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Washington University in St. Louis

Some Exercise Guidelines May Not Be Valid For Older Adults, Studies Find

Exercise guidelines that use a measure called heart-rate reserve or rely on how people rate their exertion may not be valid for older women, a new study finds. A previous, smaller study suggests this conclusion also may apply to older men. "The take-home message is that [older individuals] should use a different heart-rate method in conjunction with how difficult they perceive the exercise to be," says Wendy M. Kohrt, Ph.D., lead author of a paper in a recent issue of the Journal of the American Geriatric Society. Kohrt is research associate professor in the Division of Geriatrics and Gerontology and research assistant professor in the Program in Physical Therapy at Washington University School of Medicine in St. Louis.

In 1990, the American College of Sports Medicine (ACSM) updated its guidelines for exercise levels that maximize heart and lung function, and these guidelines appear to work well for younger people. One of the guidelines also worked well for the 60- to-72-year-old women in Kohrt's study. That guideline suggests that people exercise at a certain percentage of their maximal heart rate -- 220 minus age. But another guideline -- the heart-rate reserve method, which also considers resting heart rate -- did not work at all for this group.

"If we had used the heart-rate reserve method to prescribe exercise intensity in these older women, we would have been recommending that they exercise at a much higher intensity than we intended," Kohrt says.

Getting the volunteers to rate their exertion had the opposite effect. "Surprisingly, we found that these older women rated their exercise to be less difficult than we would have expected," Kohrt says.

Heart Rate Measured

The study, which was supported by grants from the National Institutes of Health, involved 112 healthy women between ages 60 and 72. The women walked on treadmills at four different speeds, resting for 10 minutes between each speed. While the volunteers were walking, the researchers measured heart rate and oxygen uptake. They also took blood samples for hormone and lactate measurements. At another time, they measured resting and maximal heart rates and maximal oxygen consumption -- the latter were obtained while the women exercised as hard as they could.

The researchers expressed their measurements from each exercise level as percentages of maximal heart rate or maximal oxygen uptake. The relationship between the two conformed to the ACSM guidelines. When the women exercised at a level that made their hearts beat at 92 percent of their maximal heart rate, for example, they utilized 91 percent of the oxygen they would consume if they were exercising as hard as they could.

The researchers also expressed exercise intensity as a percentage of heart-rate reserve, which they obtained by subtracting resting heart rate from maximal heart rate. In this case, the results didn't jibe. When the women were consuming 91 percent of the maximal amount of oxygen they could consume, they were exercising only at 82 to 86 percent of their heart-rate reserve. When they were consuming 55 percent of their maximal oxygen uptake, they were exercising at only 22 to 35 percent of their heart-rate reserve. "So using the heart-rate reserve method to prescribe exercise for this group would result in the exercise being performed at an intensity considerably and unpredictably higher than the intended intensity," Kohrt says.

In 1993, Kohrt performed a less detailed study on 49 men ages 60 to 72. When the men exercised at 78 percent of their maximal oxygen uptake, their heart rate was 81 percent of maximal heart rate but only 69 percent of heart-rate reserve. "This suggests that the findings from the current study also apply to older men," Kohrt says.

Perceived Exertion

The ACSM guidelines also relate heart rate and oxygen uptake to perceived exertion. Younger people, for example, consider exercise to be very light when their heart rate is 35 percent of maximum and their oxygen consumption is less then 30 percent. A heart rate of 60 percent to 79 percent and an oxygen consumption of 50 percent to 74 percent is rated as moderate or somewhat hard.

The researchers expected older women to find exercise more tiring. To their surprise, the volunteers rated each treadmill exercise as easier than a younger person would have done. Their blood samples also contained much lower levels of stress hormones and lactate, which muscles make when oxygen is in short supply. So the metabolic data complemented the subjective data of perceived exertion. "It appears that older women and men are able to exercise without inordinate fatigue at higher relative intensities than younger people," Kohrt says.

The study suggests that older people should use maximal heart rate rather than heart-rate reserve to determine how hard to work out. But the percentage of maximal heart rate that would be best for a younger person might be too low for someone in this age range, Kohrt says.

She stresses that no one should exercise hard without their doctor's permission and that sedentary people should gradually increase their pace. And if people can't do such an intensive work out -- exercise that makes them sweat and unable to converse -- there still are benefits to be had. "In general, the more you do, the better it is for you," Kohrt says. "But anything is better than nothing. Even modest activity decreases the risk of heart disease."

For more information, refer to Kohrt WM, Spina RJ, Holloszy JO, Ehsani AA, "Prescribing Exercise Intensity for Older Women," Journal of the American Geriatric Society, 46, 129-133, February 1998.

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