The hamstrings are a group of large muscles located at the back of the thigh, rather than a single muscle. Alongside the quadriceps, which are located at the front of the thigh, the hamstrings are primarily involved in walking and standing.

The hamstrings consist of three muscles: the semitendinosus, semimembranosus, and biceps femoris. These muscles counterbalance the powerful quadriceps muscles at the front of the thigh.

Compared to the quadriceps, the hamstrings are relatively weaker. Both the hamstrings and quadriceps act on the knee joint, so an imbalance between the strength of these muscles can lead to knee issues. A good rule of thumb is to ensure that the strength of the hamstrings is about two-thirds that of the quadriceps. For example, if your quadriceps can resist 100 pounds, then your hamstrings should be able to resist 66 pounds.

Biceps Femoris (Long and Short Heads)

As the name suggests, the biceps femoris has two heads: the long head and the short head. The short head originates from the linea aspera (a rough ridge on the femur) and inserts into the upper part of the fibula. It is involved in bending the knee and externally rotating the knee (turning the right foot to the right). The long head originates from the ischial tuberosity (the bone forming the base of the pelvis) and also inserts into the upper part of the fibula. The long head has multiple functions including knee flexion, hip extension, external rotation of the knee, and posterior extension of the hip when the thigh is in a fixed position.

Semimembranosus and Semitendinosus

The semimembranosus originates from the ischium and inserts into the inner part of the tibia. It shares the same functions as the biceps femoris, except for knee external rotation. Due to its insertion on the inner part of the tibia, it aids in the internal rotation of the knee (turning the right foot to the left). The semitendinosus has the same origin as the semimembranosus, but it inserts into the medial aspect of the tibia. The semitendinosus has the same functions as the semimembranosus.