The semimembranosus is a key component of the hamstring muscle group, located in the back of the thigh. Understanding which nerve provides its motor function, known as innervation, is important for understanding lower body movement and for diagnosing specific nerve-related injuries or muscle damage. This muscle plays a significant role in movements like walking and running. The nerve supply acts as the communication wire, telling the muscle when and how to contract to produce movement.
Anatomy and Function of the Semimembranosus
The semimembranosus muscle is situated on the posteromedial side of the thigh, lying deep to the more superficial semitendinosus muscle. It is distinguished by its broad, flat, and membranous tendon of origin, which is how it gets its name. This muscle, along with the semitendinosus and the long head of the biceps femoris, forms the hamstring group.
The semimembranosus originates from the superolateral facet of the ischial tuberosity (the bony prominence you sit on). It extends down the thigh and inserts onto the posteromedial aspect of the medial tibial condyle (a part of the shin bone). Because it crosses both the hip and knee joints, it is capable of producing movement at both locations.
The primary functions of the semimembranosus involve extending the hip joint and acting as a powerful flexor of the knee joint. Furthermore, it assists in the internal rotation of the leg when the knee is flexed. Working together with the other hamstrings, it helps stabilize the knee and hip joints during dynamic activities.
Nerve Pathways Supplying the Muscle
The semimembranosus muscle receives its direct command signals from the Tibial Division of the Sciatic Nerve. The sciatic nerve is the largest nerve in the human body, formed by a collection of nerve roots that exit the lower spine. It travels down the back of the thigh, where it eventually splits into the tibial nerve and the common fibular (peroneal) nerve.
The tibial division specifically supplies the muscles in the posterior compartment of the thigh, including the semimembranosus. The nerve roots that contribute to this innervation are the ventral rami of spinal nerves L5, S1, and S2.
After the sciatic nerve exits the pelvis, it descends through the posterior thigh. The branches that supply the semimembranosus typically arise high in the thigh, near the muscle belly, before the sciatic nerve fully separates. The nerve’s route exposes it to potential compression or injury along its course from the spine to the muscle.
Muscle Testing and Injury Implications
Knowledge of the semimembranosus innervation is useful for clinicians diagnosing lower limb nerve entrapment or muscle injury. Conditions like sciatica (irritation or compression of the sciatic nerve) can directly impair the function of the semimembranosus. Piriformis syndrome, where the sciatic nerve is compressed by the piriformis muscle in the buttock, may also present with weakness in this hamstring muscle.
When a nerve injury is suspected, manual muscle testing is a common clinical procedure to assess the muscle’s strength and function. The semimembranosus is tested by having the patient prone (lying on their stomach) and resisting knee flexion. Weakness or pain during this resisted movement can suggest either a muscle strain or a problem with the L5, S1, or S2 nerve roots or the tibial division of the sciatic nerve.
The semimembranosus is susceptible to hamstring strains, especially during activities that involve hyperstretching or high-speed running. An injury to the muscle or its tendon will cause pain upon palpation and during resisted contraction or stretching. Injuries to the semimembranosus, particularly those involving the proximal tendon, can sometimes require a longer recovery time compared to strains in other hamstring muscles.