Which muscle groups contribute to plantar flexion and are innervated by the tibial nerve?

• A. Anterior compartment muscles: tibialis anterior, extensor digitorum longus
• B. Posterior compartment leg muscles: gastrocnemius, soleus, and plantaris
• C. Lateral compartment muscles: fibularis longus and brevis
• D. Popliteus and plantaris only

Answer: (B)

Plantar flexion is produced mainly by the muscles of the posterior leg that cross behind the ankle and attach to the heel via the Achilles tendon. These muscles are innervated by the tibial nerve as it travels through the popliteal region into the posterior compartment. The gastrocnemius, soleus, and plantaris together make up the primary plantar flexors, with gastrocnemius and soleus providing most of the force and plantaris adding a small assist. The tibial nerve also supplies other deep posterior muscles, but they aren’t listed here. The other options point to muscles that either dorsiflex the foot (anterior compartment) or are innervated by a different nerve (fibularis muscles by the superficial fibular nerve), and popliteus is tibial-nerve supplied but does not plantar flex. Therefore, the group of gastrocnemius, soleus, and plantaris best fits both plantar flexion and tibial nerve innervation.