When astride a horse, the human body is stabilized to a considerable degree by its transverse abdominal muscles. These are the deepest layers of muscle, and they encase the entire midsection. Unlike the five other muscle types in the ab family, the transverses do not respond to the usual sit-ups and crunch work. What gets their attention are exercises involving the legs, hips, and deeper stimulation of the abdominal core.
Lie on your back with your arms by your sides. Place your feet on the floor hip-width apart. With your back straight, raise your body from the floor while pulling your navel inward. Hold this position for 30 to 60 seconds, breathing normally while keeping your back straight and parallel to the floor. Return to the starting position, then relax. Do three to five repetitions.
Lie on the floor on your back. Place your hands, palms down, beneath your buttocks while pressing your back to the floor. Raise one leg 10 inches, then slowly return it to the floor. Raise and lower your other leg the same way. Set your goal at three sets of 10 repetitions for each leg. Alternately, lift both legs at once to a 90-degree angle and lower slowly to the floor. Do as many as you can and work your way up to 15 over time.
Lie down and press your back to the floor. Bend your knees while keeping your feet planted on the floor. Slowly raise your pelvis, hold briefly at its peak, then slowly lower it back to the floor. Keep upper body stationary throughout. Seek three sets of 15 repetitions.
From Hands and Knees
Assume hands-and-knees position on the floor with hands shoulder-width apart. Your elbows should be slightly bent. Your knees are touching and slightly behind you, with your toes curled against the floor. Draw in your navel to tighten your abs, and in a smooth controlled motion, raise your knees from the floor a few inches. Your upper body remains stationary. Hold this position for 20 seconds (make 60 your goal), then slowly return your knees to the floor. Seek to repeat three sets of 10 repetitions.