BICEPS FEMORIS – Not to be confused with the bicep muscle in your upper arm!! Its main action is flexion at the knee, but it is also responsible for extending the thigh at the hip and lateral rotation at the hip and knee.
SEMITENDINOSUS – Responsible for flexion of the leg at the knee joint and extension of thigh at the hip. It also medially rotates the thigh at the hip joint and the leg at the knee joint.
SEMIMEMBRANOSUS – Lies underneath the semitendinosus. It is responsible for flexion of the leg at the knee joint, extension of the thigh at the hip and medial rotation of the thigh at the hip joint and the leg at the knee joint.
If you spend much of your day sat behind a desk, then it is not unusual for your hamstrings to feel tight – however, it might not solely be your hamstrings causing the issue. If your hip flexors and the muscles in the front of your pelvis are tight, this can elevate the attachments of your hamstrings (it causes an anterior tilt in the pelvis and basically places your hamstrings in a lengthened position) which in turn creates that feeling of tightness. Make sure you stretch your hip flexors out as well as your hamstrings before and after exercise!