The fibula is the smaller of the two long bones in the lower leg, running parallel to the larger, load-bearing tibia. This slender bone is situated on the lateral, or outer, side of the leg, extending from below the knee down to the ankle. The fibula does not bear significant body weight; its primary functions are providing stability to the ankle joint and serving as an extensive attachment site for several muscles. The fibula is organized into distinct surfaces—anterior, lateral, and posterior—each providing an anchoring point for the muscles of the leg’s three functional compartments.
Muscles Attaching to the Lateral Fibula
The lateral surface of the fibula gives rise to the muscles of the lateral compartment, which are responsible for eversion—the motion of turning the sole of the foot outward. This movement is important for balance on uneven surfaces. The two muscles are the Fibularis Longus and the Fibularis Brevis, formerly known as the Peroneus Longus and Peroneus Brevis.
The Fibularis Longus, the longer and more superficial muscle, originates high up on the head and the upper two-thirds of the lateral fibular shaft. Its tendon runs down the leg, passing behind the lateral malleolus, the bony prominence of the ankle. Besides eversion, this muscle contributes minimally to plantarflexion and helps support the arches of the foot.
The Fibularis Brevis sits deep to the Longus and originates lower down on the inferolateral surface of the fibular shaft. Its tendon also travels behind the lateral malleolus but inserts onto the base of the fifth metatarsal bone. The Fibularis Brevis is considered a strong pure evertor of the foot. The coordinated action of these two muscles is important for ankle stability, preventing excessive inward rolling of the foot during movement.
Muscles Attaching to the Anterior Fibula
The anterior surface of the fibula, along with the adjacent interosseous membrane, provides origin for three muscles of the anterior compartment. These muscles are primarily responsible for dorsiflexion—the action of lifting the front of the foot toward the shin. This movement is necessary to clear the foot during the swing phase of walking and running. The three muscles originating partly from the fibula are the Extensor Digitorum Longus, the Extensor Hallucis Longus, and the Fibularis Tertius.
The Extensor Digitorum Longus originates from the upper portion of the fibula’s medial surface and the interosseous membrane. This muscle divides into four tendons that extend the four smaller toes and assists in dorsiflexing the entire foot at the ankle joint.
The Extensor Hallucis Longus arises from the middle two-thirds of the fibular shaft’s medial surface and the interosseous membrane. The function of this muscle is the extension of the great toe, in addition to its role as a powerful dorsiflexor of the ankle.
The Fibularis Tertius is a smaller, sometimes absent, muscle often viewed as an offshoot of the Extensor Digitorum Longus. It originates from the distal anterior surface of the fibula and the interosseous membrane, inserting onto the fifth metatarsal. Unlike its lateral compartment counterparts, the Fibularis Tertius acts as a dorsiflexor, contributing to the lift of the foot, and assists in eversion.
Muscles Attaching to the Posterior Fibula and Proximal End
The posterior surface and the proximal end (head) of the fibula host a diverse group of muscles. The fibular head serves as the insertion point for the short head of the Biceps Femoris muscle, one of the hamstring group. This muscle is a primary flexor of the knee joint, responsible for bending the leg. The strong pull of this tendon on the fibular head can lead to an avulsion fracture if the muscle contracts suddenly and forcefully.
Moving down the shaft, the posterior surface of the fibula is a large origin site for deep muscles of the posterior compartment. The Soleus muscle, a wide, flat muscle deep to the Gastrocnemius, originates from the posterior surface of the fibular head and the upper third of the fibular shaft. The Soleus is a powerful plantarflexor, providing most of the force needed to propel the body forward during walking and running.
The deeper layers of the posterior compartment rely on the fibula for attachment. The Flexor Hallucis Longus, which controls the movement of the great toe, originates from the lower two-thirds of the posterior fibula. This muscle is responsible for curling the great toe and assists in plantarflexion, helping to stabilize the foot.
The Tibialis Posterior muscle, the deepest muscle in the compartment, originates from the posterior surface of the fibula, the tibia, and the interosseous membrane between them. This muscle is important for inversion of the foot and provides dynamic support to the medial arch of the foot.