At the onset of constant-load exercise, oxygen uptake (V̇O2) increases rapidly, followed by a slower rise until steady-state V̇O2 is achieved. However, during exercise where an increase in blood lactate concentration (blood [La−]) is evident, a slow, continuous increase in oxygen uptake occurs instead of steady-state V̇O2 kinetics (17). This delayed increase in V̇O2 is commonly known as the “slow component” of V̇O2 (7) and can result in a profound underestimation of exercise V̇O2. In sedentary populations, lactate threshold (LT) occurs at less than 60% of V̇O2max (16). Therefore, it is likely that a slow component of V̇O2 commonly occurs at frequently prescribed exercise intensities in this population. However, methods to reduce the slow component and elicit steady-state V̇O2 during high-intensity exercise have not been established.
Heart rate (HR) increases in direct proportion to V̇O2 during aerobic exercise (10). Thus, maintaining a constant HR could result in a constant V̇O2. Because the magnitude of the slow component is closely associated with the magnitude of blood [La−] accumulation during exercise (4,7,18), rating of perceived exertion (RPE) is also a promising tool for reducing or eliminating the slow component, as there is a strong association between RPE and blood [La−] that is evident regardless of exercise mode (8) or training status (13). Furthermore, Stoudemire et al. (15) observed that adjusting intensity to elicit a target RPE resulted in constant blood [La−] and V̇O2.
Currently, the American College of Sports Medicine (ACSM) recommends basing aerobic exercise intensity on a power output or velocity, HR, and/or RPE associated with a target V̇O2 (6). However, the comparative utility of manipulating exercise intensity based on HR and RPE to decrease the slow component of V̇O2 has not been evaluated. Therefore, the purpose of this study was to compare the use of HR and RPE in reducing the slow component of V̇O2 during high-intensity exercise.
Approach to the Problem and Experimental Design
This study utilized a repeated measures design with method of intensity adjustment (power output, HR, RPE) as the independent variables and V̇O2, HR, RPE, and blood [La−] as the dependent variables. This allowed for evaluation of the efficacy of HR and RPE in decreasing V̇O2 during exercise.
Nine nonsmoking, apparently healthy (5) male volunteers (age = 23.9 ± 4.9 yr; height = 177.4 ± 10.1 cm; weight = 75.3 ± 13.0 kg) participated in this study. Subjects were sedentary (<2 d·wk−1 of endurance exercise), not taking any medications, and free from known cardiovascular and/or metabolic disease. Written informed consent was obtained from each subject before testing. Testing procedures were approved by the University Committee on Research in Human Subjects at Michigan State University.
Maximal Exercise Test
Subjects completed a maximal, incremental graded exercise test on an electronically braked cycle ergometer (SensorMedics, Inc., Yorba Linda, CA). All subjects began exercise at a power output of 50 W for 3 min. Every 3 min, the power output increased by 50 W until the subject reached volitional exhaustion or could not maintain a pedaling cadence of at least 60 rpm. The highest min average of oxygen uptake (V̇O2) achieved throughout the test was defined as the subject’s maximal oxygen uptake (V̇O2max). Criteria for V̇O2max included the achievement of two of the following: an HR of at least 85% of age predicted maximal HR, an RER value greater than 1.10, and postexercise blood [La−] greater than 8 mmol·L−1. During the test, HR was measured every 3 min using a Polar (Gays Mills, WI) HR monitor. A SensorMedics 2900 metabolic measurement cart was used to continuously measure V̇O2. The BORG 6–20 RPE scale was used to measure RPE every 3 min (2). Standardized RPE instructions were read and thoroughly explained to each subject before the test. Blood samples were taken before exercise and every 3 min throughout the exercise test via finger stick for the determination of blood [La−], which was analyzed using a YSI Stat 2300 automated analyzer (Yellow Springs, OH).
Submaximal Exercise Tests
Each subject performed three submaximal cycle ergometer exercise tests on separate occasions. Exercise intensity for each test was determined from the V̇O2max test and based on: 1) the power output corresponding to 75% V̇O2max (PO75 test), 2) the HR corresponding to 75% V̇O2max (HR75 test), and 3) the RPE corresponding to 75% V̇O2max (RPE75 test). Seventy-five percent of V̇O2max was chosen as the intensity because it is typically above LT in untrained individuals (16) but is within ACSM guidelines (6). Subjects completed all three tests within a 15-d period, and each test was separated by at least 24 h. Subjects performed no other exercise on the day of testing. The order of three submaximal exercise tests was predetermined and nonrandomized so that the same number of subjects performed a given test (PO75, HR75, or RPE75) as their initial submaximal test. Therefore, three subjects performed the PO75 test first, three subjects performed the HR75 test first, and three subjects performed the RPE75 test first. Subjects were blinded as to which test they were completing, and were blinded to workload adjustments during the test. Pedal frequency was kept at a constant RPM among all the tests for each subject.
Each subject exercised at a power output corresponding with 75% of V̇O2max for 15 min or until the subject could no longer continue.
Each subject cycled at a power output corresponding with 75% of V̇O2max for the first 3 min and 30 s of the test. Power output was then adjusted to maintain a HR corresponding to 75% V̇O2max throughout the 15-min test. Target HR was determined using the linear relationship between HR and V̇O2 from the V̇O2max test.
Each subject cycled at a power output corresponding with 75% V̇O2max for the first 3 min and 30 s of the test. Power output was then adjusted in order to maintain an RPE corresponding to 75% V̇O2max throughout the remainder of the test. Target RPE was determined using the linear relationship between RPE and V̇O2 from the V̇O2max test.
For all submaximal tests, V̇O2, HR, RPE, and blood [La−] were obtained using the same procedures as the V̇O2max test. HR and RPE were measured every 30 s for the HR75 and RPE75 tests and every minute for the PO75 test. Blood [La−] was determined preexercise and immediately postexercise for all submaximal exercise tests.
For each subject’s HR75 and RPE75 protocols, power output was adjusted if necessary to maintain a constant HR or RPE corresponding with 75% of V̇O2max throughout the test. The amount of power output adjustment was based on the average increment in HR or RPE per stage of the V̇O2max test.
Differences in V̇O2 at 3 min and end-exercise were analyzed using a two-factor repeated-measures ANOVA. The slow component has been defined as the difference between end-exercise V̇O2 and 3-min V̇O2 (18). A one-factor repeated-measures ANOVA was used to compare end-exercise power output, HR, RPE, and blood [La−] for the three submaximal tests. For all significant main effects and interaction effects, power was between 0.77 and 1.00. Post hoc means comparisons were performed using a Fisher’s LSD test. A priori significance was established at P < 0.05.
Subjects had an average V̇O2max of 2.85 ± 0.46 L·min−1, a maximal HR of 187 ± 11 beats·min−1, and a maximal RPE of 18.6 ± 0.9. Average V̇O2 at minute 3 and end-exercise for all three submaximal tests is displayed in Figure 1. End-exercise V̇O2 was significantly (P < 0.05) higher than the respective 3-min V̇O2 for the PO75 (3-min V̇O2 = 2.22 ± 0.29 L·min−1; end-exercise V̇O2 = 2.62 ± 0.28 L·min−1) and RPE75 (3-min V̇O2 = 2.24 ± 0.26 L·min−1; end-exercise V̇O2 = 2.42 ± 0.46 L·min−1) tests but not the HR75 test (3-min V̇O2 = 2.26 ± 0.31 L·min−1; end-exercise V̇O2 = 2.35 ± 0.39 L·min−1). Furthermore, end-exercise V̇O2 was significantly (P < 0.05) greater for the PO75 test than both the RPE75 and HR75 tests. No significant differences were observed between end-exercise V̇O2 for the RPE75 and HR75 tests. Additionally, no significant differences in V̇O2 were observed between the three tests at minute 3.
Average HR and RPE for each minute along with average target RPE are displayed in Figures 2 and 3, respectively. For the HR75 test, the largest difference between average target HR and average observed HR was 3.8 beats·min−1 (minute 7). For the RPE75 test, the largest difference between average target RPE and average RPE was 0.3 RPE (minutes 9 and 14).
Average end-exercise PO, HR, RPE, and blood [La−] are displayed in Table 1. End-exercise power output, HR, and RPE were significantly (P < 0.05) higher for the PO75 test than either the HR75 or RPE75 tests. There were no significant differences between the HR75 and RPE75 tests for any of these variables. End-exercise blood [La−] values did not differ significantly among the three tests.
The major finding of the present study was that adjusting power output to maintain a target HR or RPE decreased the slow component of V̇O2 by the same magnitude. Prior research suggests that the slow component should be taken into consideration when prescribing exercise based on exercise intensity (9). The results of this study support those findings, as the V̇O2 slow component resulted in an increase in V̇O2 greater than 300 mL·min−1 from the third minute of exercise to the end of exercise. The relationship between HR and V̇O2 is well established (10). However, before the present study, it was unknown if maintaining a constant HR would eliminate or reduce the slow component of V̇O2. Our results suggest prescribing exercise based on an individual’s HR response corresponding with 75% V̇O2max is effective in reducing the slow component of V̇O2.
Furthermore, these results suggest that maintaining a constant RPE is also effective at reducing the slow component. It was hypothesized that maintaining a constant RPE would result in a constant blood [La−] and thus decrease the slow component. Previous studies have shown a strong association between blood [La−] and RPE that occurs regardless of exercise mode (8) and training status (13). In addition, Stoudemire et al. (15) reported constant [La−] and V̇O2 values when treadmill speed was adjusted to keep subjects at a constant RPE. However, the RPE75 test in the present study decreased the slow component but did not cause a reduction in end-exercise blood [La−].
A possible explanation for this observation is that blood lactate concentrations in the present study were much higher than in the Stoudemire et al. study (15). Gaesser and Poole (7) have described V̇O2 kinetics during exercise intensities within three distinct “domains.” The heavy exercise domain is characterized by work rates exceeding lactate threshold but occurring below maximal lactate steady state. A slow component is evident in the heavy exercise domain but resembles a delayed steady state (7). However, as postexercise blood [La−] was well over 7.0 mmol·L−1, subjects in the present study were likely in what Gaesser and Poole (7) characterize as the “severe-intensity” domain, which is below V̇O2max but above maximal lactate steady state. If the initial intensity was indeed above maximal lactate steady state, it is possible that the rate of increase in blood [La−] was too large to be compensated for by decreasing power output. Therefore, it is possible that the RPE/blood [La−] relationship is altered in the severe exercise intensity domain.
Previous research suggests the V̇O2 slow component-blood [La−] relationship is not causal (11,18), and recent evidence (1,3,12,14) suggests increased recruitment of fast-twitch motor units may be the primary cause of the slow component. Saunders et al. (12) reported that increased fast-twitch motor unit recruitment was responsible for the close relationship between the slow component and blood lactate at intensities above LT. The present study supports the lack of a causal relationship between the slow component of V̇O2 and blood [La−] as the slow component was reduced in the HR75 and RPE75 tests without accompanying decreases in blood [La−].
The ACSM recommends basing exercise intensity on a power output, HR, and/or an RPE corresponding with a target V̇O2 (6). However, data from the present study suggest that in untrained healthy males exercising at 75% V̇O2max, use of power output corresponding with a target V̇O2 may result in a significant slow component, which may compromise exercise tolerance and cause an underestimation of the metabolic cost of the exercise session. It appears that use of both HR and RPE are more effective than keeping a fixed power output for eliciting a target V̇O2.
A limitation of this study was that the duration of the submaximal protocols was less than the duration traditionally used in an exercise prescription based on ACSM guidelines. However, the PO75 test was quite difficult for the subjects to complete and a duration of 20 min would potentially have been prohibitive to successful completion of the PO75 test. This lends support to the contention that the slow component may compromise exercise tolerance, even when exercise is performed according to ACSM guidelines. Furthermore, exercising at a longer duration would likely have elicited an even larger slow component in the PO75 test and an even greater reduction in V̇O2 for the HR75 and the RPE75 tests. Additionally, the results of the present study cannot be generalized across a variety of intensities, as they only apply to exercise at 75% V̇O2max in sedentary males.
Because of the slow component, the metabolic cost of an exercise bout is often greater than prescribed. Although the RPE75 test did not entirely eliminate the slow component, this study’s results suggest prescribing exercise based on HR or RPE rather than power output improves the accuracy of the exercise prescription if the individual is exercising above lactate threshold. This may lead to improved exercise program adherence, as a more accurate prescription will reduce fatigue, as well as lead to a more accurate estimation of the caloric cost of exercise. HR and RPE appear to be effective tools of exercise prescription for lowering the slow component of V̇O2 and establishing a more appropriate level of intensity in untrained males exercising in the severe intensity exercise domain.
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Keywords:©2003The American College of Sports Medicine
SLOW COMPONENT; CYCLE ERGOMETRY; TESTING; SUBMAXIMAL EXERCISE