You wouldn't start your car, shift into drive and step on the gas without warming the engine up first. But many people will jump directly into a vigorous exercise program without giving the body a chance to adjust.

Exercise warm-ups and cool-downs -- basic and necessary parts of any exercise program -- are easy to forget or ignore when you are intent on packing a lot of exercise into a short time.

"The public does not understand a warm-up at all," said Dr. Allan Levy, director of the Department of Sports Medicine at Pascack Valley Hospital in Westwood, N.J., and associate team physician for the New York Giants. "They confuse stretching with warm-ups."

Warm-ups prepare the body for hard, physical activity. An effective warm-up means elevating the body's temperature from 1 1/2 to 2 degrees, said Levy.

When the body warms up, muscles, fiber and tendons become more fluid and can be stretched easily. Also, the muscles use energy more efficiently when they are warm. When you break into a sweat, you are at the point where your body is warmed up for exercise, Levy said.

This state can be achieved in a number of ways. Levy recommends walking fast, doing calisthenics or jogging slowly.

"Warm-ups depend on the exercise task," said Peter Van Handel, senior sports physiologist with the U.S. Olympic Committee in Colorado Springs. "If a person is going to bike ride, the first five or 10 minutes can be spent doing warm-up cycling."

A study at the University of California at Los Angeles looked at the effect of sudden, vigorous exercise on people who did not warm up first. The subjects were suddenly placed on a treadmill for 15 seconds. More than 70 percent of the participants had abnormal electrocardiogram responses or irregular heartbeats, said Dr. John Cantwell, director of the Preventive Medicine Center at Georgia Baptist Hospital in Atlanta.

"There are good physiological reasons to support warming up first," said Cantwell.

First, the breakdown of oxygen from the red blood cells is increased by a warm-up and more oxygen gets into the muscle. As the temperature of the muscle rises, muscle thickness decreases and mechanical efficiency improves.

So called passive warm-ups in which the body temperature is elevated through external means such as a hot shower or sauna are not practical, Cantwell said. And people should warm up within 45 minutes of the activity.

Levy recommends warming up first and then stretching to prepare the muscles. Both should be done before high-intensity exercise such as aerobics begins. When stretching, don't bounce. Slowly stretch the muscle and then maintain that position for 10 or 15 seconds, said Levy. Bouncing during a stretch can tear muscle fibers.

The Olympic Committee's Van Handel suggests establishing a consistent routine that incorporates a warm-up with stretching. "Start with one area and work your way through the whole body," he said.

For example, a person could start at the neck, rotating the head, then do shoulder shrugs, lateral bends, trunk rotations and leg exercises.

By establishing a consistent routine, you don't forget the warm-up.

"I try to enforce the idea that warm-ups and cool-downs are a part of the exercise activity itself," Van Handel said.

Equally important to a good fitness routine is the cool-down. It gives the body a chance to recover from the exercise.

Cool-downs also involve movement but at a much slower pace. "I call it a warm-down," said Pascack Valley Hospital's Levy. "Warm-down carries the connotation that there is some activity involved, as in the warm-up."

Simply walking or using the muscles you have been exercising with in a less vigorous way can help the body cool down gradually. "It depends on the muscles you are using because those are the ones you have to keep going," said Levy.

Cool-downs help re-direct blood flow back to the heart and brain after exercise.

After running, the heart rate is high because it has been pumping a lot of blood to the legs. The body has no mechanism to get the blood back up to the heart against gravity, Levy said.

When the leg is moving, the calf muscles contract and squeeze the blood upward. If you suddenly stop and stand still, you lose the effect of the muscle contraction.

"You can actually pool several pints of blood down in the legs," said Levy, "and there may not be enough to supply the heart and brain." The result can be a fainting spell or in rare instances, a heart attack.

Cool-downs also help muscles recover. During exercise, the body begins to burn fat after it exhausts its supply of carbohydrates. One waste product produced from burning fat is lactic acid, which accumulates in the muscle and causes muscle fatigue and soreness. The only way the body rids the muscle of lactic acid is by releasing it into the bloodstream.

"The longer you keep the exercise going, the more lactic acid you are going to get rid of and the less stiff you are going to feel tomorrow," Levy said.

If time is short, it is better to cut down on the amount of vigorous exercise than to eliminate the warm-up and cool-down, said Van Handel.