An explanation for various rectus femoris strain injuries using previously undescribed muscle architecture.

We performed cadaveric dissection of the rectus femoris muscle to correlate the various lesions of strain injury seen with imaging studies to the muscular anatomy. The proximal tendon is composed of a superficial, anterior portion from the direct head, and a deep intramuscular portion from the indirect head. The muscle fibers arising from the anterior superficial tendon of the direct head travel in a posterior and distal direction to insert on the posterior tendon of insertion, giving the proximal muscle a unipennate architecture. Muscle fibers from the intramuscular tendon of the indirect head originate on both the medial and lateral sides of the tendon and insert on the distal posterior tendon to create its bipennate structure. Three chronic strain injuries involving the midmuscle belly substance were explored grossly and microscopically. It appears that one type of acute strain injury occurs in the midmuscle belly with disruption of the muscle-tendon junction of the intramuscular tendon resulting in local hemorrhage and edema. More chronically, this hematoma organizes into a fatty, loose connective tissue encasement of the deep intramuscular proximal tendon. Serous fluid from the hematoma may remain within the connective tissue sheath, creating a pseudocyst with the deep intramuscular tendon of the indirect head at its center. The muscle's anatomy helps to explain a different rectus femoris strain injury.