While there will always be a place for sit-ups for ab definition, many trainers now suggest standing exercises that not only train abs but back muscles, too.
They’re more practical than getting down on the floor, and they may also be more effective because the moves strengthen the body’s core muscles, making everyday movements — forward, backward and side-to-side — easier. Training them may also help you avoid the low-back pain that typically comes with age.
Start with side bends. Stand with feet shoulder-width apart and hold a light dumbbell in each hand, hands at your sides. Slowly bend to the right as far as you can without twisting your upper body. Your right hand will dip slightly toward your right knee. Hold, slowly return to start position and then bend to the left. Repeat 10 times to each side.
Next, move to hip hinges. Stand facing a wall, feet shoulder-width apart. Press your hands into the wall and contract your abs. Bend and lift your right knee in front of you until your thigh is parallel to the ground. Moving like a hinge from the hip, open your knee as far to the right as you can, then move back to center and across to the left. Repeat the entire sequence 10 times with each leg.
For the standing twist, knees are slightly bent and hands are behind your head. Put your weight on your left leg and lift your right knee in front of you to hip level as you bring your left shoulder down to try to meet it. Return to start. Repeat for a total of 20 reps, and then repeat on opposite side.
The American Council on Exercise has more on ab exercises.