Gracilis muscle

Origin and insertion

Gracilis is a thin, flat, long muscle that attaches to the coxal bone and tibia. It starts out broad and then tapers off as it approaches its insertion point. The muscle originates through a thin aponeurosis from three sites located on the ischium and pubis:

• medial margins of the lower half of the anterior body of pubis
• the entire surface of the inferior pubic ramus
• a small portion of the ramus of ischium close to its adjoining point with the inferior pubic ramus

The muscle fibers travel inferiorly and eventually blend into a round tendon which courses posterior to the sartorius tendon and passes the medial condyle of femur. At the level of the proximal part of tibia, the gracilis tendon curves and then fans out around the medial condyle of tibia. Here, it joins the pes anserinus, which represents a conjoined tendon comprising the tendons of three different muscles; gracilis, sartorius and semitendinosus. 

The muscles that comprise the pes anserinus can be easily remembered with a following mnemonic;

Say Grace Sarah

• Sartorius muscle
• Gracilis muscle
• Semitendinosus muscle

Pes anserinus, and hence the gracilis muscle, attaches to the medial surface of proximal tibia inferior to the condyle. Within the pes anserinus, the gracilis tendon attaches more proximally than the tendons of semitendinosus and sartorius. Some tendinous fibers of the gracilis slip into the deep fascia of the lower leg while others can often blend with the tendon of the medial head of gastrocnemius muscle. 

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Relations

Gracilis is the most superficial muscle of the medial (adductor) compartment of the thigh. It is overlaid superficially by the skin and subcutaneous tissue, while its medial part is also covered by the deep layer of fascia lata. The portion of the fascia lata between the sartorius and gracilis tendons is pierced by the saphenous nerve, which exits the adductor canal to become subcutaneous. The saphenous branch of the descending genicular artery also courses between sartorius and gracilis.

Adductor brevis and adductor magnus muscles are located deep to the gracilis. However, the spiral trajectory of this muscle can result in some complex and changing relations. Therefore, the gracilis is also neighboring the medial border of adductor magnus, the lower border of adductor brevis, and it is located medial to adductor longus. The tibial collateral ligament is located deep to the gracilis tendon, being separated from it by the anserine bursa.

Innervation

Gracilis muscle is innervated by the obturator nerve, which is a branch of the lumbar plexus. It arises from the L2-L3 spinal nerves.

Blood supply

Gracilis receives the majority of its vascular supply from the ‘artery to the adductors’, which is a branch of the deep femoral artery. The ‘artery to the adductors’ enters gracilis via its lateral surface, approximately one third away from its origin. 

The proximal portion of the muscle also receives a small proportion of blood supply from the medial circumflex femoral artery. The distal third of the gracilis is also supplied by a minor branch of the femoral artery.

Don't forget to quiz yourself on the gracilis and other muscles of the thigh to consolidate your knowledge!

Functions

Gracilis acts on the hip and knee joints, resulting in several movements:

• Strong leg flexion and medial (internal) rotation around the knee joint when the knee is in a semiflexed position.
• Weak thigh flexion and adduction around the hip joint, simply aiding the other, more powerful thigh adductors.

The most important function of the gracilis is to help the hamstring muscles flex the knee, for example during the initial swing phase in walking, or during boat rowing. The medial rotation of the leg also becomes evident during walking, when the foot is solidly planted on the ground. When the lower extremity is fixed, the gracilis muscle laterally rotates the femur and pelvis around the tibia, which acts as a fulcrum. This action is important to balance the trunk. However, all movements of the gracilis become evident during horse riding, when the muscle helps the rider to grip the horse (thigh adduction) and control the flexed knee. 

For more information about the gracilis muscle and the other muscles of the hip and thigh, take a look at the following resources: