Exercise intensity refers to how much work the body is doing during exercise; exercise intensity training methods represent different means by which this work can be prescribed, monitored, and quantified. Exercise intensity training methods can fall into either objective or subjective categories. Examples of objective exercise intensity training methods include heart rate and oxygen consumption. Examples of subjective exercise intensity training methods include rating of perceived exertion (RPE) and the talk test.
Exercise intensity is arguably the most critical component of the exercise prescription F.I.T.T. model. Failure to meet minimal threshold values may result in lack of a training effect, while too high of an exercise intensity could lead to overtraining and negatively impact adherence to an exercise program. Furthermore, exercise intensity that is too high could also elicit adverse cardiovascular-related events. Consequently, it is paramount for the correct range of exercise intensity to be established—one that is appropriate on an individual basis according to differences in age, sex, training status, medical condition, and goals. The purpose of exercise intensity training methods is to provide individuals and fitness/exercise professionals with an array of instruments for which safe and effective exercise intensity can be prescribed, monitored, and quantified.
There are various methods available to prescribe exercise intensity for training. These methods are:
Exercise intensity can be prescribed according to the oxygen uptake reserve (VO2R) method. This method establishes the target workload (i.e., exercise intensity) according to a percentage of the VO2R. The VO2R is determined by taking the difference between maximal oxygen uptake (VO2max) and resting oxygen uptake. The recommended exercise intensity for healthy adults in terms of VO2R is 40%–85%. However, in individuals with poor levels of fitness, an exercise intensity as low as 30% of VO2R has been shown to be sufficient for eliciting a training effect. One of the premier benefits of using the VO2R method is that energy expenditure can be determined relatively easily.
Another method for prescribing exercise intensity is the heart rate reserve method, or the Karvonen method. The heart rate reserve method establishes the target workload (i.e., exercise intensity) according to a percentage of the heart rate reserve. The heart rate reserve is determined by taking the difference between maximal heart rate and resting heart rate. The maximal heart rate is most commonly determined by subtracting the age of an individual from 220. However, this method for estimating maximal heart rate can be off by as many as 10–15 beats per minute for some individuals. Preferably, a maximal heart rate obtained from a maximal exercise test can be determined. An accurate measurement of resting heart rate is also required to ensure accuracy with the heart rate reserve method of prescribing exercise intensity. Resting heart rate should be measured as a 60-second count of heart rate recorded after resting quietly for at least five minutes in a seated position. The recommended exercise intensity for healthy adults in terms of heart rate reserve is 40%–85%. However, in individuals with poor levels of fitness, an exercise intensity as low as 30% of heart rate reserve has been shown to be sufficient for eliciting a training effect. Percentages of heart rate reserve have been shown to accurately reflect equivalent percentages of VO2R. The heart rate reserve method is more practical than the VO2R method for use in fitness and clinical settings.
A third method for prescribing exercise intensity is the maximal heart rate method. This method establishes the target workload (i.e., exercise intensity) according to a percentage of the maximal heart rate. The recommended exercise intensity for healthy adults in terms of maximal heart rate is 55%–90%. Although the percentage of maximal heart rate method for prescribing exercise intensity is relatively easy to utilize, it is important to recognize that it is less accurate than the heart rate reserve method. This is because of the variation in resting heart rate amongst the population. For instance, an individual with a maximal heart rate of 180 beats per minute could be asked to exercise at 55% maximal heart rate, which would be 99 beats per minute; however, it is possible that the individual has a resting heart rate of 100 beats per minute. This example may seem extreme, but the reality is that it simply highlights the importance of considering resting heart rate when formulating the target exercise intensity. Nonetheless, establishing exercise intensity according to the maximal heart rate method is convenient and preferable in certain situations, such as when instructing group exercise classes.
The metabolic equivalents (METs) method establishes the target workload (i.e., exercise intensity) according to a percentage of an individual's peak MET level; the peak MET level is determined by dividing the VO2max by 3.5. For example, the peak MET value for an individual with a VO2max of 35 mL/kg/min would be 10 METs (35 divided by 3.5). Various exercises and activities that correspond to the prescribed MET workload can be identified using the Compendium of Physical Activities ( http://www.sites.google.com/site/compendiumofphysicalactivities/home.html ). An advantage of using the MET method is that energy expenditure can be determined relatively easily.
Perceived exertion methods include the talk test and rating of perceived exertion (RPE) scale. The talk test is used to prescribe a moderate level of exercise intensity. Individuals are instructed to work at levels that result in sensations of increased breathing; however, the individual is working at a level that also permits them to speak full sentences. Two primary RPE scales can be used to prescribe exercise intensity; the 6 to 20 scale, with 6 equating to rest and 20 representing maximal effort, and the 0 to 10 scale, with 0 equating to rest and 10 representing maximal effort. The recommended exercise intensity for healthy adults in terms of RPE is 12–16 (on the 6 to 20 scale) and 2–6 (on the 0 to 10 scale).
Exercise intensity that is too high can lead to overtraining, negatively impact adherence to an exercise program, or evoke adverse cardiovascular-related events. Accordingly, it is important to recognize various factors that may influence oxygen consumption or heart rate beyond exercise itself, and therefore potentially compromise the accuracy of a prescribed exercise intensity range. For instance, the day-to-day variability in exercise heart rate ranges from two to four beats per minute under laboratory-controlled conditions. Other physiological and environmental conditions can influence resting and exercise heart rate to a greater extent and hence change the interpretation. After several minutes of moderate-intensity exercise, there is a gradual decrease in stroke volume accompanied by a concomitant rise in heart rate. This physiological phenomenon has been termed cardiovascular drift. Both heart rate and oxygen consumption values have been reported to rise up to 15% during submaximal exercise bouts lasting 5–60 minutes. Dehydration and heat stress can exacerbate the magnitude of cardiovascular drift. When planning for a target heart rate or oxygen consumption, cardiovascular drift should be considered. Dehydration, independent of hyperthermia, can markedly influence the heart rate response to exercise; with research showing an increase in exercise heart rate up to 7.5%. Monitoring heart rate to gauge exercise training intensity will become increasingly unreliable under conditions of worsening dehydration. It has consistently been reported in the literature that there is an increase in heart rate and oxygen consumption performed in hot environmental conditions for the same given workload. Similarly, exercise at altitude elicits higher heart rate and oxygen consumption values for comparable sea-level workloads due to the decreased oxygen-carry capacity of blood at greater elevations. Fitness and exercise professionals need to be mindful of these factors when employing the various exercise intensity training methods to ensure safe and effective exercise is being performed.
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Lance C. Dalleck, BA, MS, PhD