Question: What are the primary muscles that control eversion of the foot?

Answer: Peroneus longus and Peroneus brevis.

These muscles are located on the lateral aspect of the lower leg (Figure 1). The peroneus longus is a superficial muscle that originates at the head and upper two-thirds of the lateral surface of the fibula, as well as the adjacent intermuscular septa. Lying underneath the longus, the peroneus brevis is a shorter, smaller muscle originating from the lower two-thirds of the lateral surface of the fibula. The tendons of these two muscles run together behind the lateral malleolus. They are held in place by the superior peroneal retinaculum, a fibrous band anchored to the lateral malleolus and the calcaneus. If you evert your foot, you can feel the tendons pop out as they pass behind the ankle bone. They feel like one at this juncture because the longus is cradled into the brevis so they travel together as one unit. After this point, they go their separate ways. While the brevis inserts at the base of the fifth metatarsal bone, the longus extends obliquely through the foot to insert at the base of the first metatarsal bone and the first cuneiform bone.

When the foot is in dorsiflexion, the peroneus brevis is the prime mover in eversion; in plantar flexion, the longus plays the larger role. In addition to controlling eversion, these two muscles also perform other important functions. They contribute to plantar flexion of the foot, and they work together to provide lateral stability - keeping us from falling over on the lateral side of the foot and spraining the ankle. Working in conjunction with the posterior tibialis, which helps stabilize the medial side of the foot, they help us maintain our balance as we walk, stand, dance and run.

Question: What orthopedic assessment test is used to determine whether these muscles have been injured?

Answer: Resisted eversion.

To perform this test, place one hand on the medial aspect of the calcaneus and the other hand on the lateral aspect of the forefoot. Ask the person to resist your pressure as you push the forefoot medially. Note that in dorsiflexion, resisted eversion tests the peroneus brevis (Figure 2), and in plantar flexion, it tests the longus (Figure 3).