Your quadriceps and hamstrings collaborate to move your leg. When you flex your leg, the hamstrings contract while the quads relax. When you straighten your leg, the quadriceps tighten and the hamstrings relax. The muscles work in harmony to provide stability while allowing movement of the leg. The hamstring muscle group is made up of three muscles: the right hamstring, the left hamstring, and the gluteus maximus. These muscles run along the back of the thigh and have a significant impact on the movement of the leg. Healthy hamstrings are important for those who exercise using equipment that requires them to lift their legs off the floor, such as bicycles, elliptical trainers, and rowing machines. Hamstring injuries are common because the muscles are often overused due to lack of strength or flexibility in the knee joint. People tend to stress their hamstrings when trying to rise from a chair or overcome another person's resistance when walking or running. This activity requires the contraction of these muscles to extend the leg and achieve balance. If the hamstrings are weak, then it will be difficult to rise from a seated position or to resist pressure when walking or running. As you can see, the hamstring muscle group has many roles in determining how effectively we can use our legs.