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Harvard Commentaries
Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School

Focus on Fitness Focus on Fitness

The Aerobic Side of Fitness

July 09, 2013

By Howard LeWine M.D.

Brigham and Women's Hospital

What Defines a Fit Person?

When describing someone who is fit, we might say that he or she looks healthy, appears relatively lean, and has some muscular definition. Implied in this definition is a sense that the person has healthy cardiovascular function. We use the terms fit, in good shape, and in good physical condition interchangeably.

For competitive athletes, this definition would not come close to describing the level of conditioning they strive for. So fitness must be defined personally, based on the level of physical functioning that supports what the individual wishes to achieve. A competitive weight lifter will have a very different personal fitness goal than a marathon runner. The weight lifter, like a high jumper or a football defensive lineman, will need muscle power that provides short bursts of high energy and intensity. The long-distance runner wants just the opposite: muscles that can continue to perform after two hours as well as when the race started.

For most of us, our fitness goals are much more modest. We want the physical capabilities, including strength and endurance, to engage in recreational sports or just to make it easier to perform daily activities. A well-balanced fitness program combines aerobic exercise three to five times per week and strength training two to three times per week.

In a previous column I discussed strength training. Today's column focuses on aerobic exercise.

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"Aerobics" Use Oxygen

The strict definition of an aerobic exercise is muscular movement that uses oxygen to burn both carbohydrates and fats to produce the energy needed to contract muscle cells. During anaerobic exercise muscle cells burn only carbohydrates to produce energy without using oxygen. For everyday purposes, aerobic exercise means repetitive muscular activity that causes the heart rate to accelerate and remain higher than usual over an extended period of time.

In general, you lose weight with prolonged aerobic exercise, while you build muscle strength with anaerobic exercise. The reality is that most recreational exercises, such as vigorous walking, running, swimming and cycling, are primarily aerobic. If you occasionally increase the pace and intensity, you will start to go anaerobic in some muscle groups. The reality is that you can't stay anaerobic for very long; it hurts too much from buildup of lactic acid, a normal byproduct of anaerobic metabolism. Anaerobic muscle activity will exhaust you in 60 seconds or less, often much less.

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Monitor Heart Rate To Measure Exercise Intensity

In an exercise laboratory, technicians can assess exercise intensity by measuring physiological functions such as oxygen consumption to derive metabolic equivalents (METs). But for everyday purposes heart rate is the simplest way to track your exercise intensity.

You gauge your heart rate during exercise as a percent of your maximum heart rate. The standard to find your max heart rate is 220 beats per minute minus your age in years. So an average 50-year-old would use 170 beats per minute as an initial heart-rate maximum. To qualify as moderate-intensity exercise, heart rate should remain at 50% to 75% of maximum for the duration of the exercise routine. Heart rates above 75% of max are considered high intensity. Above 90% usually put you into an anaerobic mode. A healthy, fit individual may do bursts of anaerobic activity, quickly backing off to recover in an aerobic zone.

You can measure your heart rate by feeling your pulse frequently (best to feel the pulse in your neck) or wearing a heart-rate monitor (about $50 for a basic model). Many health centers have aerobic exercise machines with built-in heart-rate monitors.

Another commonly used indicator that you are staying aerobic is "the talk test" — simply the ability to talk while exercising. You may be breathing fast and hard, but if you can still carry on a conversation, you are still aerobic. If you are breathing so hard that you can't talk, you are more likely to be anaerobic. Failing the "talk test" lets you know that you may be going anaerobic.

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Warming Up and Steady State

Warm up for five to seven minutes for a 30- to 60-minute aerobic exercise routine. During these first few minutes your heart rate will increase quite quickly and then begin to level off. From here, the heart rate will tend to rise very gradually, assuming your work load remains constant or becomes slightly more strenuous. You have reached a steady state. The slow rise in heart rate from here is related to the normal increase in body temperature and release of "fight or flight" hormones from your adrenal glands (called catecholamines).

As you become more conditioned, you will experience a decrease in:

  • Your resting heart rate
  • How quickly your heart rate rises during the warm-up
  • Your peak heart rate, even though you are working harder and longer than ever

Maximal heart rate declines with increasing aerobic fitness.

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How Much and How Often?

Health benefits appear to be equal whether you do continuous aerobic exercise or intermittent. I prefer getting it all done at one time, because you can take better advantage of a good warm-up. If you do opt for shorter walks or runs, they need to be at least 10-minute stretches of elevated heart rate, and you need to do at least 30 minutes combined.

My own aerobic exercise prescription includes some dedicated aerobic time every day and strength training two to three times per week (no more frequently than every other day). On days you do strength training, 10 to 20 minutes of low to moderate aerobic activity (55% to 60% of max heart rate) is OK. On the other days, aim for 40 to 60 minutes of dedicated aerobics.

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Howard LeWine, M.D. is chief editor of Internet publishing, Harvard Health Publications. He is a clinical instructor of medicine at Harvard Medical School and Brigham and Women's Hospital. Dr. LeWine has been a primary care internist and teacher of internal medicine since 1978.

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