The hamstrings are the big muscles on the back of your thighs. Together with the quadriceps, the hamstrings make walking and standing possible.
The hamstrings are made up of the following muscles - the biceps femoris (short and long head), the semimembranosus, and the semitendinosus.
The hamstrings are a weak muscle when compared with the quadriceps (front thigh muscle). Both the hamstrings and the quadriceps pull on the knee joint, so if one of those muscles is stronger than the other, then knee problems are bound to arise. A good rule of thumb is to make sure that the hamstrings are 2/3 as strong as the quadriceps. For example, if you can resist 100 lbs with your quads, then you should be able to resist 66 lbs with your hamstrings.
As the name implies, the biceps femoris has two heads. The short head originates at the linea aspera (roughened ridge on the femur) and inserts into the upper part of the fibula. The short head involves flexing of the knee and external rotation of the knee (point your right foot to the right).
The long head originates from the bone that forms the base of the pelvis (ischium) and inserts into the upper part of the fibula. The long head serves many functions including: knee flexion, hip extension, knee external rotation, and back extension at the hip joint (when the thighs are in a fixed position).
The semimembranosus originates from the ischium and inserts into the internal part of the tibia. The semimembranosus serves the same function as the biceps femoris, with the exception of knee external rotation. Because it inserts into the internal part of the tibia, the semimembranosus aids in internal rotation on the knee (point your right foot to the left).
The semitendinosus has the same origin point as the semimembranosus, but its insertion point is at the middle portion of the tibia. The semitendinosus has the exact same
functions as the semimembranosus.