Peroneus Longus Muscle
The lateral compartment of the leg contains two peroneal muscles. The function of this compartment is to protect the underlying neurovasculature, as well as allow the foot to evert and the ankle to plantarflex. The peroneus longus is more superficial of the two muscles of the lateral compartment of the leg. In this article we will discuss the anatomy of the muscle, as well as its clinical relevance.
Peroneus longus is also known as fibularis longus. It arises from the head, superior part of the lateral fibular shaft, deep surface of deep fascia, anterior and posterior crural intermuscular septa and occasionally from the lateral condyle of tibia.
The tendon of the muscle descends down the leg, and runs posterior to the lateral malleolus along with the tendon of the peroneus (fibularis) brevis. These two muscles run within a tunnel in a common synovial sheath covered by the superior fibular retinaculum, and the tendon of fibularis longus is thickened as is passes through.
The tendon of the muscle crosses the lateral side of the cuboid, and then proceeds to run against its inferior surface. It is thickened as it turns laterally against the surface of the cuboid, and is often the site for a sesamoid bone. This groove is converted into a tunnel by the long plantar ligament.
The muscle then continues to run from the lateral side of the cuboid more medially, and inserts onto the lateral surface of the medial cuneiform as well as lateral surface of the first metatarsal. It rarely sends slips to the second metatarsal. Its route along the lateral part of the foot and insertion onto the lateral surfaces of the medial cuneiform and first metatarsal, indicate its functional role in maintaining the transverse arch. Fibularis tertius is an occasional accessory muscle that lies in the anterior compartment of the leg, and causes dorsiflexion.
Functionally, the lateral compartment of the leg causes eversion and plantarflexion of the ankle. Recall that the talocrural joint (also known as the ankle joint), slows for flexion and extension only, and inversion and eversion in fact occur at the subtalar joint. Muscles that cause inversion include tibialis anterior, and tibialis posterior. The lateral compartment muscles work with tibialis posterior (which is also a plantarflexor) to oppose the actions of the dorsiflexor muscles (tibialis anterior and fibularis tertius).
The muscle gets its blood supply from the fibular artery, which is a branch of the posterior tibial artery. The artery in fact runs within the posterior compartment of the leg, but sends perforating arteries through to the lateral compartment.
The superficial branch of the common fibular nerve (arises from the ventral rami of L4-S3, and is a branch of the sciatic nerve which also arises from the ventral rami of L4-S3) innervates the muscle. It also supplies motor innervation to the fibularis brevis muscle, and sensation to the anterior and lateral surfaces of the leg.