Your legs are made up of dozens of muscles. Some of these muscles are long and narrow, others short and thick. Each helps you walk, run, jump and bend down. The back of your thigh is made up of hamstrings, a group of three major muscles, one of which is a muscle called the biceps femoris. Together with two other muscles in the hamstrings, the biceps femoris is involved in major muscle movements of the upper leg.
The long- and short-headed biceps femoris muscle located on the back of the thigh originates at the buttocks and extends to an insertion point at the back of the knee. Specifically, the upper portion or head of the biceps femoris starts at the bottom end of the back of the pelvis at a point called the ischial tuberosity, near the coccyx or tailbone. The other end of the muscle attaches to the top ends or the heads of the fibula and tibia just below the back of your knee joint.
As the biceps femoris muscle contracts, the muscle shortens, allowing you to lift your leg or bend your knee. As the muscle elongates or stretches, you're able to extend the leg away from your body. The long head of the biceps femoris muscle allows you to extend your leg while the short head biceps femoris allows you to rotate your knee laterally and flex your knee.
Extension and Flexion
The biceps femoris muscle works in conjunction with multiple thigh and hip muscles. The main function of this muscle is to enable you to extend your leg and thigh at the hip joint. The biceps femoris allows you to lift your leg upward from the floor, to kick a ball and any other movement that requires extension of the leg away from your body. Lifting your knee requires flexion, initiated by a shortening of the hamstring muscles, including the biceps femoris.
Your biceps femoris muscle allows you to rotate your knee away from the body toward the side. You use this muscle to sit cross-legged on the floor, to perform plie squats and to perform the mountain pose in yoga. Bending your legs and pressing your feet together is made possible by this muscle, as are a number of other lateral rotation functions of the knee and hip working together.