Which muscle is innervated by the L4 nerve root and contributes to ankle inversion?

• A. Extensor digitorum longus
• B. Tibialis anterior
• C. Gastrocnemius
• D. Peroneus tertius

Answer: (B)

Ankle inversion is produced by muscles that pull the foot inward, mainly tibialis anterior and tibialis posterior. The L4 nerve root contributes fibers to the deep peroneal nerve, which innervates tibialis anterior. This muscle inverts the foot as it dorsiflexes, so it satisfies both criteria: innervation by the L4 root and a role in inversion.

Extensor digitorum longus mainly dorsiflexes the ankle and extends toes and tends to assist eversion, with innervation that includes L5-S1 via the deep peroneal nerve, not as cleanly linked to L4. Gastrocnemius plantarflexes the foot and is innervated by the tibial nerve (S1-S2). Peroneus tertius dorsiflexes and everts, innervated by the deep peroneal nerve, but it everts rather than inverts.