MUSCLES ENGAGED Hamstrings The hamstrings play a crucial role in many daily activities, such as, running, jumping, walking and cycling. Strengthening your hamstring muscles help to protect your knees from injury by absorbing the stress placed on the knee. Weak hamstrings can lead to knee injury. Glutes Gluteal Muscles (Glutes) are one of the strongest muscles in the body and are the connecting point between your legs and back muscles. Glutes are used in a variety of actions from moving the leg to extending and rotating the hip, as well as extending and rotating the trunk of the body. Equipment Used Stability Ball A stability or exercise ball is a ball constructed of soft elastic and filled with air until firm.Most stability balls range from a diameter of approximately 35 to 85 centimeters (14 to 34 inches). Exercise Instructions Step 1 Start with your legs up on the stability ball and your hips raised several inches off of the floor. Step 2 Curl the stability ball in towards your buttocks and then drive your hips up towards the ceiling. Fully contract your buttocks at the top of the motion and try to create a straight line from shoulders to knees. Step 3 Slower lower your hips down towards the mat as your release the ball back to the starting position using your hamstrings and glutes. Make sure to keep control throughout the entire exercise.