Origin: The long head originates at the ischial tuberosity – the aspect of the pubic bone that comes in contact with your seat when you sit down. The origin of the short head is at the linea aspera on the body of the femur.
Insertion: The insertion of the biceps femoris is at the styloid process on the head of the fibula and the lateral collateral ligament and the lateral tibial condyle.
Action: The biceps femoris flexes and laterally rotates your knee. In other words, it bends your knee so that your foot moves toward that back of your body and rotates it outward. It also extends your hip, bringing your leg toward the back of your body.
Origin: Like the biceps femoris, the semitendinosus originates at the ischial tuberosity.
Insertion: The semitendinosus attaches at the proximal tibia, medial to the tibial tuberosity.
Action: The semitendinosus helps to flex and medially rotate your knee. It also extends your hip.
Origin: Like the other two hamstrings, the semimembranosus originates at the upper, outer quadrant of posterior surface of ischial tuberosity.
Insertion: Semimembranosus inserts at the medial condyle of the tibia, below the articular margin, and at the fascia over the popliteus and oblique popliteal ligament.
Action: This muscle flexes and medially rotates your knee, as well as extends your hip.
You will recognize the hamstrings as the three large muscles in the back of your thigh. Trigger points in the biceps femoris refer pain to the popliteal area at the back of the knee. Trigger points in the semitendinosus and semimembranosus refer pain to the gluteal fold – the area where the tissues of your buttock meet your thigh.
Weakness and inflexibility in the hamstrings has the potential to cause pain in the lower back. Upon movement, the inflexibility of the hamstring will put stress on the hips and pelvis, which can impact the angle of the lumbar curve.
This is why it is important to stretch and strengthen your muscles. Of course, this does not necessarily mean that you need to become a body builder, but allowing your muscles enough definition to keep stability will help prevent injury and pain.
The Whole Story on Hamstrings by Stana Landon for Active.com