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A peroneal tendon describes any of three tendons that attach to the peroneus muscles in the lower leg: the peroneus longus, peroneus brevis, and peroneus tertius. Like any tendon, it is a band of fibrous connective tissue linking its attaching muscle to a specific bone or bones, thus acting like a lever that translates actions initiated by muscle contractions into movements at one or more joints. Each peroneal tendon connects its muscle, which begins on the fibula bone on the outside of the lower leg, to the foot and therefore produces motion at the ankle. Due to their vulnerable location on the outer ankle, these tendons are susceptible to inversion ankle sprains, which occur when the bottom of the foot is rolled inward and the tendons are overstretched.
All three situated vertically along the lateral or outer side of the leg above the ankle, the peroneus muscles share a name but are three separate muscles, each with its own peroneal tendon. The peroneus longus is the largest, highest, and most superficial, meaning that it lies closest to the skin. Arising at its topmost point from the head of the fibula just below and to the outside of the knee, it narrows as it descends to form a tendon that crosses behind the lateral malleolus, the large bony projection of the bottom of the fibula felt on the outside of the ankle. From there its tendon crosses obliquely under the foot to attach to the first metatarsal, the long bone aligning with the big toe. The peroneus longus tendon pulls the sole of the foot outward, an action known as eversion, as well as assists in plantar flexion of the foot, as in pointing the foot downward at the talocrural, or ankle, joint.
Originating beneath the peroneus longus on the bottom two thirds of the lateral side of the fibula, the peroneus brevis is a smaller muscle that descends just deep to the longus. Forming its peroneal tendon just above the ankle, it too runs behind the lateral malleolus, where along with the tendon of the peroneus longus it traverses the groove between the malleolus and the calcaneus, or heel bone. From there the tendon wraps under the foot and inserts at the base of the fifth metatarsal, found on the side of the pinky toe. While the peroneus brevis similarly assists in plantar flexion, as its tendon crosses the talocrural joint, its main function is eversion of the foot. This action occurs at the subtalar joint, which is the articulation found immediately beneath the ankle between the calcaneus and the talus, the bone between the calcaneus and the base of the tibia and fibula.
The last of the peroneal muscles is the peroneus tertius, which is the smallest of the three and which originates below and just anterior to or in front of the peroneus brevis. Arising from the bottom third of the fibula, its peroneal tendon enters the foot via the anterior side of the ankle joint alongside the tendon of the extensor digitorum longus muscle and inserts near the peroneus brevis tendon on the fifth metatarsal. The tendon’s location allows for minor assistance in everting the foot as well as in dorsiflexing the ankle, which is the upward hinging of the foot at the ankle joint.