The accessory peroneus quartus muscle lies posterior and medial to the peroneus brevis and longus tendons. With respect to the Achilles tendon, it lies anterior and lateral. This muscle may present as a mass or incidental finding on MRI. It occurs in 13-15% of people. Most often, this muscle is asymptomatic; however, in some people it may be associated with lateral ankle pain or ankle joint instability. It may also predispose an individual to subluxation of the peroneal tendons due to its mass effect within the peroneal retinaculum.