Now that we’ve seen these fascial structures, let’s get back to the muscles, the ones that produce inversion and eversion. There are two muscles that produce inversion, tibialis anterior, which we’ve seen already, and tibialis posterior.
Here’s tibialis posterior. Tibialis posterior arises from the back of the tibia, the back of the fibula, and from the interosseous membrane in between. Its tendon passes immediately behind the medial malleolus, through a fibrous tunnel that’s covered by the flexor retinaculum. Beyond the malleolus the tendon of tibialis posterior fans out. It has a wide insertion, here, on the navicular and first cuneiform bones and also under here, on the bases of the second, third and fourth metatarsals. Here’s the action of tibialis posterior: it inverts the foot
The other muscle that can act as a foot invertor is tibialis anterior, which inserts so close to tibialis posterior that it has almost the same line of action. We looked at tibialis anterior, in its role as an ankle dorsiflexor, earlier in this section.
Now we’ll move on, to look at the three muscles that evert the ankle: peroneus longus, brevis, and tertius. Here’s peroneus brevis. Peroneus brevis arises from here on the distal fibula.
Lying on top of peroneus brevis, is peroneus longus. Peroneus longus arises from here on the proximal fibula. Its origin extends up onto the head of the fibula, with a gap here.
The deep peroneal nerve passes under the upper end of peroneus longus here, as we’ll see. The other muscle in the picture here is tibialis anterior.
At the ankle, the tendons of peroneus longus and brevis pass behind the lateral malleolus and beneath the peroneal retinaculum, longus behind, brevis in front. Peroneus brevis runs forward to insert here, on the base of the fifth metatarsal. To see the remarkable course of the peroneus longus tendon, we have to remove the entire sole of the foot. Peroneus longus runs around the cuboid bone, and along a deeply placed fibrous tunnel, to insert right over here, on the base of the first metatarsal.
Lastly, in front of peroneus brevis and longus, here’s peroneus tertius. Peroneus tertius arises from here on the fibula. The tendon of peroneus tertius passes under the extensor retinaculum, and in front of the lateral malleolus to insert here, on the base of the fifth metatarsal, next to peroneus brevis. The action of all three of the peroneal muscles is to evert the foot.
In addition, peroneus tertius, acting along with its anterior neighbors, can help to dorsiflex the ankle. The muscles of inversion and eversion are important, because they enable us to stay balanced and upright on a surface that tilts to one side, or to the other.