Stability ball leg curls
>> How Lie face-up on the floor with legs extended and heels on top of a stability ball. Lift your hips off of the floor and pull your heels toward your butt, keeping your toes and knees pointed up. Hold your hips high and straighten your legs; do three sets of 10. When that move becomes easy, try them one-legged: Extend one leg toward the ceiling while you roll the ball toward your butt with the other.
>> Why Each time you step down, the hamstrings contract and pull the lower leg beneath you, “like doing leg curls all the way down the mountain,” says Snyder. This exercise mimics that on-trail movement to strengthen hamstrings and prevent the knee pain caused when the quads are disproportionately stronger than the muscles on the back of the leg.