The hamstrings are a group of three muscles located at the back of the thigh. These muscles include the biceps femoris, semitendinosus, and semimembranosus.[1] They play a vital role in the movement and stabilization of the hip and knee joints. Each muscle has its origin at the ischial tuberosity of the pelvis and inserts on different parts of the lower leg bones, including the tibia and fibula. The hamstrings are essential for activities such as walking, running, and jumping.
Location
The hamstrings are located on the posterior side of the thigh, extending from the pelvis to the lower leg, crossing both the hip and knee joints.[4]
Structure and Anatomy
Muscle Composition
The hamstrings consist of three primary muscles located on the posterior aspect of the thigh:
Biceps Femoris:
- Long Head: Originates from the ischial tuberosity of the pelvis and inserts on the head of the fibula.
- Short Head: Originates from the linea aspera of the femur and also inserts on the head of the fibula.
Semitendinosus:
Originates from the ischial tuberosity and inserts on the medial surface of the tibia, just below the knee. It has a long, cord-like tendon that is easily palpable.[8]
Semimembranosus:
Originates from the ischial tuberosity and inserts on the posterior aspect of the medial tibial condyle.[6] This muscle is broader and flatter compared to the semitendinosus.
Tendons
The hamstring muscles transition into tendons near their insertion points. These tendons cross both the hip and knee joints, allowing the muscles to influence movements at both joints. The tendons are particularly robust near the knee.
Innervation
The hamstrings are primarily innervated by the sciatic nerve, with:
- The tibial division of the sciatic nerve innervating the biceps femoris (long head), semitendinosus, and semimembranosus.[7]
- The common fibular division of the sciatic nerve innervating the biceps femoris (short head).
Blood Supply
The hamstrings receive their blood supply from the:
- Deep femoral artery (profunda femoris): Provides branches such as the perforating arteries that supply the muscles.
- Inferior gluteal artery: Contributes to the proximal portions of the hamstrings.
Attachments and Landmarks
The hamstrings originate from the pelvis and span across the posterior thigh to insert on the tibia and fibula. Their main anatomical landmarks include:
- Ischial Tuberosity: The origin point for all hamstring muscles except the short head of the biceps femoris.
- Head of the Fibula: Insertion point for the biceps femoris.
- Pes Anserinus: A tendinous structure where the semitendinosus shares an insertion with other muscles (sartorius and gracilis) on the medial tibia.
Fascial Enclosure
The hamstrings are enclosed by the fascia of the thigh, known as the fascia lata, which helps maintain their position and aids in force transmission during muscle contraction.[5]
Regional Differences
- The biceps femoris is located laterally on the posterior thigh, while the semitendinosus and semimembranosus are positioned medially.
- The semimembranosus lies deeper than the semitendinosus, contributing to the bulk of the medial hamstring region.
Function
Hip Extension
The hamstrings extend the hip joint by pulling the thigh backward. This action is essential for movements such as walking, running, jumping, and climbing, where the leg moves behind the body.
Knee Flexion
The hamstrings flex the knee joint by bringing the lower leg closer to the thigh. This movement is crucial for activities like walking, squatting, and cycling.
Control of Movements
The hamstrings help control the rate of knee extension during activities such as descending stairs or landing from a jump by acting eccentrically (lengthening under tension).
Stabilization
The hamstrings stabilize the pelvis and prevent it from tilting excessively during walking or running.[3] They work in coordination with other muscles to maintain proper posture and balance.
Rotation of the Knee
- The biceps femoris assists with lateral rotation of the knee when it is flexed.
- The semitendinosus and semimembranosus assist with medial rotation of the knee when it is flexed.
Dynamic Support of the Lower Limb
The hamstrings act dynamically to support and stabilize the lower limb during fast or complex movements, providing both strength and flexibility.
Clinical Significance
The hamstrings are crucial for lower limb movement and are prone to various injuries and conditions due to their role in dynamic activities.
Hamstring Strains
One of the most common sports-related injuries, often caused by overstretching or sudden, forceful movements. Strains typically occur in high-speed activities like sprinting or jumping.
Tendinopathy
Overuse or repetitive stress can lead to tendinopathy, particularly at the proximal tendon near the ischial tuberosity.[1] This condition causes pain and reduced flexibility.
Tears and Avulsions
Severe injuries, such as complete tears or avulsion injuries (where the tendon pulls away from the bone), may require surgical intervention.
Tightness and Reduced Flexibility
Tight hamstrings are a common issue that can contribute to lower back pain, poor posture, and increased risk of injuries in the lower back and knees.
Role in Rehabilitation
The hamstrings are a focus in rehabilitation programs for knee and hip injuries, as their strength and flexibility are vital for restoring normal function.