The muscle group situated at the back of the thigh, extending from the pelvis down to the shin bone, is collectively known as the hamstrings. This group of muscles spans the area between the hip joint and the knee joint, forming a thick, prominent mass on the posterior side of the upper leg. The hamstrings are involved in nearly every movement of the lower body, making them one of the most frequently used and sometimes injured muscle groups. Understanding the specific structure of these muscles helps explain their function in movement and their susceptibility to strain.
Anatomy of the Hamstring Muscles
The hamstring group is composed of three distinct muscles: the semitendinosus, the semimembranosus, and the biceps femoris. They are generally considered “biarticular” because they cross and act upon two joints, the hip and the knee. All three muscles share a common origin point on the lower, posterior part of the pelvis called the ischial tuberosity, often referred to as the “sit bone.”
The semitendinosus and semimembranosus are located on the medial, or inner, side of the back of the thigh. The semitendinosus inserts onto the upper-inner surface of the tibia, the larger lower leg bone, as part of a structure known as the pes anserinus. Lying deeper and slightly more to the outside is the semimembranosus, which attaches to the medial tibial condyle, a bony prominence at the top of the shin bone.
The biceps femoris muscle is positioned on the lateral, or outer, side of the posterior thigh. This muscle is unique because it has two heads, a long head and a short head. The long head originates from the ischial tuberosity alongside the other hamstrings, but the short head originates lower down on the femur, the thigh bone. Both heads converge to insert primarily onto the head of the fibula, the smaller bone of the lower leg, near the knee joint.
Essential Roles in Movement
The primary functions of the hamstrings are directly related to their attachments above the hip and below the knee. Their most powerful action is knee flexion, which is the movement of bending the knee and pulling the heel toward the buttock. They also play a major role in hip extension, which involves moving the leg backward away from the body, a movement performed repeatedly during walking and running.
Beyond these primary movements, the hamstrings contribute to the rotation of the lower leg when the knee is bent. The semitendinosus and semimembranosus are responsible for medially rotating the tibia, turning the lower leg inward. Conversely, the biceps femoris causes lateral rotation, turning the lower leg outward.
These muscles are particularly active in controlling movement during high-speed activities like sprinting. As the leg swings forward, the hamstrings engage in an eccentric contraction, meaning they lengthen while contracting, to decelerate the forward swing of the lower limb. This action controls the body during running and prevents the knee from hyperextending. The dual action of the hamstrings across both joints allows them to stabilize the pelvis and contribute to overall lower body posture and locomotion.
Common Hamstring Injuries
The most frequent issue affecting this muscle group is a hamstring strain, commonly referred to as a pulled hamstring. This injury occurs when the muscle fibers are overloaded and stretched beyond their capacity, resulting in a tear. Healthcare providers typically classify these strains into three grades based on the extent of the damage.
Grades of Hamstring Strain
A Grade 1 strain is a mild injury involving microscopic tearing of the muscle fibers, resulting in minor discomfort and little loss of strength. A Grade 2 strain is more moderate, involving a partial tear of the muscle belly and causing noticeable pain, swelling, and a clear reduction in strength. The most severe injury, a Grade 3 strain, represents a complete rupture of the muscle, which can lead to a total loss of function and may require a much longer recovery period.
These strains often occur during activities that require sudden, forceful movements, such as sprinting or rapid acceleration. The biceps femoris muscle is particularly susceptible to these sprint-type injuries. Other risk factors include insufficient warm-up before physical activity, general muscle tightness, and a strength imbalance where the quadriceps muscles on the front of the thigh are significantly stronger than the hamstrings.