Which muscle is primarily involved in plantar flexion of the ankle according to the provided material?

• A. Popliteus
• B. Peroneus longus
• C. Tibialis anterior
• D. Soleus

Answer: (D)

Plantar flexion at the ankle is driven mainly by the calf muscles, collectively known as the triceps surae. Among them, the soleus does the bulk of the work because of its size, structure, and constant role in pushing the foot downward, especially during standing and slow, steady movements. The soleus is a strong, endurance-oriented muscle that remains highly active regardless of knee position, making it the primary plantarflexor in many everyday tasks.

The gastrocnemius also helps with plantarflexion, but it crosses the knee. When the knee is bent, this biarticular muscle loses some of its tension, so its contribution to pushing the foot downward is reduced. That’s why the soleus is identified as the primary plantarflexor in contexts where knee position limits gastrocnemius action.

Other muscles listed don’t primarily plantarflex. The popliteus mainly unlocks the knee and assists with rotation rather than moving the ankle into plantarflexion. The tibialis anterior dorsiflexes the foot (pulls the toes upward) rather than pointing the toes downward. The peroneus longus does assist in plantarflexion but its primary action is everting the foot and supporting the arches, not generating the main plantarflexion force.