How are the hamstrings used in running?
Another important muscle group used in running is the hamstring. There are four muscles in the hamstrings. The inner two, the semimembranosus and the semitendinosus, have their origins in the pelvis and they insert at the tibia. The outer two muscles are the two heads of the biceps femoris. The long head of the biceps femoris has its origin on the pelvis, the short head has its origin on the femur, and both insert at the fibula. I think it is interesting that only one of the muscles attaches to the femur. When these muscles contract, they bend the knee and pull the lower leg backward.
While running, these muscles are used together with the glutes to pull the femur backward and to pull on the ground with the foot. If the hamstrings are passive as the glutes begin to pull the femur downward, the lower leg will extend, lengthening the leg and slowing it down. This may also allow the foot to contact the ground early, and too far in front of the hips contributing to further slowing of the foot. If the hamstrings are active while the glutes contract, the leg can extend gradually to whatever its appropriate amount of extension is as the foot contacts the ground under the hips. The hamstrings are also important in curling the leg at the end of the ground contact. This provides additional horizontal force and also begins to shorten the leg. After the foot leaves the ground, the hamstrings continue to curl and shorten the leg until the calf contacts the hamstrings allowing the leg to quickly rotate forward with minimal leverage due to length.