Q:HOW CAN INTENSITY BE DETERMINED FOR AEROBIC EXERCISE?
A:For many years, the use of the FITT principle of exercise prescription has been used (4). FITT reflects the frequency (F), intensity (I), time (T), and type (T) of exercise in an individualized exercise program (4). More recently, the concept has been expanded to FITT-VP with the addition of volume (V) and progression (P) (see Box 1 for information on the components of FITT-VP) (3).
With regard to prescribing intensity for cardiorespiratory (aerobic) exercise, determining a target intensity is the first step and then a method of monitoring intensity needs to be selected (5). Rather than using the same absolute workload for everyone, exercise prescriptions are individualized by using relative intensity in an attempt to provide a similar exercise stress for individuals of differing physiological and functional capacities (12).
Various methods of calculating relative intensity have been used, including the following: percentage of maximal oxygen uptake (%V˙O2max), percentage of maximal heart rate (%HRmax), percentage of oxygen uptake reserve (%V˙O2R), and percentage of heart rate reserve (%HRR) (see Box 2 for calculations associated with these methods) (3). Other researchers point to the value of using aerobic threshold or anaerobic threshold as a reference point for relative exercise prescription, although disadvantages are acknowledged related to threshold calculation methods and the need for multiple laboratory-based exercise trials for threshold verification (12). The 9th edition of ACSM’s Guidelines for Exercise Testing and Prescription suggests that %HRR and %V˙O2R methods “may be preferable” compared with simply taking a percentage of maximal heart rate or oxygen uptake, although acknowledging the lack of universal acceptance and potential influence of measurement methods on accuracy (3). See Box 3 for background and research related to methods of determining intensity.
Researchers have tried to identify a minimum threshold intensity, below which no improvements in cardiorespiratory fitness will result. Current fitness level seems to be a critical factor; the Table provides suggested minimum intensity levels needed for fitness improvements (5). For those with a lower initial aerobic capacity, the minimal threshold will be lower; for those with a higher aerobic fitness, the intensity will need to be higher to see improvements (e.g., training with aerobic intervals) (5). Interval training with near-maximal intensity has the potential to bring greater aerobic fitness improvements and may be worth considering as part of a training program that also includes continuous training at moderate or vigorous intensities (5). See Box 4 for a brief summary of one study examining the impact of different intensities on aerobic capacity (9). As always, safety is a priority, as pointed out in ACSM’s Resource Manual for Guidelines for Exercise Testing and Prescription, 7th edition, which states (5), “Vigorous and higher intensities should not be used in the fitness setting with moderate-risk or high-risk clients, even though they may have no known disease, unless clearance is obtained from a physician.”
Exercise professionals should, therefore, consider the client’s current health status, risk classification, fitness level, and future goals when making recommendations on exercise intensity. ACSM’s Position Stand “Quantity and Quality of Exercise for Developing and Maintaining Cardiorespiratory, Musculoskeletal, and Neuromotor Fitness in Apparently Healthy Adults: Guidance for Prescribing Exercise” highlights the following relative intensity ranges (8):
- Light intensity = 30% to 39% HRR or V˙O2R
- Moderate intensity = 40% to 59% HRR or V˙O2R
- Vigorous intensity = 60% to 89% HRR or V˙O2R
As is clear from this list, there is a range given for each general intensity level, allowing for intensity to be individualized for each person. Calculation of a target heart rate or target workload are two ways to translate intensity into practical terms (5).
Heart rate (HR) and oxygen uptake (V˙O2) have an almost linear relationship (1). Plotting the relationship between HR and V˙O2 from an exercise test can be useful (e.g., for individuals taking β-blockers, this may be particularly valuable as the HR response to exercise is attenuated), although such data are not always available (3). Various factors can influence HR as well as the relationship between HR and V˙O2, including hydration status (dehydration elevates HR, making HR less reliable as a marker of intensity), altitude (V˙O2 and HR are still linearly related, but HR will be higher at a given V˙O2; V˙O2max is lower), and temperature (hot conditions elevate HR, cold conditions alter the V˙O2 and HR relationship because V˙O2 is elevated because of shivering) (1). Thus, although various calculations are used to determine targets, recognize potential limitations and use HR and workload ranges as helpful guides rather than rigid requirements. The exercise professional should be prepared to make adjustments in exercise intensity based on individual responses to a given exercise bout as well as due to changes in fitness or health status over time. See Box 5 for an example of prescribing exercise intensity using target HR and target workload.
In addition to %HRR and V˙O2R, exercise intensity also can be based on how hard the exercise subjectively feels; two examples of this are the talk test and ratings of perceived exertion (RPE). The talk test refers to the ability to converse during exercise or, more specifically, that an individual can “just respond to conversation” (14). The ability to speak comfortably reflects being at a low intensity; the ability to speak, but with some difficulty, is suggested to reflect moderate-intensity exercise (15). When individuals are no longer able to speak comfortably, the intensity appears to be beyond lactate and ventilatory thresholds (15). When using the RPE scale, the focus should be on overall sensation of effort; RPE 12 to 13 (on the 6 to 20 RPE scale) is associated with moderate intensity, and RPE 14 to 17 is associated with vigorous intensity (5).
Determining a target exercise intensity should be made with the individual’s health status and fitness goals in mind. With the use of various calculations (e.g., %HRR, %V˙O2R), target intensity ranges can be determined. Using HR and workload targets, along with subjective effort monitoring (e.g., talk test), can help guide in the selection of exercise intensity as part of an individualized FITT-VP exercise plan.
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