Objective. Quantify the presence of the fibularis quartus muscle and correlate it with the individual's sex and concomitant presence of the fibularis tertius muscle. Materials and Methods. We evaluated 211 magnetic resonance examinations (43.13% men and 56.87% women) on the ankle and hindfoot, produced between 2009 and 2011. Results. The fourth fibularis muscle was found to be present in 7.62% of the examinations and 75% of these also contained the fibularis tertius muscle. Conclusion. The incidence of the fourth fibularis muscle is variable; moreover, its incidence does not depend on the individual's gender or the presence of the fibularis tertius muscle. 1. Introduction The fibularis quartus muscle was first described by Otto in 1816 [1] and was subsequently studied in detail by Hecker in 1923 [2]. It forms part of one of the three groups of muscle variations that occur in the ankle: the group of muscle-tendon variants of the fibular muscles [3]. In most cases, the presence of this muscle is asymptomatic, and it is detected by chance during the examination or surgical procedure [4–6]. However, in some cases, its presence is associated with certain symptoms: pain in the ankle, with or without anterior trauma; displacement, spraining, or tearing of the fibularis brevis tendon; tendon calcification; or painful hypertrophy of the retrotrochlear eminence [4, 5, 7]. The presence of the fibularis quartus muscle may simulate a longitudinal tear in the fibularis brevis tendon, but is differentiated by the presence of the muscle, when a muscle belly separated from the muscle belly of the fibularis brevis is present [8]. In most cases, the origin of the fibularis quartus muscle is posterior to the bifurcation of the belly of the fibularis muscles, or in the fibularis brevis muscle. It may also originate in the posterior dace of the fibula or in the fibularis longus muscle. Its insertion is also variable, thus explaining the variety of names that it has received: accessory fibularis muscle (insertion into the tendon of the fibularis longus, in the sole of the foot); fibulocalcaneus muscle (insertion into the retrotrochlear eminence); fibulocuboid muscle (insertion into the tuberosity of the cuboid bone, inferiorly); or fibular-fibularis longus muscle (insertion into the tendon of the fibularis longus muscle or into the inferior retinaculum of the fibularis muscles, adjacent to the retrotrochlear eminence). These last two variants are the ones with lowest incidence [6, 9, 10]. The fibularis quartus muscle acts predominantly as a pronator for the foot, as do the
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