A high-intensity exercise for one person may be low-intensity for another; it depends on the person’s fitness and aerobic capacity. Someone in poor shape may find brisk walking difficult, whereas Hubbell would barely break a sweat on a power walk. Many different scales are used to estimate exercise intensity in an individual, including the Borg Rating of Perceived Exertion Scale and a measure called maximal oxygen consumption (VO_2max), which is the maximum amount of oxygen that can be used at the peak of intense aerobic exercise.

The harder an athlete works during an activity, the more oxygen is consumed and the faster her heart beats until she reaches her VO_2max, or her maximum heart rate. Exercise intensity is often estimated as a percent of one’s VO_2max, or as a percent of one’s maximum heart rate.

The accuracy of determining exercise intensity using heart rate depends on the accuracy of estimating maximum heart rate (MHR). One of the best methods for estimating maximum heart rate is MHR = 208 − (age × 0.7). However, this is only an estimate of MHR, and it does not work well for everyone. For example, older individuals who have remained fit throughout their adult life typically have a MHR well-above what is predicted by this equation. For an average 22-year-old, vigorous-intensity exercise—such as a running a 5K race—would equate to exercising at 77% to 95% of her maximum heart rate, which occurs at heart rates between 148 and 183 beats per minute. Sedentary 22-year-olds may not have the fitness to run a 5K race and could see a similar increase in heart rate by brisk walking the same distance. Increased fitness enables a person to engage in higher-intensity activity more comfortably. (INFOGRAPHIC 16.7)

Question 16.5