V̇O2 reserve and the minimal intensity for improving... : Medicine & Science in Sports & Exercise (original) (raw)
In their landmark paper on exercise prescription in 1957, Karvonen et al. (23) reported that a threshold exercise intensity existed for the improvement of cardiorespiratory fitness. The lowest intensity that produces a training effect can be termed a “threshold” if intensities below this level fail to produce improvement. Karvonen et al. found that an intensity of at least 70% of the difference between maximal and resting heart rate (i.e., 70% of heart rate reserve, HRR) was required to produce a training effect in young male adults (an observed decrease in heart rate at a fixed submaximal work rate, which suggested an increase in maximum oxygen consumption, V̇O2max). Subsequent research has led the exercise science community to revise the intensity threshold downward. In its 1990 position stand (1), the American College of Sports Medicine (ACSM) suggested a threshold intensity for training at 50% of HRR or V̇O2max for most adults and 40% of HRR or V̇O2max for individuals with a low initial level of aerobic fitness. Thus, it appears that the threshold training intensity may vary according to the pretraining V̇O2max, or level of habitual physical activity.
Recent research has demonstrated that %HRR does not provide equivalent exercise intensities to %V̇O2max but is instead equivalent to a percentage of the difference between maximum and resting oxygen consumption, i.e., to a percentage of oxygen consumption reserve, %V̇O2R (33,34). Thus, in its 1998 position stand (28), the ACSM revised its exercise prescription recommendations to use %V̇O2R, rather than %V̇O2max, as a means for establishing exercise intensity.
A considerable discrepancy can exist between %HRR or %V̇O2R units and %V̇O2max units. At rest, an individual is by definition at 0% of both HRR and V̇O2R but is at some finite value above 0% of V̇O2max. This latter value is inversely related to fitness, in that an individual with an aerobic capacity of 10 metabolic equivalents (METs; 1 MET = 3.5 mL·min−1·kg−1) is at 1/10 or 10% of V̇O2max at rest, whereas an individual with an aerobic capacity of only 5 METs is at 1/5 or 20% of V̇O2max at rest. Maximum exercise elicits 100% of HRR, V̇O2R, and V̇O2max for individuals of all fitness levels. Thus, at intensities between rest and maximum, the discrepancy between %HRR or %V̇O2R units and %V̇O2max units varies with both the fitness level of the individual and the specific intensity within the range between rest and maximum (33,34). For example, an individual with a 5-MET capacity would be at 52% of V̇O2max when at 40% of HRR or V̇O2R, whereas an individual with a 10 MET capacity would be at 46% of V̇O2max when at 40% of HRR or V̇O2R. Note that a discrepancy of 12 percentage units (52% vs 40%) when at 40% of HRR translates into a 30% discrepancy in the actual exercise intensity (12/40 = 30%).
Changing the basis of exercise prescriptions from %V̇O2max to %V̇O2R has the advantage of placing clients of varying fitness levels at equivalent relative intensities above rest and provides more accurate translations of intensity, expressed as V̇O2, into target heart rates using the %HRR method. However, the use of %V̇O2R raises a question. Because %V̇O2R and %V̇O2max are not equivalent units of intensity, if there is a threshold intensity for aerobic training at 40–50% of V̇O2max, what is this threshold in %V̇O2R units? This is an important question to address, because many of the initial studies, such as the one by Karvonen et al., established exercise intensity as a % of HRR, which more accurately reflects %V̇O2R rather than %V̇O2max. Other studies used varied methods to establish exercise intensity. This analysis was undertaken to translate the training intensities in previous studies to %V̇O2R units to determine threshold intensities for improving cardiorespiratory fitness among subjects with varied baseline V̇O2max values.
METHODS
The following steps were used to include research studies that evaluated the impact of low-to-moderate exercise training intensities on the V̇O2max of healthy adults. First, references in the ACSM’s 1990 and 1998 position stands (1,28) regarding the intensity threshold were obtained. Second, a MEDLINE search was performed using the search words “exercise,” “training,” and “maximum oxygen consumption.” Third, the reference lists of articles obtained in the first two steps were cross-referenced for additional studies.
Only studies that measured pre- and post-training V̇O2max and that had at least one group of subjects who exercised at an intensity that approximated 60% or less of V̇O2max were included for analysis. Criteria for the attainment of V̇O2max varied. Most studies used the widely accepted criteria of a plateau in oxygen consumption and/or a respiratory exchange ratio ≥ 1.10 (2–4,7,9,17,20,25,29–31), although some of these also included a criterion regarding age-predicted maximal heart rate (2,20,25,29,30) or the attainment of a high blood lactate concentration (30,31). The remaining studies used volitional fatigue as the criterion for maximal effort (8,11,12,14,24) or did not state any criteria (18,19). In almost all studies, the mode of exercise during the maximal test was the same as during exercise training. The exceptions were one study that trained with walking or jogging but tested on a cycle ergometer (31) and one study that trained on a cycle ergometer and did not state the mode during testing (18). Several early studies were not included in the analysis because they did not measure V̇O2max but reported improvements in fitness as reductions in heart rate at a fixed submaximal workload or by increases in physical work capacity at a given submaximal heart rate.
The following methods were used to translate the exercise intensity used in training studies to %V̇O2R units. Some studies reported the intensity as %HRR (2,3,12,14,24,30), and these were assumed to provide equivalent values in %V̇O2R units (33,34).
Other studies reported the intensity as %V̇O2max(4,7,9,17,19,20,31). In these cases, the intensity percentage was multiplied by the reported mean value of V̇O2max for the subjects to obtain the gross V̇O2 during exercise training in mL·min−1·kg−1. %V̇O2R was then determined by the formula: %V̇O2R = (gross exercise V̇O2 −3.5)/(V̇O2max −3.5), in which the value 3.5 mL·min−1·kg−1 was assumed to be the average resting V̇O2 of the subjects.
One study reported the exercise intensity for different training groups as various speeds of walking (11). Each walking speed was translated to a gross exercise V̇O2 by using the ACSM metabolic equation for walking (15), and this value was converted to a %V̇O2R as described above.
Some studies reported the exercise training intensity as a % of maximal heart rate (8,25,29), whereas one reported specific heart rates (18). In the latter case, %HRmax was calculated by dividing the specified exercise heart rates by an estimate of mean maximal heart rate. Because no age was reported for the subjects in this study, and they were identified to be “university students,” a mean age of 20 yr and a maximal heart rate of 200 beats·min−1 were assumed for the group. Once exercise intensity was expressed as %HRmax for all of these studies, the intensity was converted to %V̇O2R by using the formula: %V̇O2R = 1.667(%HRmax) −70%. This formula was derived from data obtained in a study that evaluated the heart rate/V̇O2 relationship (32). A nearly identical formula was derived from a separate data set by Howley (22). Both Howley and the current authors found that fitness level has only a minimal effect on the relationship between %HRmax and %V̇O2R, thus supporting the use of this formula for the conversion of intensities in the current analysis.
RESULTS
Table 1 presents a summary of the studies that were evaluated, with specific reference to subject demographics, initial or baseline V̇O2max, the mode, frequency, duration and overall length of the training program, the reported training intensity, the training intensity translated to %V̇O2R, and the percentage improvement in V̇O2max after training. Fifteen of the 18 studies compared the effects of training at two or three intensities. Within studies, the low-intensity groups generally performed a greater duration or frequency of exercise to accomplish the same total amount of work as did the higher-intensity groups. Between studies, the duration, frequency, and total length of the training programs varied, making comparisons between studies tenuous at best.
Summary of low-to-moderate intensity training studies, with reported intensity converted to %V̇O2R.
A careful examination of Table 1 reveals a clear difference in training response between subjects with baseline V̇O2max values above and below 40 mL·min−1·kg−1. These results have been collated in Table 2. Groups with initial V̇O2max values below 40 mL·min−1·kg−1 always exhibited significant improvements in V̇O2max after training, even with exercise intensities as low as 28–32% V̇O2R. Groups with mean initial V̇O2max values above 40 mL·min−1·kg−1 exhibited an intensity-dependent response, in that those who exercised at intensities below 46% V̇O2R consistently demonstrated a lack of improvement, whereas those who exercised at intensities above this value consistently experienced improvements in V̇O2max. To establish that a threshold training intensity exists, there must be studies that placed subjects at a low intensity and obtained no improvement in V̇O2max. Thus, a threshold was found only for groups with mean initial V̇O2max values above 40 mL·min−1·kg−1, at approximately 45% V̇O2R.
Collation of exercise groups based on initial V̇O2max and training intensity expressed as %VO2R.
Table 1 reveals a trend for greater percentage improvements in V̇O2max when training at higher versus lower exercise intensities. Five of the 15 studies that compared two or more progressive intensities found a statistically greater improvement in the higher-intensity group (and three of these five studies controlled the total amount of work between groups). Of the 10 remaining studies, 8 exhibited a nonsignificant greater improvement in the higher-intensity group (and 7 of these 8 studies controlled the total amount of work between groups). If there were no intensity effect, the 10 studies should not show a preponderance of results in the same direction.
DISCUSSION
The 1957 study by Karvonen et al. (23) is a landmark contribution to the field because it established the use of heart rate reserve for exercise prescription. The authors also observed that intensities below 70% of HRR were ineffective for the development of aerobic fitness in healthy young men. However, only six subjects participated in the experiment, and only three of these trained at an intensity less than 70% of HRR. Moreover, the maximum oxygen consumption of the subjects was not measured. Thus, although the study serves as the foundation for later work, it did not firmly establish a threshold intensity.
The current analysis suggests that a threshold intensity may exist at approximately 45% of V̇O2R for individuals who begin training with an aerobic capacity greater than 40 mL·min−1·kg−1. However, the three studies that reported no improvement in V̇O2max after training (8,9,18) had only four to nine subjects per group. If a greater number of subjects had been tested, statistically significant improvements in V̇O2max may have been attained. Also, if the training regimens had been greater in terms of frequency, duration, or total length, this might have produced greater results. These three studies utilized a total volume of approximately 100 min·wk−1 for 5, 8, or 10 wk. However, of the studies which evaluated similarly low intensities in less fit subjects, three used comparable volumes (78, 102, and 120 min·wk−1) and overall lengths (8–10 wk) and did achieve statistically significant increases in V̇O2max(2,3,14), lending support to a threshold for the higher fit subjects. Nevertheless, it is difficult from a statistical perspective to have a high degree of confidence in a negative result, i.e., the failure to obtain a significant improvement. Rather than stating that 45% of V̇O2R represents a true threshold for this population, it might be more appropriate to simply state that exercise training above 45% of V̇O2R generally results in improved aerobic capacity. Thus, this training level may be considered the minimal effective intensity for this population of moderate-to-high fit subjects.
No threshold intensity was apparent for less fit subject groups, as all studies using subjects with aerobic capacities less than 40 mL·min−1·kg−1 found statistically significant increases in V̇O2max, regardless of training intensity. However, training intensities below 30% of V̇O2R were not routinely evaluated. It would be accurate to simply state that training intensities of 30% or more of V̇O2R routinely resulted in improved aerobic capacity. This level of training may currently be considered the minimal effective intensity for this population of low fit subjects. Additional research with lower training intensities could revise this value. As illustrated in Figure 1, the minimal effective training intensities identified in this analysis vary in a direct manner with the pretraining aerobic capacity. The trends found in this analysis suggest that severely deconditioned subjects may respond to very low training intensities, whereas highly conditioned subjects may require greater training intensities to produce improvements in V̇O2max.
The minimal effective training intensities identified in this analysis for low fit subjects (mean V̇O2max of ∼29 mL·min−1·kg−1, range of mean values from earlier studies 13–39), and moderate-to-high fit subjects (mean V̇O2max of ∼45 mL·min−1·kg−1, range 41–51).
The current analysis of threshold intensities is limited by the indirect means of estimating %V̇O2R values in previous studies. Those studies that reported exercise intensity as a percentage of HRR are likely to be most accurately translated into %V̇O2R values, given the equivalence of these terms. Studies that reported exercise intensity as a percentage of V̇O2max should provide fairly accurate translations into %V̇O2R units, as the only assumption needed to make this conversion is that mean resting V̇O2 for the subjects was 3.5 mL·min−1·kg−1. The least accurate translations are likely to be for those studies that reported exercise intensity as a specific heart rate, or as a percentage of HRmax. Nevertheless, even these studies should provide reasonable estimates of the range of intensities in %V̇O2R units, if not precise values. A further limitation in this analysis is that all of the translations of intensity into %V̇O2R units used group mean values. It would be more accurate to convert data from individuals, had such data been available, and report the mean of the resulting intensity translations. Thus, present findings should be considered preliminary until they are confirmed or refuted by training studies that establish exercise intensity in %V̇O2R units.
Previous reviews have concluded that there is a threshold intensity for the improvement of V̇O2max(1,28) but not all reviewers have agreed that a threshold exists (5). Much of the early work supporting a threshold used small sample sizes and did not measure V̇O2max(10,13,23). Of all studies that have supported a threshold, only the three presented in Table 1 measured V̇O2max(8,9,18), and of these, only one expressed the exercise intensity as a percentage of V̇O2max(9). Thus, it is understandable that controversy existed over both the presence of a threshold and the value of that threshold in %V̇O2max units. The current analysis has established that a reasonable level of support exists for a threshold at 45% of V̇O2R for subjects of a moderate-to-high initial fitness level (defined here as > 40 mL·min−1·kg−1), although it would be prudent to refer to this as an effective minimal training intensity rather than as a threshold per se.
If one wishes to compare the %V̇O2R recommendations from this analysis with the earlier ACSM %V̇O2max guidelines, one must recognize that the discrepancy between %V̇O2R units and %V̇O2max units is affected by both aerobic capacity and the intensity of exercise (33,34). This discrepancy is greater for individuals with lower aerobic capacity, and it is greater at lower intensities than at higher intensities. This analysis found that the minimal effective training intensity for subject groups with V̇O2max values below 40 mL·min−1·kg−1 was approximately 30% V̇O2R. The average V̇O2max for all of these groups was 28.9 mL·min−1·kg−1. Translating the training intensity to %V̇O2max units yields: [0.30(28.9 − 3.5) + 3.5]/28.9 = 38% of V̇O2max. This value compares favorably with the ACSM’s earlier position that 40% of V̇O2max is the threshold for adults with an initially low fitness level (1). This analysis found that the minimal effective training intensity for subject groups with V̇O2max values above 40 mL·min−1·kg−1 was approximately 45% V̇O2R. The average V̇O2max for all of these groups was 44.7 mL·min−1·kg−1. Translating the training intensity to %V̇O2max units yields: [0.45(44.7 − 3.5) + 3.5]/44.7 = 49% of V̇O2max. This value compares favorably with the ACSM’s earlier position that 50% of V̇O2max is the threshold for most healthy adults (1).
This analysis found fairly strong support for the thesis that training at higher intensities results in greater percentage improvements in aerobic capacity than does training at lower intensities, even when the lower intensity exercise is performed with a sufficient duration to accomplish the same total amount of work (i.e., duration was varied inversely with intensity so that work output was held constant). A preponderance of the 15 studies that compared more than one training intensity found that the higher intensity resulted in either significantly greater gains (N = 5) or a trend for greater gains (N = 8). Taken together, these results imply that the studies that did not reach statistical significance would have done so if they had used larger sample sizes.
Wenger and Bell (35) evaluated studies that included much higher intensities than those reviewed in the current analysis and concluded that the greatest gains in V̇O2max occur when training is performed at intensities of 90–100% of V̇O2max. Due to convergence at very high intensities, 90–100% of V̇O2max is approximately equal to 90–100% of V̇O2R. However, the safety of high-intensity exercise has been challenged, due to the potential for cardiovascular complications (16) and musculoskeletal injury, and because extremely vigorous intensities are likely to discourage participation.
Relative to exercise benefits, increased cardiorespiratory fitness has traditionally been emphasized more than the potential for improved health and disease prevention. Consequently, many lay persons consider exercise as being synonymous with vigorous physical activity, like jogging or running. There are, however, numerous health benefits that can be derived from more moderate exercise intensities, including favorable changes in body composition, bone density, glucose tolerance, and coronary risk factors, as well as a reduction in cardiovascular-related mortality. Thus, it appears that many health benefits of exercise may occur at lower levels or intensities of exercise than are generally prescribed for cardiorespiratory conditioning, especially if the frequency and/or duration of training are increased appropriately.
A critical question, however, is whether higher training intensities evoke greater health benefits when the total amount of work or calories expended is controlled. Results from the Harvard Alumni study suggest an important role for higher intensities (26). Activity performed at intensities below 4 METs (“light”) was not associated with a reduction in all-cause mortality, regardless of the total number of calories expended per week, whereas activity performed from 4 to nearly 6 METs (“moderate”) was somewhat beneficial, and activity performed at ≥ 6 METs (“vigorous”) was highly correlated with reduced mortality. Other studies have suggested that vigorous intensities may not necessarily offer additional advantages in treatment of hypertension (21), and improvements in high-density lipoprotein cholesterol and reduction in fat stores (6). High-intensity exercise may elicit favorable changes in body composition, but if the primary purpose of the training program is to promote reductions in body weight and fat stores, then regimens of greater frequency and duration at moderate intensities are recommended. Although the potential added value of vigorous intensity over moderate or light intensities of exercise is still under debate, recent public health statements (27) suggest that regular, moderate-intensity physical activity, compatible with the minimal effective intensities identified in the present study, provides substantial health benefits.
CONCLUSIONS
This analysis has found that individuals who begin training with an aerobic capacity greater than 40 mL·min−1·kg−1 can expect improvements in V̇O2max by using training intensities of at least 45% of V̇O2R, provided sufficient training frequency and/or duration are employed. Individuals with baseline aerobic capacities below 40 mL·min−1·kg−1 obtain improvements in V̇O2max with training intensities as low as 30% of V̇O2R. A further conclusion is that higher intensities of training are generally more effective at improving V̇O2max; however, unconventionally vigorous exercise may not be recommended in consideration of injury, cardiovascular complications, and compliance issues.
Address for correspondence: David P. Swain, Ph.D., FACSM, Wellness Institute and Research Center, Old Dominion University, Norfolk, VA 23529-0196; E-mail: [email protected].
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Keywords:
MAXIMUM OXYGEN CONSUMPTION; OXYGEN UPTAKE; OXYGEN UPTAKE RESERVE; AEROBIC CAPACITY; EXERCISE TRAINING; EXERCISE PRESCRIPTION
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