The hamstring muscles, like many other muscles in our body, are used constantly. They not only help us walk and run, they help with posture. In many cases, the hamstrings are overworked, which makes them vulnerable to injury. Running can especially stress the hamstrings due to the decelerating and accelerating of the legs with each stride. Normally, the hamstrings can handle this, but when they are weak and overworked, trouble arises. By strengthening the hamstrings, you will set yourself up for better postural mechanics and less injury.
Many people I see at the gym can point to their hamstrings, which are located on the back of the thighs, but few know where they actually attach. The hamstrings run from the bottom of the hip bone, beneath the glutes, to the back of the shin bone, below the back of the knee. This long muscle has two primary functions. One is to bend the knee joint, and the other is to extend the hip joint. To illustrate the knee function of the hamstring, just bend the knee while standing on one leg. The second function is illustrated when bending over a small fence to reach something on ground. During this act, you will bend completely from the hips as you lean forward over the fence. Your hamstrings are partly responsible for keeping you from falling forward while doing this, and are also working hard while raising you back to the upright position.
Usually during movement, the functions are used together to create more complex movements such as squatting, lunging, walking and running. With weak hamstrings, many of these motions will be compromised. This is why it is so important to keep them strong.
One of the best exercises I know to strengthen not only the hamstrings, but the entire motion that the hamstrings are a part of, is to do a supine ball leg curl. During this motion, the hamstrings, glutes and calves are exercised. Items needed are a 55-centimeter Swiss ball, which you can buy at just about any fitness store.