You don’t have to do hundreds of sit-ups and all manner of crunches to see abdominal muscle definition. The following moves are done standing and holding a weight, such as a 5-pound ball with an easy-to-grip handle.
For the standing twist, feet are hip-width apart, toes turned slightly out to the sides for better balance. Holding the weight with both hands, extend your arms straight out in front of you, just below shoulder level. Squeeze your glutes to steady your hips and visualize pressing your abs into your spine as you twist your upper body to the right, then all the way to the left and finally back to center for one rep. Repeat up to 15 times per set. Gradually progress until you can complete three sets.
For the overhead lean, stand in the same starting position, holding the weight, then straighten your arms all the way up toward the ceiling. With control, lean your upper body to the right, then all the way to the left and back to center for one rep. Repeat up to 15 times per set. Gradually progress until you can complete three sets.
For side bends, hold the weight in your right hand. Place your left hand behind your head or on your waist for better balance. Keeping your hips, back and head aligned, slowly lean to the right. Hold for two counts and return to start. Repeat up to 15 times, switch sides and repeat on the left side. Alternate to the right and to the left to complete up to three sets of 15 on each side.
The American Council on Exercise has more on these and other standing ab exercises and how to do them safely.