It is situated at the upper part of the lateral side of the leg, and is the most superficial of the three peroneus muscles.
It is innervated by the superficial fibular nerve (superficial peroneal nerve).
It arises from the head and upper two-thirds of the lateral surface of the body of the fibula, from the deep surface of the fascia, and from the intermuscular septa between it and the muscles on the front and back of the leg; occasionally also by a few fibers from the lateral condyle of the tibia. Between its attachments to the head and to the body of the fibula there is a gap through which the common peroneal nerve passes to the front of the leg.
It ends in a long tendon, which runs behind the lateral malleolus, in a groove common to it and the tendon of the Peronæus brevis; the groove is converted into a canal by the superior peroneal retinaculum, and the tendons in it are contained in a common mucous sheath.
The tendon then extends obliquely forward across the lateral side of the calcaneus, below the trochlear process, and the tendon of the peroneus brevis, and under cover of the inferior peroneal retinaculum.
It crosses the lateral side of the cuboid, and then runs on the under surface of that bone in a groove which is converted into a canal by the long plantar ligament; the tendon then crosses the sole of the foot obliquely, and is inserted into the lateral side of the base of the first metatarsal bone and the lateral side of the medial cuneiform.
Occasionally it sends a slip to the base of the second metatarsal bone.
The tendon changes its direction at two points: first, behind the lateral malleolus; secondly, on the cuboid bone; in both of these situations the tendon is thickened, and, in the latter, a sesamoid fibrocartilage (sometimes a bone), is usually developed in its substance.
The Peronæus longus also everts the sole of the foot, and from the oblique direction of the tendon across the sole of the foot is an important agent in the maintenance of the transverse arch.
Taking their fixed points below, the Peronæi serve to steady the leg upon the foot.
This is especially the case in standing upon one leg, when the tendency of the superincumbent weight is to throw the leg medialward; the Peronæus longus overcomes this tendency by drawing on the lateral side of the leg.