leg and passes through the tarsal tunnel alongside the main flexor tendons, sometimes as a separate muscular belly.
| Origin | Fibula or posterior tibia in the deep posterior compartment, proximal to the flexor digitorum longus |
|---|---|
| Insertion | Joins the flexor digitorum longus tendons in the tarsal tunnel or foot |
| Nerve Supply | Tibial nerve |
| Blood Supply | Posterior tibial artery |
| Actions | Assists toe flexion; functions as an accessory to flexor digitorum longus |
|---|
This accessory muscle is a recognised cause of tarsal tunnel syndrome, as its additional bulk within the tarsal tunnel compresses the tibial nerve during exercise. It appears as an unexpected muscular mass on MRI of the tarsal tunnel in symptomatic patients. Surgical decompression includes excision of the accessory muscle belly to decompress the tunnel.
Not reliably palpable due to deep location within the lower leg and tarsal tunnel.
Tibial nerve compression within the tarsal tunnel from the anomalous flexor digitorum accessorius longus muscle belly, producing medial ankle pain and plantar numbness that worsens with activity.
