A fitness ball can be a great tool for toning your abdominal muscles (abs), as well as other core muscles.

Fitness balls — also known as stability balls — come in various sizes. Choose a fitness ball that allows your knees to be at a right angle when you sit on the ball with your feet flat on the floor.

To tone your abdominal muscles using a fitness ball, try the abdominal crunch:

• Sit on the fitness ball with your feet resting on the floor, about hip-width apart.
• Keeping your back straight, cross your arms on your chest and tighten your abdominal muscles.
• Lean back until you feel your abdominal muscles further tighten.
• Hold for three deep breaths.
• Return to the starting position and repeat.

Start with up to five repetitions, as long as you're able to maintain good form and technique. Over time, as you strengthen your abs and core muscles, gradually increase to 12 to 15 repetitions on the fitness ball.

For most people, a single set of 12 to 15 repetitions until the muscle is fatigued can build strength and improve fitness as effectively as can multiple sets of the same exercise. Aim to do muscle strengthening exercises at least twice a week. Remember to breathe freely and deeply while using the fitness ball.