Hamstring exercise. The legs of a young man perform a hamstring exercise. The man is lying on his front with one ankle crossed over the other. As he raises the lower leg, the upper leg provides resistance. In this way he exercises the hamstring muscles at the back of each thigh which bend the knee and swing the legs backwards from the thigh. The muscles will become stronger and more flexible. This type of exercise can be used to improve health, for recreation, or during rehabilitation from a physical injury.