June 1, 2011
Maximum heart rate is the maximum attainable heart rate for a person usually reached during all-out maximal exercise. Percent of maximum heart rate is often used as a marker of intensity and has even been used to prescribe exercise intensity. However, making a prescription based on heart rate alone, while precise, is not always very accurate.
The most accurate way of prescribing exercise intensity is ventilatory threshold. However, measuring ventilator threshold is difficult, time consuming, and requires a fully equipped exercise physiology laboratory. Ventilatory threshold is measured using collected exhaled air as you exercise at higher and higher intensities. By analyzing breathing rate and exhaled air, the two ventilatory thresholds, VT1 and VT2, can be measured. The ventilatory thresholds correspond to how your body is creating energy and how the heart and lungs are working. Training below VT1 (Zone 1) can be sustained for long periods of time, but usually doesn’t lead to much improvement in cardiovascular fitness. Training between VT1 and VT2 (Zone 2) cannot be sustained for very long, but leads to greater improvements in cardiovascular fitness. Training above VT2 (Zone 3) can only be sustained for very short periods of time and is most effective in maximizing work capacity. Heart rate matched to the laboratory measured ventilatory threshold is a very accurate way of prescribing cardiovascular intensity.
However, since we don’t all have access to an exercise physiology lab, heart rate prediction formulas have been used to prescribe exercise intensities by estimating maximum heart rate. The traditional maximum heart-rate estimation is 220 minus age. For example, somebody who is 25 years old would have an estimated maximum heart rate of 195 beats per minute. Recently, a more accurate formula for women has been suggested, 206 minus 88 percent of age. A 25 year old woman would then have an estimated maximum heart rate of 198 beats per minute. While these calculations are usually easy to compute and provide an easy marker from which you can determine exercise intensity, they should be questioned due to inaccuracies. The estimation may be 10 or 20 beats per minute more or less than your actual maximum heart rate. Basing exercise intensity on an inaccurate estimation of maximum heart rate can mean straying from your training prescription and training in a different heart rate zone than intended.
A better and more accurate way to monitor intensity during exercise is by using the Talk Test. Research has shown that one’s ability to talk correlates with VT1 and VT2. The point at which talking first becomes slightly difficult was at the same intensity as VT1. The point at which talking becomes impossible or very difficult was at the same intensity as VT2, according to the ACE-sponsored research study. Using the talk test, while not as specific as a number on a heart rate monitor, is a much more accurate way of prescribing exercise intensity. This talk test is best used when cardiovascular training is highly specific and competitive, for example an endurance athlete training for a triathlon.
Another way to monitor exercise intensity is by the Rating of Perceived Exertion (RPE), or Borg scale. Using the Borg scale, with values from 0 to 20, can help you monitor your cardiovascular intensity. A rating of 7 corresponds to very, very light training intensity. A rating of 12 corresponds to moderate intensity. A rating of 17 corresponds to very hard intensity. A rating of 20 corresponds to maximum all-out intensity. Notice how, as your fitness increases, your exertion level at a previously difficult intensity starts to decrease. For example, running a 10 minute/mile pace used to be a level 16 exertion level for you. Two months after exercising regularly, you notice that the same 10 minute/mile pace is only an exertion level 12. RPE is a useful way of monitoring progress and keeping intensity appropriate. It is best used when cardiovascular training is more generalized and less specific. A person most likely to benefit from using the Borg RPE scale is following a general weight loss program.
By focusing on exercise intensity as a personal measure that is different for different individuals, rather than a number to attain, you can set better exercise intensity prescriptions. As you become better trained, the intensities at which VT1 and VT2 are reached will increase. This means that performing the same intensity that used to be difficult is becoming easier and easier, meaning an increase in cardiovascular fitness!