An excellent exercise for your hamstrings with no weight.
Lie on your back on a mat, placing your lower heels on the top of the stability ball. Your feet should be lined up with your hips and your toes pulled slightly toward the ceiling. Contract your abs and extend your arms out to your sides with palms turned to the floor to help stabilize your body during the exercise.
Lift your hips up off the floor. Press your heels into the ball for additional stability. Do not lift the hips so high that the lower back begins to arch. Continue to press upwards until your body is in a straight line from your heels to your shoulder blades.
Exhale and gently bend your knees and pull the ball toward your hips. Your toes may point away from your body in this movement. The hips will continue to lift as you pull the ball toward your hips. Keep the torso stable.
Inhale and slowly press the ball away from your hips until the knees are straight, stabilizing with the core, hamstrings, glutes, and arms. Repeat.