Which muscle on the back of the lower leg is primarily involved in plantarflexion?

• A. Soleus
• B. Gastrocnemius
• C. Tibialis posterior
• D. Flexor digitorum longus

Answer: (B)

Plantarflexion at the ankle is dominated by the calf muscles, the gastrocnemius and the soleus, which together form the triceps surae and attach to the calcaneus via the Achilles tendon. The muscle on the back of the lower leg that is primarily involved is the gastrocnemius because it is the prominent superficial muscle crossing both the knee and ankle. When you rise onto your toes with the knee extended, the gastrocnemius contracts strongly to lift the heel. The soleus also plantarflexes, especially for endurance tasks and when the knee is bent, but it is deeper and less dominant in powerful, quick plantarflexion. The tibialis posterior and flexor digitorum longus assist with foot movements and toe flexion rather than serving as the main plantarflexors.