In 1957 Karvonen described a method of prescribing exercise in which a target heart rate was determined at a given percentage of the difference between resting heart rate and maximal heart rate (4). This has since come to be known as the% heart rate reserve (%HRR) method, and many users (1) assume that it provides an equivalent exercise intensity as the same percentage of maximal oxygen consumption(%˙VO2max), although this claim was not made by Karvonen.
A recent study from this laboratory (7) demonstrated that the values for%HRR do not correspond to the equivalent values of%˙VO2max during cycling exercise. Rather, it was shown that%HRR was equivalent to the values of%˙VO2reserve (%˙VO2R), i.e., to a percentage of the difference between resting and maximal oxygen consumption. Specifically, the regression of%HRR versus%˙VO2R yielded a slope and intercept that were indistinguishable from the line of identity. This finding was expected because the comparison of%HRR with%˙VO2max begins by aligning resting HR with zero˙VO2, whereas a comparison of%HRR with%˙VO2R aligns resting HR with resting ˙VO2.
The purpose of the current study was to extend these findings to treadmill exercise, testing the hypothesis that%HRR is equivalent to%˙VO2R, not to%˙VO2max. A second hypothesis was that the discrepancy between%HRR and%˙VO2max would be inversely related to fitness level, as previously shown for cycling exercise (7)
Subjects. All subjects were between 18 and 40 yr of age and were apparently healthy as defined by the American College of Sports Medicine(1). All provided informed consent in accordance with institutional guidelines for research with human subjects. A total of 50 subjects participated in the study (26 males, 24 females). A summary of their physical characteristics is presented in Table 1. Physiological characteristics of the subjects recorded during testing are presented in Table 2.
Protocol. Each subject's height, weight, and skinfold measurements (the latter to estimate percent fat (5)) were recorded. Subjects were fitted with a face mask (Hans Rudolph, Kansas City, MO), and ECG electrodes were placed in a lead II configuration. Each subject performed an incremental exercise test in 3-min stages (Bruce protocol) on a motorized treadmill (SensorMedics 2000, Yorba Linda, CA). Ambient temperature during testing was 25.0 ± 0.3 °C. Subjects were asked to abstain from alcohol, caffeine, and other drugs for 24 h and not to eat for at least 1 h before testing.
The exercise test was preceded by 5 min of seated rest. All subjects exercised until they reached exhaustion and under their own volition decided to stop. This was followed by 2-3 min of active cooldown.
Data collection and analysis. Heart rate was measured continuously on an automated ECG system (SensorMedics Max-1). Expired gases were collected continuously and analyzed for the determination of ventilation(˙VE), oxygen consumption (˙VO2), and carbon dioxide production (˙VO2) using a metabolic cart (SensorMedics 2900c), whose O2 and CO2 analyzers were calibrated before each test against known gas concentrations, and whose flowmeter was calibrated at least once per day against a 3.0-L syringe. Maximal oxygen consumption was defined as the highest ˙VO2 obtained over any continuous 30-s time period, provided respiratory exchange ratio (RER) was ≥ 1.10. (Two individuals who obtained maximal RER values of 1.08 were included in the study because it was felt that their high level of endurance conditioning (both attained˙VO2max values greater than 70 mL·min-1·kg-1) prevented them from reaching 1.10 despite obvious duress.) Maximal heart rate was defined as the highest value recorded over any continuous 30-s period during exercise.
The average values of heart rate and ˙VO2 recorded over the last 2 min of seated rest were considered to be the resting values. The heart rates and ˙VO2s obtained during the last 30 s of each stage were recorded and expressed as percentages of their respective ranges, i.e.,%HRR and%˙VO2R. Furthermore, the end-of-stage ˙VO2s were expressed as percentages of their maxima, i.e.,%˙VO2max.
Statistics. For each subject individually, two linear regressions were performed: one on the values of%HRR versus%˙VO2max, and one on the values of%HRR versus%˙VO2R. In each case, data obtained at rest, at each completed stage of exercise, and at maximum were entered into the regressions. The mean (± SE) values for intercepts, slopes, and Pearson r correlations were determined for the two sets of regressions. Student t-tests were used to determine whether the mean intercepts and slopes differed from 0 and 1, respectively (i.e., whether they differed from the line of identity). Furthermore, dependent t-tests were used to determine whether the slopes and intercepts differed between the two regressions.
To test the relationship between fitness level and the discrepancy between%HRR and%˙VO2max, a regression was performed on each subject's intercept (from their%HRR versus% ˙VO2max regressions) versus their ˙VO2max expressed in mL·min-1·kg-1.
The regressions of%HRR versus%˙VO2max yielded a mean intercept of -6.1 ± 0.7, mean slope of 1.10 ± 0.01, and mean r of 0.990± 0.002 (Fig. 1). The mean intercept was significantly different from 0 (P < 0.001), and the mean slope was significantly different from 1 (P < 0.001).
The regressions of%HRR versus%˙VO2R yielded a mean intercept of 1.5 ± 0.6, mean slope of 1.03 ± 0.01, and mean r of 0.990± 0.002 (Fig. 2). The mean intercept was significantly different from 0 (P < 0.02) and the mean slope was significantly different from 1 (P < 0.001).
The regression of%HRR versus%˙VO2R was significantly closer to the line of identity than was the regression of%HRR versus%˙VO2max, in that the intercepts and slopes of these two regressions were significantly different from each other (P < 0.001).
There was a significant inverse relationship between fitness level(˙VO2max) and the intercept of the%HRR versus%˙VO2max regressions (r = 0.32, P < 0.05).
The current study has found that although%HRR is highly correlated with%˙VO2max during treadmill exercise, the regression of these two variables differed significantly from the line of identity.%HRR was more closely matched to%˙VO2R, in that this regression was significantly closer to the line of identity than was%HRR versus%˙VO2max.
These findings are very similar to those recently reported by Swain and Leutholtz for cycling exercise (7), in which there was a significant discrepancy between%HRR and%˙VO2max, whereas there was no difference between%HRR and%˙VO2R. Previous studies using treadmill exercise also support these findings. Belman and Gaesser demonstrated a difference between%HRR and%˙VO2max(2), and Davis and Convertino reported an equivalency between%HRR and%VO2R(3).
The findings in the current study differ in one respect from the previous study on cycling from this same laboratory. In the previous study(7), the%HRR versus%˙VO2R regression was indistinguishable from the line of identity, whereas in the current study it was slightly different from the line of identity. This may be an effect of the mode of exercise. However, the work of Davis and Convertino(3) argues against such a mode effect. In 1975, they studied nine male subjects during treadmill exercise and found no difference between%HRR and%˙VO2R at four workloads ranging from approximately 30% to 80% of ˙VO2R (referred to as net ˙VO2 in that study).
The slight difference between%HRR and%˙VO2R in the current study may be a result of a temperature effect. In the cycling study, ambient temperature averaged 21.8 °C (7), whereas it averaged 25.0 °C in the current study. This small, but statistically significant, difference in temperatures might have resulted in a somewhat elevated heart rate response during exercise, without altering ˙VO2(6). Higher heart rates at any given ˙VO2 should result in a difference between%HRR and%˙VO2R values. Furthermore, since%HRR is typically lower than%˙VO2max at any given intensity, elevated heart rates resulting from a temperature effect would minimize this difference. In the current study, the mean intercept for%HRR versus%˙VO2max was -6.1, whereas it was -11.6 in the cycling study. However, since core temperatures were not measured in either study, this difference is not conclusively temperature related.
As in the cycling study (7), a fitness effect was observed on the discrepancy between%HRR and%˙VO2max, in that the less fit subjects had a greater intercept than the high fit subjects. This means that exercise prescriptions based on an assumed equivalency between%HRR and%˙VO2max produce greater errors for low fit clients.
There seem to be at least three advantages to prescribing exercise based on%˙VO2R rather than%˙VO2max, in regards to: target heart rate, relative exercise intensity, and net caloric expenditure. 1)Target Heart Rate: If the exercise prescription is to be translated into a heart rate value, the close relationship between%HRR and%˙VO2R provides a more accurate intensity than does the relationship between%HRR and%˙VO2max. 2) Relative Exercise Intensity: The use of%˙VO2max does not provide an equivalent relative exercise intensity for individuals of different fitness levels, while%˙VO2R does. To illustrate, consider two patients, one with a 4 MET capacity and another with a 10 MET capacity. At rest, the 4 MET patient is at 25% of˙VO2max (1 MET/4 METs), while the 10 MET patient is at 10% of˙VO2max (1 MET/10 METs). If the exercise prescription is set at 40% of ˙VO2max, the 4 MET patient only increases by 15%, whereas the 10 MET patient must increase by 30%, in%˙VO2max terms. However, if the prescription is set at 40% of VO2R, both patients make similar adjustments in relative effort. Of course, this is achieved by the current practice of prescribing exercise by%HRR. The problem with the current practice is in assuming that this reflects a percentage of ˙VO2max. 3) Net Caloric Expenditure: Only the net caloric expenditure associated with exercise should be considered for weight loss purposes, as the resting caloric expenditure would have occurred regardless of the client's activity. Exercise prescriptions based on%˙VO2R (a net term) rather than%˙VO2max (a gross term) can be more directly translated into net caloric expenditure and are thus more appropriate for this purpose.
%HRR is widely used in exercise prescriptions, generally under the assumption that it provides an intensity equivalent to the same%˙VO2max value. The current study using treadmill exercise, and a previous study using cycling exercise (7), have found that these terms are not equivalent. The current study found a much closer relationship between%HRR and%˙VO2R, which is supported by two previous studies which found these terms yielded equivalent exercise intensities (3,7). It is recommended that%HRR continue to be used in exercise prescriptions, but its use should be based on target values of%˙VO2R rather than%˙VO2max.
1. American College of Sports Medicine. ACSM's Guidelines for Exercise Testing and Prescription,
5th Ed. Baltimore: Williams and Wilkins, 1995, pp. 12-19, 158-168.
2. Belman, M. J. and G. A. Gaesser. Exercise training below and above the lactate threshold in the elderly. Med. Sci. Sports Exerc.
3. Davis, J. A. and V. A. Convertino. A comparison of heart rate methods for predicting endurance training intensity. Med. Sci. Sports
4. Karvonen, M. J., E. Kentala, and O. Mustala. The effects of training on heart rate: a longitudinal study. Ann. Med. Exp. Biol. Fenn.
5. Pollock, M. L., D. H. Schmidt, and A. S. Jackson. Measurement of cardiorespiratory fitness and body composition in the clinical setting. Comp. Ther.
6. Rowell, L. B. Human cardiovascular adjustments to exercise and thermal stress. Physiol. Rev.
7. Swain, D. P. and B. C. Leutholtz.% Heart rate reserve is equivalent to% ˙VO2
reserve, not to%˙VO2max
. Med. Sci. Sports Exerc.