To avoid problems that can accompany intense athletic activity, sports medicine specialist Dr. Gabe Mirkin lists two basic rules:

1. Get your pulse up to at least 120 beats per minute, for 30 minutes, at least three times a week for cardiovascular fitness.

"There is no need to go beyond 30 minutes . . . or to get up to 180 beats. Too much of anything is not good for you.

"But exercise must be regular. It is impossible to be fit, or maintain fitness, on one workout a week. Although you can on two, three times is much better."

2. Alternate between hard and easy workouts.

"Your body can't take hard exercise day in and day out. Even world-class atheletes use the hard-easy principle -- running a 4-minute-mile pace one day and an 8- or. 12-minute-mile pace the next.

"So many people end up in physician's offices because they don't understand that it takes 48 hours for skeletal muscles to heal. It's ideal to do something like hard bike riding Monday and hard swimming Tuesday. But do not use the same muscle group aggressively without a 48-hour recovery."

And, says Mirkin:

"Making love is not an effective cardiovascular exercise. The fast heartbeat is due to hormones in the brain, not usually from increased blood flow . . . unless you're a marathon runner who makes love vigorously."