Start lying on your back with arms extended straight in the air with wrists over shoulders. Bend your knees to a tabletop position, making 90-degree angles with your shins parallel to the floor. While pushing your low back against the ground, extend your right arm overhead and your left leg outward until they hover a few inches off the floor, but not so low that your back arches. Immediately return to the starting position, and repeat on the opposite side. Do 10 reps on each side, alternating arms and legs each rep.
Keep your arms extended by your sides on the ground or directly overhead. Bring your legs into that same tabletop position and — keeping your knees bent at 90 degrees — lower your right heel to tap the ground. Return your leg to the starting position, and repeat with your left leg. By shortening the length of your legs, you reduce the load on your core, thus requiring less stability, and you’re using your arms to help you balance. Do 10 reps on each side, alternating legs each rep.
While holding a five- to 10-pound medicine ball (a kettlebell or single dumbbell does the job, too), extend your arms directly above your head and perpendicular to the floor. Keep your arms in place while extending one leg at a time. The goal here is to create maximum tension in the body, so imagine you’re trying to crush the ball between your hands. This tension helps stabilize your core and adds work for your upper-body muscles. Do 10 reps on each side, alternating legs each rep.