Muscle herniation can be defined as protrusion of a portion of muscle through an acquired or congenital defect of enclosing fascia. Although it is usually a cosmetic problem, it can lead to local pain and tenderness after prolonged exertion. In this report, we present a case of flexor digitorum superficialis muscle herniation in a 58-year-old man. The radiographic, ultrasonographic and magnetic resonance imaging findings are described with dynamic examination, permitting demonstration of muscle herniation through the fascial defect during muscle contraction.