The narrow FL extends from the head of the fibula to the sole of the foot. The fibularis longus (FL) is the longer and more superficial of the two fibularis muscles, arising much more superiorly on the shaft of the fibula. To test the fibularis longus and brevis, the foot is everted strongly against resistance if acting normally, the muscle tendons can be seen and palpated inferior to the lateral malleolus. ![]() When standing (and particularly when balancing on one foot), the fibularis muscles contract to resist medial sway (to recenter a line of gravity, which has shifted medially) by pulling laterally on the leg while depressing the medial margin of the foot. In practice, the primary function of the evertors of the foot is not to elevate the lateral margin of the foot (the common description of eversion) but to depress or fix the medial margin of the foot in support of the toe off phase of walking and, especially, running and to resist inadvertent or excessive inversion of the plantarflexed foot (the position in which the ankle is most vulnerable to injury). From the neutral position, only a few degrees of eversion are possible. As evertors, the fibularis muscles act at the subtalar and transverse tarsal joints. However, because the fibularis longus and brevis pass posterior to the transverse axis of the ankle (talocrural) joint, they contribute to plantarflexion at the ankle – unlike the postaxial muscles of the anterior compartment (including the fibularis tertius), which are dorsiflexors. ![]() Both muscles are evertors of the foot, elevating the lateral margin of the foot.Äevelopmentally, the fibularis muscles are postaxial muscles, receiving innervation from the posterior divisions of the spinal nerves, which contribute to the sciatic nerve. These muscles have their fleshy bellies in the lateral compartment but are tendinous as they exit the compartment within the common synovial sheath deep to the superior fibular retinaculum. ![]() The lateral compartment of leg contains the fibularis longus and brevis muscles. Here, the tendons of the two muscles of the lateral compartment of leg (fibularis longus and brevis) enter a common synovial sheath to accommodate their passage between the superior fibular retinaculum and the lateral malleolus, using the latter as a trochlea as they cross the ankle joint. The lateral compartment of leg ends inferiorly at the superior fibular retinaculum, which spans between the distal tip of the fibula and the calcaneus. It is bounded by the lateral surface of the fibula, the anterior and posterior intermuscular septa, and the deep fascia of the leg. The lateral compartment of leg, or evertor compartment, is the smallest (narrowest) of the leg compartments.
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