Interval training allows more intense exercise in less time

Q: What is interval training? What are the benefits of exercising this way?

A: Interval training simply means alternating between short bursts of intense exercise and brief periods of rest (or a less-intense activity). The payoff is improved cardiovascular fitness with shorter workouts.

Aerobic activities such as walking, biking, running and swimming make the heart and lungs work harder, which increases cardiovascular endurance. They also help burn calories and pare down unwanted fat. Aerobic activity is an important part of any well-rounded exercise program.

"High intensity" basically means exercising at an intensity that you could not sustain for more than five to 10 minutes before becoming exhausted. In interval training, you do high-intensity exercise for a minute or so, then rest and repeat.

I spoke to Howard Knuttgen, research associate in physical medicine and rehabilitation at Harvard-affiliated Spaulding Rehabilitation Hospital. He said the seesaw relationship between exercise intensity and duration is what makes interval training work.

Here are a few examples of what interval training would look like in practice:

Swimming: Swim one lap as fast as you can. Rest for about the same time as it took you to swim the lap. Repeat.

Walking: Walk as fast as you can for a minute or two. Then walk at a leisurely pace for the same period. Repeat.

Gym machines: Treadmills, elliptical trainers and stationary cycles often have a built-in interval training function to put you through your paces.

Interval training allows you to accomplish the same amount of exercise "work" in less time. That could make workouts easier to fit into a busy day.

As fitness improves, you should feel better, with greater endurance. But no matter what type of cardiovascular training you choose, you need to do it regularly to see improvement.

If you're otherwise healthy, interval training shouldn't present major risks, as long as you start gradually. If a person's heart has normal blood flow, interval training carries minimal risk, and indeed helps keep the heart healthy.

On the other hand, if you have heart disease, high blood pressure, diabetes or other risk factors for heart disease, talk to your doctor before starting interval training.

The possible risk is this: First, a brief period of intense physical work will cause the heart to work very hard. If heart disease has slowed the supply of blood to part of the heart, that part of the heart may not get the blood it needs to do the work. That, in turn, could start a dangerously irregular heart rhythm, one that could lead to cardiac arrest.

You can reduce the risk of straining a muscle or joint by starting with a gentle warm-up before your workout to loosen your joints and get your blood flowing.

Be prepared to "feel the burn" after your challenging new workouts. It's normal to feel sore for a day or two as your muscles recover. A day of rest between workouts is also a good idea.

• Dr. Anthony Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.