Fitness; Target Heart Rate: Is It Important, Or Overrated?
By WILLIAM STOCKTON
Published: April 4, 1988
DISPLAYED on the walls of most health clubs are posters that depict the ''target'' heart rates that someone should achieve while exercising. People who exercise regularly soon learn to figure this rate: Subtract your age from 220, and calculate 85 percent of that value for the top of the zone and 65 percent for the bottom.
But an increasing number of cardiologists believe that target heart rates have little meaning and that recreational athletes who slavishly take their pulses as they exercise probably are wasting their time. They believe that the body knows what its target heart rate is and that someone who exercises soon learns how to sense this level of activity.
''I've run in marathons and been very active, but I can admit to having taken my training heart rate three times in my life,'' said Dr. Paul D. Thompson, an assistant professor of medicine at the Miriam Hospital of Brown University whose research specialty is exercise and the heart. ''If you exercise until you are slightly breathless, then you are at your anerobic threshhold. The body soon learns what this is, and you stay below it.''
Not all cardiologists and research physiologists completely agree with Dr. Thompson's contention that monitoring one's pulse is unnecessary.
''For some people, it's a useful guide to how hard they should work when they exercise,'' said Dr. William Haskell, an associate professor of medicine at Stanford University. ''Someone who might overexert could use it to control his exercise. But for the general population, a sense of fatigue is just as useful a guide.''
A primary reason to exercise, of course, is to achieve cardiovascular fitness. It is well established that the fit person's heart beats more slowly, both at rest and during exercise. A resting heart rate for a sedentary person might be 70 beats per minute, while a highly fit person's rate might be 50.
During moderate exercise, the sedentary person's rate will shoot up to 150, while the fit person's might reach only 100. Over a lifetime, a fit person's heart beats perhaps 1 billion fewer times in a comparable lifespan. The assumption is that a heart that works less takes longer to wear out, although there are no definitive medical studies that prove this.
But the real measure of fitness is not heart rate, but the body's ability to take up oxygen through the respiratory system and deliver it to the cells, where it combines with glycogen provided by food to create energy. The more fit we are, the more blood the heart can pump and the more oxygen can be carried to the cells.
If you gasp for breath during a modest jogging session and feel inferior to joggers who are not panting, it is because their cardiovascular and respiratory systems have no difficulty sending enough oxygen to the cells. Because your body is less fit, oxygen supplies at the cellular level become a problem more quickly. When you slow down, or stop for a minute or two and walk, oxygen supply to the muscle cells has an opportunity to catch up.
With too much exertion, even in a fit person, the oxygen supply is depleted, and metabolism crosses a threshhold from aerobic to anerobic. This is the breathlessness that maximum exertion brings.
In the research laboratory, oxygen delivery can be measured precisely by collecting and analyzing exhaled breath during exercise on a treadmill or stationary bicycle. But without such expensive equipment, doctors rely on the heart rate, since increases in the heartbeat roughly parallel oxygen transport.
If you are 40 years old, your theoretical maximum heart rate is 220 minus age, or 180. The 85 percent upper-level target rate is 153. The 65 percent rate is 117. If you were to exercise so hard that the heart rate reached 180, you doubtlessly would be over the anerobic threshhold, If you exercised until it reached 153, you would be exercising near the limit of your body's ability to deliver oxygen to the cells without moving to anerobic exercise, although there is much variability in these heart rate levels from person to person.
But cardiologists such as Drs. Thompson and Haskell say this theoretical notion founders on the realities of human variability. Target heart rate is based on all people, but in the individual it can vary plus or minus about 15 or 20 beats per minute. ''The target heart rate is really the grossest approximation,'' Dr. Thompson said. ''A patient will tell me, 'I'm pushing as hard as I can, and I still cannot get up to my target heart rate.' That's because of the variability from person to person.
''People need to realize it's just another tool to assist somebody in trying to establish a suitable exercise intensity level,'' he said. ''In our experience, when people are taught to use the target heart rate in this way, they learn to sense a general level of fatigue when they exercise and don't need to take their heart rate.''
Dr. Jack H. Wilmore, chairman of the department of kinesiology and health education at the University of Texas in Austin, said he has found that patients who adjust their exercise to attain certain levels on a Swedish exercise scale can come within a beat or two of their theoretical target heart rates.
The scale runs from 6 to 20. A 6 is someone at rest; a 20 is total exhaustion from exercise. In between are graded definitions of exertion. A 7 is very, very light exertion, while an 11 is fairly light. A 17 is very hard, and a 19 is very, very hard.
''Once someone understands the scale, if you tell them to give you an exercise level between 12 and 15, they will reach a heart rate that is right in the training heart rate zone,'' Dr. Wilmore said. ''They don't have to think about it or monitor it.''