To perform a Plank with an alternating Leg Raise on a yoga ball with feet on the ground and elbows on the ball, begin by assuming a plank position with your elbows resting on the ball and your feet on the ground. Engage your core muscles to stabilize your body. Lift one leg off the ground, extending it straight behind you while maintaining your balance on the ball. Hold for a moment, then lower the leg back down to the ground and repeat the movement with the opposite leg. Alternate legs while focusing on maintaining a straight line from your head to your heels and keeping your core engaged throughout the exercise. This variation adds an extra challenge to your core and balance as you work to stabilize your body on the unstable surface of the yoga ball.