Playing tennis or racquetball is an enjoyable way to boost the intensity of your fitness program. It can also improve your balance, strength, and agility.

Racquet sports alternate bursts of high-intensity exercise while you score points, with brief rest periods while you pick up the ball and serve. This stop-and-start activity is similar to interval training.

Playing racquet sports, or any active sport, 3 hours a week can cut your risk of developing heart disease and lower your blood pressure. One key to getting a good aerobic workout in tennis or racquetball is to keep your rest periods brief. Your heart will continue to work at an aerobic level, but without the sustained stress.

It's also important to prepare for your game as you would for a workout. Warm up for at least 10 minutes before you begin, followed by another 10 minutes of stretching. Check with your doctor before you begin an exercise program.

Playing tennis at a moderate-to-vigorous intensity burns calories. It also builds strength in your upper body, legs, hips, and belly, and improves your speed and overall flexibility.

To get the most from your workout, you and your opponent should agree to play for the aerobic benefit, as well as for fun. Instead of firing aces past each other, plan on a volley-and-return match that keeps you both moving. Scatter your shots around the court to make the most of the distance you both run. Also limit your number of serves. Or play for total points instead of using traditional scoring.

To keep your game injury-free, follow these suggestions from the American Academy of Orthopaedic Surgeons:

• Warm up before you begin to play. Jumping jacks, stationary bike riding, or running in place for 3 to 5 minutes are good warm-up activities.
• Do slow, gentle stretches after you've warmed up.
• Play on a court with a forgiving surface, if possible. Stay off hard surfaces like cement and asphalt.
• Wear shoes that offer good support for your ankles.
• Dry your racket handle often to prevent blisters.
• Protect your back by bending your knees and raising your heels when serving or hitting the ball overhead.

Racquetball has plenty of fitness benefits. If you play to 15 points, it usually takes about 20 minutes. A typical match of 3 games usually lasts an hour. You can burn lots of calories. Because racquetball demands many twists, turns, and dives, the game also helps you maintain flexibility and fine-tune your concentration, balance, and reaction time.

• Always wear eye protection when playing, says the American Academy of Ophthalmology. Buy eye guards that have been certified by the American Standard of Testing and Materials. If you wear eyeglasses, wear eye guards over them. Your eyeglasses aren't made to give enough protection.
• Drink plenty of fluids before you play. Drink 16 to 32 ounces of water or other fluids 1 to 2 hours before you begin.
• If you feel pain or cramping, stop the game.
• Keep at least a foot-and-a-half of space between you and your opponent. If you think you're too close to swing your racket without hitting them, hold your swing or stop the game.
• After the game, allow time to cool down before showering, and do some stretching exercises.