The peroneus brevis tendon is a band of fibrous connective tissue that attaches to the peroneus brevis muscle in the lower leg. Like any tendon, it connects 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. In this case, the peroneus brevis tendon connects the muscle, which begins at the fibula bone in the lower leg, to the foot and therefore produces motion at the joint it crosses: the ankle.
Originating on the bottom two thirds of the lateral or outer side of the fibula, which is the smaller bone in the lower leg lateral to the tibia, the peroneus brevis is one of three peroneus muscles. The peroneus longus and tertius are also found on the outside of the lower leg, above the bony prominence on the outside of the ankle known as the lateral malleolus. All three descend to cross the ankle joint.
The peroneus brevis, specifically, forms its tendon just above the ankle, 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. The last of five long, skinny foot bones, the fifth metatarsal is found on the side of the pinky toe.
While the peroneus brevis and peroneus longus assist the other muscles of the posterior compartment of the lower leg in plantar flexion, or the downward hinging of the ankle joint as in pointing the toes, their main function is eversion of the foot. Eversion is the action of rolling the foot outward at the ankle, so that the sole of the foot faces laterally or away from the midline of the body. 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. Because the peroneus brevis tendon crosses this joint as well as the talocrural joint of the ankle, where plantar flexion takes place, it is involved in movements at both joints but is more closely linked to eversion.
The location of the peroneus brevis tendon also makes it susceptible to inversion ankle sprains, the most common type of ankle sprain. As the ankle is most likely to turn with the foot inverting or rolling inward, as in landing badly from a jump, the ligaments on the outside of the ankle can easily become overstretched. Recommended treatment for this and other tendon sprains includes the RICE method — rest, icing, compression, or elevation — which in the absence of a tear should allow the tendon to heal on its own.