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Measuring Exercise Intensity
Tuesday, August 28, 2007
There are different ways of measuring the intensity of physical activity. A commonly used method is to calculate a target heart rate during exercise. This method relies on the fact that the maximal heart rate roughly correlates with a person's age. A simple formula for calculating maximal heart rate is to subtract the person's age in years from the number 220. When engaging in moderately intense physical activity, a person's heart rate should be approximately 50 to 70% of the maximum heart rate. For example, for a 50-year-old person, the estimated maximum heart rate would be 170 beats per minute (220 – 50 years). The target heart rate for moderately intense physical activity for a 50 year old would be 85 to 119 beats per minute (50% of 170 = 85 beats per minute; 70% of 170 is 119 beats per minute). For vigorous physical activity, the target heart rate should be approximately 70 to 85% of the maximum heart rate. For a 40 year old, this would be 126 to 153 beats per minute.
To measure the heart rate during aerobic activity, briefly pause the activity and take the pulse at the wrist. Do this by placing the index and middle finger of left hand over the thumb side of the right wrist, where the radial pulse can be felt. Press lightly. Count the number of beats for 15 seconds and multiple this number by four. This will give the number of heart beats per minute. (If you check the pulse for only 10 seconds, you can multiply the number of beats obtained during this period by 6 to obtain the number of heartbeats per minute.)
Although calculating a target heart rate is a beneficial way of determining the exercise intensity, it does have some drawbacks. It requires effort on the part of the exerciser. The formula for calculating exercise intensity is useful, but it is an approximation. Many fit individuals can actually obtain a higher heart rate than predicted by the equation, for a given level of exertion. In those who are taking medications that can affect the heart rate, the target heart rate method of calculating exercise intensity can be misleading. In particular, there is a class of medications used to treat high blood pressure, known as beta-blockers, which can limit the increase in heart rate that usually occurs with exercise. These medications decrease the force of contraction of the heart, and they also slow the pulse. In individuals taking beta-blocker medications, the pulse does not increase by the same amount during exercise as in normal individuals not taking this medication. A person on this medication may not elevate his or her heart rate into the target range, even though exercise is at the appropriate level of intensity.
A useful subjective measure of exercise intensity can be achieved using a “rating of perceived exertion”. Perceived exertion is how hard you feel like your body is working. It is based on the sensation a person experiences during physical activity. It includes such things as increased heart rate, faster breathing, sweating, and muscle fatigue. It is subjective, but it is a good indicator of how hard a person is working out. In the original formulation of the scale, developed by Borg, exercise was rated on a scale of 6 to 20, with 6 being no exertion at all and 20 being maximal exertion. A high correlation exists between a person's perceived exertion rating on this scale and the actual heart rate during physical activity. The heart rate is often about 10 times the number selected on this scale. For example, a person rating the difficulty level of exercise as 12 on this scale often has a heart rate on the order of 120 beats per minute. Moderate intensity physical activity using a traditional Borg scale is in the range of 12 to 14. If you desire moderate intensity physical activity, but estimate that you are at an intensity level of 10 on this scale, you should increase your exertion (such as walking faster on a treadmill) until you feel that your intensity has increased to 12-14.
A modified Borg scale has been developed. It rates exercise intensity from 0 to 10. The higher the number, the greater the effort. In the modified rating of perceived exertion scale, a 0 corresponds to no exertion at all. Light intensity activity is rated as a 2. Moderate intensity is roughly 3 to 5. Vigorous activity is greater than 7. For most individuals, in whom a moderate level of physical activity is desired, 3-5 would be the desired range on this scale.
The rating of perceived exertion has several advantages. It is quick and easy to use. It does not require stopping exercise to check, as is necessary with heart rate monitoring. It also does not require a watch, clock, or heart rate monitor. This way of calculating exercise intensity can be employed by most people engaging in physical activity. It is the preferred method for calculating exercise intensity in those on beta-blocker medications or other medications that can slow or limit the pulse.
Using a rating of perceived exertion is helpful in determining if you are exercising at the appropriate level of effort. The effectiveness of this method shows how well our bodies can innately tell how hard we are exercising.Monitoring exercise intensity using a rating of perceived exertion: