Unilateral short rectus femoris muscle belly

Introduction Rectus femoris is a spindle shaped muscle forming part of the quadriceps muscle group in the anterior compartment of the thigh. Traditionally, it is described as having two heads which arise as a straight tendon from the anterior inferior iliac spine and a reflected tendon from the ilium just above the acetabulum. Recent studies have shown that the anatomy of the muscle is more complex than previously understood [1–3]. The straight tendon covers the anterior aspect of the proximal third of the muscle, whilst the reflected tendon is deeper and extends intramuscularly for about two-thirds of the length of the muscle [1]. The distal tendon arises as a broad aponeurosis on the posterior surface of the distal two-thirds of the muscle, narrows to a thick flat tendon about two-thirds of the way down the thigh, and inserts into the base of the patella. In the proximal third of the muscle, fibers from the superficial anterior tendon of the straight head pass obliquely to the posterior distal tendon, giving a unipennate appearance. Distal to the superficial proximal tendon, muscle fibers from the central intramuscular tendon of the reflected head run bilaterally to the posterior tendon creating the classical bipennate structure [1]. In a study of 48 adult cadavers, Tubbs et al. [2] found that rectus femoris had a third head in the majority of individuals, arising from the iliofemoral ligament and tendon of gluteus minimus. The distal tendon of rectus femoris combines with the tendons of vastus medialis, intermedius and lateralis to form a bior trilaminar quadriceps tendon (rectus femoris tendon being most superficial) that inserts into the base of the patella [3]. From here, fibers continue to the tibial tuberosity via the patellar ligament. The quadriceps femoris is a powerful knee extensor but rectus femoris also contributes to hip flexion since it crosses the hip joint anteriorly. Despite its complexity, anatomical variants of rectus femoris are rare. To our knowledge, a short muscle belly has not been previously described.