The semimembranosus muscle is one of the three muscles comprising the hamstring group located at the back of the thigh. It is a large, flat, and elongated muscle, important for both knee and hip movements. The semimembranosus is notable for its extensive membranous tendon from which it gets its name, playing a crucial role in the extension of the hip and the flexion and internal rotation of the knee.
Location
The semimembranosus is located in the posterior compartment of the thigh. It originates from the ischial tuberosity of the pelvis and inserts into the medial condyle of the tibia. It lies medial to the semitendinosus and biceps femoris muscles, making up the medial part of the hamstring group.
Anatomy
Origin
The semimembranosus originates from the ischial tuberosity, which is the lower and back part of the hip bone. It shares this common origin point with another hamstring muscle, the semitendinosus.
Insertion
The muscle extends down the back of the thigh and inserts mainly into the medial condyle of the tibia. The insertion is characterized by a broad, flat tendon. Some fibers also extend to the fascia covering the popliteus muscle on the back of the knee and to the tibial collateral ligament of the knee joint, contributing to the stability of the joint.
Structure
The semimembranosus muscle starts with a thick, fleshy upper portion and transitions into a broad, flat tendon as it approaches the knee. This structure is why it’s termed “semimembranosus,” as it has a membranous appearance towards its insertion.
Relations
- Medially: It lies deep to the semitendinosus muscle and superficial to the adductor magnus muscle.
- Laterally: It is located medial to the biceps femoris, another hamstring muscle.
- Superiorly: In its upper part, it is covered by the gluteus maximus muscle when the hip is extended.
Blood Supply
The blood supply to the semimembranosus comes primarily from the profunda femoris artery, a branch of the femoral artery, and from the inferior gluteal artery.
Innervation
The muscle is innervated by the tibial component of the sciatic nerve (L4, L5, S1, S2 nerve roots). This nerve runs down the back of the thigh and branches out to supply the hamstrings.
Function
The semimembranosus muscle, as part of the hamstring group, has several important functions related to movement and stabilization of the lower limb:
- Hip Extension: The semimembranosus aids in extending the hip joint, which involves moving the thigh backward. This action is crucial for activities such as walking, running, and climbing stairs.
- Knee Flexion: The muscle contributes to bending the knee, an essential movement for walking, sitting, and squatting.
- Medial Rotation of the Knee: When the knee is flexed, the semimembranosus helps medially rotate the tibia (turning the shin inward), which is important for stabilization during walking and changes in direction.
- Joint Stabilization: By spanning the hip and knee joints, the semimembranosus helps stabilize these joints, particularly during dynamic movements and weight-bearing activities, reducing the risk of injuries.
Clinical Significance
Hamstring Injuries: The semimembranosus, as part of the hamstring group, is susceptible to strains and tears, particularly in athletes or during activities involving sudden acceleration or deceleration. Injuries to this muscle can result in pain, swelling, and limited mobility, significantly impacting physical performance and daily activities.
Knee Joint Stability: Given its attachment to the knee, the semimembranosus contributes to knee stability. Dysfunction or weakness in this muscle can lead to increased susceptibility to knee injuries, including ligament sprains or meniscal tears.
Posterior Thigh Pain: Issues with the semimembranosus can contribute to conditions like sciatica or lower back pain due to its proximity to the sciatic nerve and its pelvic origin. Accurate diagnosis is crucial for effective treatment.
Rehabilitation and Recovery: Understanding the anatomy and function of the semimembranosus is vital for designing effective rehabilitation programs for hamstring injuries or post-surgical recovery involving the knee or hip. Strengthening and flexibility exercises targeting this muscle can aid in recovery and prevent future injuries.
Surgical Considerations: In surgeries involving the posterior thigh, knee, or hip, the anatomy of the semimembranosus must be carefully considered to avoid iatrogenic injury and ensure optimal postoperative outcomes.
