Q. What's the best way to warm up before a half-hour of vigorous aerobics? In most of my classes, we start with about five minutes of stretching. It doesn't make sense to try to stretch "cold" muscles. What's your opinion?


Falls Church

A. It's okay to stretch before you start your aerobics class. As a matter of fact, it's okay to stretch any time of the day or night. If performed properly, stretching exercises are extremely safe to execute.

What's the purpose of stretching? To maintain or improve range of motion of the muscles surrounding a joint. I'd recommend that you stretch daily to become more flexible. And stretching can be part of your warm-up. I just wouldn't recommend it as a warm-up. Why? Stretching doesn't meet the physiological requirements that constitute a warm-up.

What are those requirements? The temperature within the body must increase. In a normal environment, perspiration is the accepted rule of thumb. In other words, once you "break a sweat" from a warm- up, you usually can assume you're ready to begin a workout.

An increase in rectal temperature is a more reliable indicator of proper warm-up. Experts say that a one- two-degree (Fahrenheit) increase is preferred.

In their Textbook of Work Physiology (McGraw-Hill), the bible of exercise physiology, Astrand and Rodahl state that for each degree of internal temperature increase, the body's metabolic rate increases by about 13 percent. How will this help you? Higher internal temperatures increase the efficiency of the exchange of oxygen from the blood to the tissues. In addition, the lowered viscosity of intracellular fluids means nerve messages travel faster, bringing improvements in reaction time and movement time. Total plmonary resistance is decreased with warm-up. All of which means that, like a car on a cold winter day, your body will run better after it's warmed up.

Of course, even though it's the most scientific approach, most people find it difficult to whip out their rectal thermometer and get a quick reading just before exercising. Breaking a sweat's the most practical guideline. Eventually, experience will determine your best warm-up protocol. Please realize, however, that studies have shown flexibility exercises will not provide the increase in deep-tissue temperatures that you're looking for.

The duration and the intensity of the exercise activity to come will dictate how much warm-up is needed. Five minutes is enough warm-up for aerobics. During this period, your goal should be to slowly bring your heart rate up to the target rate (70 to 85 percent of maximum heart rate).

For example, if you plan to run nine-minute miles, your goal during the warm-up period is to gradually bring your resting heart rate up to the rate your heart will be beating while running at that pace. Some people may require more warm-up than others.

In your case, after you've stretched for five minutes, I'm sure your aerobics instructor gradually eases the class into the aerobics phase. Each exerciser in the class has a different maximum heart rate, and their target heart rates will also be different. You may have to adjust the intensity of the warm-up period (or the actual exercise period) to reflect your particular condition. Don't expect to warm up (or exercise) at the same pace as your aerobics instructor if she has a svelte 110- pound chassis and you're hauling 145 pounds.

To sum up: Stretching exercises should be used only to increase flexibility. There's nothing wrong with stretching during your warm-up (or any convenient time of the day or night). But don't use stretchers as warm-ups. In fact you should warm up before stretching, if possible. One of the best times to stretch is after exercising. If your only choice is between stretching cold muscles and not stretching at all, then stretch them cold. But if you can warm up before stretching, then do so.