The peroneal muscles, more formally known as the fibularis muscles in modern anatomical terminology, are a crucial group of structures located in the lower leg . They play an essential role in the movement and stabilization of the ankle and foot. Comprising three distinct muscles—the peroneus longus, peroneus brevis, and peroneus tertius—this group is fundamental for bipedal locomotion, balance on uneven surfaces, and maintaining the structural integrity of the foot’s arches. A thorough understanding of their anatomy is key to comprehending both their function and their vulnerability to injury.
Anatomical Compartments and Composition
The peroneal muscles are situated primarily within the lateral compartment of the leg, a fascial-bound space on the outer side of the fibula . This compartment is home to the peroneus longus and peroneus brevis muscles. The peroneus tertius, however, is an exception, as it is typically found in the anterior compartment of the leg alongside the extensor muscles . The group’s nomenclature is currently in a state of transition; while “peroneus” (derived from the Greek word for “pin” of a brooch) is a traditional and widely used term, “fibularis” is now the preferred name in the international standard, Terminologia Anatomica, as it more accurately reflects their origin on the fibula bone .
Origins and Insertions: The Path to the Foot
Each peroneal muscle has a unique origin and insertion point, dictating its specific action on the foot. The peroneus longus is the most superficial and superior of the group. It has a broad origin from the lateral condyle of the tibia, the head and proximal two-thirds of the fibula, and the adjacent intermuscular septa . Its long tendon then embarks on a complex journey. It descends the leg, passes posterior to the lateral malleolus (the bony bump on the outside of the ankle), and courses along the lateral side of the calcaneus. It then turns sharply under the cuboid bone, running through the peroneal sulcus, to finally insert on the plantar surface of the medial cuneiform bone and the base of the first metatarsal . This oblique path across the sole of the foot effectively creates a sling for the foot’s arches .
The peroneus brevis, as its name suggests, is shorter than the longus. It originates from the distal two-thirds of the lateral fibula, lying deep to the peroneus longus muscle . Its tendon also passes behind the lateral malleolus. Crucially, in the retromalleolar groove, the brevis tendon is typically located anteromedial to the longus tendon, nestled against the bone . It then travels superior to the peroneal tubercle of the calcaneus to insert onto the prominent tuberosity at the base of the fifth metatarsal bone .
The peroneus tertius is a variable muscle, not present in all individuals. It is often considered a part of the extensor digitorum longus muscle . It originates from the distal anterior surface of the fibula and the interosseous membrane. Its tendon inserts onto the dorsal surface of the base of the fifth metatarsal, alongside or near the insertion of the peroneus brevis .
Structural Relationships and Stabilizing Features
The anatomy of the peroneal tendons is defined by their intricate pathway and the structures that stabilize them. Distally, the tendons of the peroneus longus and brevis share a common synovial sheath as they round the ankle . They pass through a critical area known as the retromalleolar groove, a fibro-osseous tunnel located on the posterior aspect of the lateral malleolus . This groove is not uniform in all people; it can be concave, flat, or even convex, with a flat or convex shape predisposing individuals to tendon instability .
The tendons are held in place by fibrous bands called retinacula. The superior peroneal retinaculum is the primary stabilizer, originating from the fibula and inserting onto the calcaneus and lateral Achilles sheath to prevent the tendons from bowstringing or dislocating anteriorly . More inferiorly, the inferior peroneal retinaculum secures the tendons as they pass across the lateral calcaneal wall . As the tendons diverge near the calcaneus, the peroneal tubercle separates the brevis tendon (passing above it) from the longus tendon (passing below it), and the common synovial sheath divides into two separate sheaths .
Innervation and Vascular Supply
The motor and sensory functions for these muscles are supplied by branches of the common peroneal nerve. The common peroneal nerve, derived from the sacral plexus (L4-S1), winds around the fibular neck before dividing . The superficial peroneal nerve provides motor innervation to the peroneus longus and brevis muscles, as well as sensory innervation to the skin over the anterolateral leg and dorsum of the foot . The deep peroneal nerve innervates the peroneus tertius in the anterior compartment . The blood supply to the lateral compartment muscles and tendons is primarily derived from the fibular artery (peroneal artery) and branches of the anterior tibial artery . It is important to note that there is a relatively avascular “watershed” zone in the peroneus brevis tendon near the retromalleolar groove, which is a common site for degeneration and tearing .
Anatomical Variants
The peroneal region is known for its anatomical variability, which can sometimes predispose individuals to pathology. The most common variant is the peroneus quartus, an accessory muscle found in 12-22% of individuals, which can crowd the retromalleolar groove and lead to tendon attrition . A low-lying peroneus brevis muscle belly, where the muscle extends too far distally, can have a similar effect . Additionally, an os peroneum is a sesamoid bone that may form within the peroneus longus tendon, visible in about 20% of foot radiographs. If fractured or irritated, it can be a source of lateral foot pain, a condition known as painful os peroneum syndrome . A hypertrophied peroneal tubercle on the calcaneus can also impinge on the peroneus longus tendon, causing chronic irritation .
Functional Significance
Collectively, the peroneal muscles are the primary evertors of the foot, turning the sole outward . The peroneus longus and brevis are also weak plantarflexors (pointing the toe), while the peroneus tertius acts as a dorsiflexor (pulling the foot up) . Beyond these gross movements, they provide critical dynamic stability to the lateral ankle, acting as antagonists to the invertor muscles to prevent the ankle from “rolling over.” The peroneus longus, with its unique course under the foot, plays a pivotal role in supporting both the longitudinal and transverse arches of the foot . It functions in concert with the tibialis posterior to create a “stirrup” that maintains the foot’s structural integrity during gait.
The peroneal muscles and tendons form a complex and elegant anatomical system. From their origins on the fibula to their diverse insertions on the foot, their path is meticulously guided by bony grooves and strong retinacula. Their function extends far beyond simple eversion, encompassing critical roles in arch support and ankle stabilization. A deep appreciation of this intricate anatomy is essential for clinicians seeking to diagnose and treat the myriad of pathologies, from tendinosis and tears to dislocations, that can afflict this vital muscle group .