If you don't have a stability ball I highly suggest going out and buying one for this 11 minute sequence alone! The stability ball is an inexpensive piece of exercise equipment that is a great tool to help you get in shape. The stability ball forces your body to stabilize itself while doing exercises. Whether you've recently recovered from a hamstring injury or you're starting a new exercise routine, keeping your hamstrings strong and flexible will help prevent future injury. The hamstrings are the three muscles that make up the back of the upper thigh. Weight-bearing and stretching exercises that target the hamstrings will improve your range of motion, muscle strength and agility. This 11 minute hamstring circuit consists of 3 moves repeated 3 times. The backs of your legs will be screaming but remember to work through the burn!