Core-strength exercises strengthen your core muscles, including your abdominal muscles, back muscles and the muscles around the pelvis. You can do many core-strength exercises with a fitness ball.

In general, use a fitness ball sized so that your knees are at a right angle when you sit on the ball with your feet flat on the floor. Do each core-strength exercise five times and maintain good form and technique. As you get stronger, gradually increase to 12 to 15 repetitions. For most people, a single set of 12 to 15 repetitions can build strength and improve fitness as effectively as can multiple sets of the same exercise.

Breathe freely and deeply and focus on tightening your abs during each core-strength exercise. If you have back problems, osteoporosis or any other health concern, talk to your doctor before doing these core-strength exercises.