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The peroneus tertius is the smallest of the three peroneus muscles in the lower leg. Situated on the lateral side of the leg between the calf and shin, it is found near the fibula bone just below the extensor digitorum longus muscle and to the inside of the peroneus brevis. Though it is narrow and band-like in shape, its fibers run not parallel to the muscle’s direction but obliquely like the fibers in a feather. This muscle plays a minor role in eversion and dorsiflexion of the foot at the ankle and in some individuals is absent all together.
With fibers that traverse the muscle diagonally rather than arise from a single tendon above and descend with the muscle, the peroneus tertius finds its origins along multiple locations in the lower leg. The most posterior portion of the muscle arises from the interosseous membrane, a sheath of connective tissue stretching between the tibia and fibula bones like a wall that separates the posterior compartment of the leg from the anterior compartment. An intermediate portion originates along the lower part of the shaft of the fibula along its front side. Still another portion of the muscle arises from fascia, or fibrous tissue, enclosing the adjacent peroneus brevis and dividing it from the peroneus tertius. This tissue is known as the intermuscular septum and is the most lateral of the muscle’s points of origin.
After these fibers travel inward and downward they converge above the ankle into a single tendon that crosses the joint anteriorly, or on its front side, alongside the tendon of the extensor digitorum longus. These tendons are held in place on the front of the ankle and foot by a transverse or horizontal band known as an annular ligament. From here the tendon of the peroneus tertius inserts about midway down the foot on the top surface of the near end of the fifth metatarsal. The fifth metatarsal is the long bone of the foot situated just beneath the pinky toe.
As the muscle extends to the foot, it is involved in eversion, which is the lateral rolling of the ankle so that the sole of the foot faces outward. This action, however, is initiated mostly by the peroneus longus and brevis; the tertius only weakly assists. Since it crosses to the front of the ankle, the peroneus tertius is also involved in dorsiflexion, or the flexing of the ankle joint so that the top of the foot is brought closer to the shin. Again, however, larger and stronger muscles like the tibialis posterior perform most of this action, with the tertius relegated to a smaller role.