If the superficial branch of the common peroneal nerve is injured, which motion would be lost?

The superficial branch of the common peroneal nerve primarily innervates the muscles that are responsible for eversion of the foot, particularly the fibularis longus and fibularis brevis. When this nerve is injured, the ability to evert the foot is significantly compromised. Eversion is the movement where the sole of the foot turns outward, which is essential for maintaining balance and stability, especially on uneven surfaces.

In contrast, dorsiflexion of the ankle is primarily controlled by the deep branch of the common peroneal nerve, which innervates the tibialis anterior muscle and several others critical for lifting the foot. Plantarflexion of the ankle is mainly controlled by the tibial nerve, innervating muscles like the gastrocnemius and soleus. Inversion of the subtalar joint is also innervated primarily by the tibial nerve through the tibialis posterior.

Therefore, the loss of the ability to evert the foot following an injury to the superficial branch of the common peroneal nerve highlights the crucial role this nerve plays in lateral stability and movement of the ankle and foot.