Stretch out those hamstrings so you can move easier and reduces chances of low back injury and running related injuries. In the modern world most people sit for long hours resulting in tight hamstrings which restricts proper movement. Hamstrings are group of three muscles that sit on the back of the thigh. They cross two joints, the hip and the knee…and their function is to bend the knee and extend the hip. Hamstring tightness can increase risk of injury and impairs optimal performance for athletes.