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Abstract
Background: A plateau in oxygen consumption (o2) is the primary means of confirming that maximal oxygen uptake (o2max) is attained during incremental exercise to fatigue. However, it is still unresolved what causes expression of a plateau in o2 at the end of incremental exercise.
Objectives: To elucidate incidence and identify determinants of the o2 plateau in subjects of varying fitness.
Methods: Thirty subjects (mean (SD) age and o2max 26.9 (9.8) years and 3.4 (0.8) litre/min respectively) were separated into three groups: endurance trained (ET; n = 9), recreationally active (Rec; n = 11), and strength/sprint trained (STR; n = 10). During three separate visits, subjects completed incremental treadmill exercise during which breath by breath gas exchange data were obtained. Body composition was measured using a three site skinfolds model. Force production of the knee extensors and flexors was assessed using isokinetic dynamometry.
Results:o2max was significantly higher (p<0.05) in the ET group vs STR and Rec. The change in o2 (Δo2) at o2max was not different (p>0.05) in the ET group (33.0 (27.3) ml/min) compared with the Rec group (30.3 (24.1) ml/min) and the STR group (44.4 (23.8) ml/min). No correlations (p>0.05) were evident between Δo2 at o2max and o2max (r = 0.05), fat-free mass (r = 0.12), and muscular strength (r = −0.12).
Conclusions: The incidence of a plateau in o2 at o2max (from correlation data) is not due to factors related to training status or physical fitness of subjects, but is altered by analysis and interpretation of gas exchange data.
- FFM, fat-free mass
- o2max, maximal oxygen consumption
- maximal oxygen uptake
- incremental exercise
- sampling interval
- treadmill running
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In healthy people, maximal oxygen consumption (o2max), the maximal rate of oxygen consumption by active muscle during exercise to fatigue, is an index of endurance performance as well as a measure of maximal cardiopulmonary function. In the laboratory setting, o2max is commonly measured during incremental exercise to fatigue on a treadmill or cycle ergometer, during which expired air is analysed by gas analysers. Despite the fact that the o2max concept has existed for over 80 years, there are no standardised guidelines to confirm incidence of o2max. The most widely used criterion for confirming attainment of o2max is a levelling off (plateau) in oxygen consumption at o2max, yet the magnitude of this plateau used varies among scientists. Classic work1 developed the most widely accepted o2 plateau (change in o2 (Δo2) at o2max ⩽150 ml/min) from multiday, discontinuous exercise testing in 115 healthy men. Later work2 suggested a Δo2 ⩽80 ml/min, whereas other work3 in six men described a plateau in o2 as no significant change in the slope of o2 versus time. Data from our laboratory4 in 16 subjects completing maximal cycle ergometry revealed 100% incidence of a plateau in o2 when a more stringent plateau criterion (Δo2 ⩽50 ml/min at o2max) was used. Consequently, it is evident that no single Δo2 exists to confirm incidence of o2max. It is therefore recommendable to examine this o2 plateau phenomenon in a large, more diverse population of subjects after repeated o2max tests.
Previous work5 suggests that the incidence of the o2 plateau is lower in the unfit, elderly, and the young. Research in equally trained men and boys6 showed low incidence (23.8% and 12.5% respectively) of a plateau in o2 (final Δo2 less than 50% of the average Δo2 per stage) in response to incremental treadmill testing, similar to data (29% incidence) from Paterson et al7 in 10–12 year old boys. However, in elderly men and women giving a “good effort,” a plateau in o2 (Δo2 ⩽2.0 ml/kg/min) was evident in 55–78.9% of subjects.8 Similarly, data4 in young, recreationally active subjects completing maximal cycle ergometry reported the universal incidence of a plateau in o2 when stringent plateau criteria were used. Data showed that the sampling interval dramatically affects the prevalence of a plateau in o2 at o2max. For example, the incidence of a plateau in o2 was 100% when data were obtained using shorter sampling intervals (breath by breath and 15 seconds), yet the incidence was lower (57% and 8% respectively) with longer (30 and 60 seconds) sampling intervals. These results suggest that it is the sampling interval and not subject status that explains the dissimilar incidence of a plateau in o2 across various studies. Moreover, the incidence of this plateau phenomenon varies across studies because of different protocols, subjects, and plateau criteria used. This indicates that a priori measurement issues selected by the researcher may determine the presence or absence of a plateau in o2 at o2max. It is also unclear if a plateau in o2 exists in subjects unfamiliar with graded exercise testing.
Previous work5 supporting one minute sampling intervals as the optimal duration for acquiring gas exchange data suggests that subjects may have to maintain maximal to supramaximal work rates for two to three minutes at the end of a test to achieve a plateau in o2. This is physiologically impractical and unlikely, especially in non-athletes, children, and the elderly. However, it does intimate that factors including o2max, fat-free mass (FFM), and muscular strength may determine the incidence of a plateau in o2 at o2max. At the end of a o2max test, there is a greater contribution of non-oxidative pathways to ATP provision, hence greater muscle mass and force production may allow a subject to tolerate the ever increasing ATP demand, exercise longer, and perhaps reveal a higher incidence of a plateau in o2 at o2max.
The primary aim of this investigation was to examine the incidence of a plateau in o2 at o2max in subjects from a variety of exercise backgrounds, and to identify determinants of this plateau by correlating measures of body composition, cardiovascular fitness, and muscular strength with Δo2 at o2max. A secondary aim was to discern if repeated o2max testing increases the incidence of a plateau in o2 at o2max by a testing effect.
MATERIALS AND METHODS
Subjects
Thirty healthy, active men and women (mean (SD) age 26.9 (9.8) years) were recruited from the university community. During their initial visit to the Human Performance Laboratory, subjects provided written informed consent and filled out a health/history questionnaire to confirm that they were free from any conditions that prevented them from fulfilling any requirements of the study. Subjects were separated into three groups: an endurance trained group (ET; n = 9) consisting of competitive triathletes, cyclists, and runners; a recreationally active group (Rec; n = 11) active in athletics and regular cardiovascular/weight training exercise, and a strength/sprint trained group (STR; n = 10) comprising sprinters and people participating in heavy resistance training four to six days a week with minimal (⩽30 minutes a week) cardiovascular exercise.
Assessment of body composition
Body height and mass were measured with the subjects dressed in shorts and T shirt. Body composition was assessed using skinfold callipers (Lange Beta Technology, Cambridge, Maryland, USA). The primary investigator measured subcutaneous fat at three sites on the body (chest, abdomen, and thigh for men, and triceps, thigh, and anterior suprailiac for women) following standardised procedures.9 Sum of skinfolds was converted into percentage body fat (%BF) using a common nomogram.10 FFM was calculated by subtracting fat mass from body mass. Two measurements at each site were taken in rotational order, and a third measurement was recorded if the first two measures deviated by more than 10%.
o2max testing
Initially, subjects completed a familiarisation test of o2max on a treadmill (Quinton Instruments Q55, Series 90, Seattle, Washington, USA). During testing, expired breath was measured using infrared gas analysers, and heart rate was recorded continuously throughout exercise (Polar Electro Inc, Woodbury, New York, USA). During o2max testing, subjects expired through a plastic mouthpiece and low resistance valve into tubing connected to a mixing chamber. Measurements of ventilation, o2, and carbon dioxide production were obtained breath by breath throughout exercise by a metabolic cart (SensorMedics Model 29C, Yorba Linda, California, USA). Throughout the test, treadmill speed remained constant at a speed selected by the subject, and grade was increased at a rate of 0.5–1.0% each minute until fatigue. o2max was confirmed by the following criteria: (a) a plateau in o2 (Δo2 ⩽50 ml/min); (b) respiratory exchange ratio greater than 1.10.4 Test duration was 8–14 minutes.11 o2max (l/min) was defined as the workload attained when the subject requested to stop exercising, and represented the mean of o2peak and the closest neighbouring data point at volitional fatigue. During exercise, rating of perceived exertion12 was obtained, and the subject was given verbal encouragement to exercise to volitional fatigue. Subjects were given no feedback during testing.
At the same time of day during the next two visits (separated by at least 48 hours), subjects repeated the o2max test on the treadmill. Treadmill speed and grade increment were identical to the initial test. Before each visit, the subject refrained from consumption of alcohol and caffeine for 48 hours and strenuous exercise for 12 hours. Subjects were required to maintain a normal dietary and exercise regimen during participation in the study, which was confirmed with 24 hour food and exercise logs.
Assessment of muscular force
On a separate occasion, muscular force of the knee extensors and flexors was determined using an isokinetic machine (BioDex System 2, Shirley, New York, USA). This required concentric maximal flexion and extension of the knee of the dominant leg at various contraction velocities (60, 90, 120, 180, 225, and 300°/s). Before testing, subjects warmed up on a cycle ergometer for five minutes, and stretched the knee extensors and flexors. They were then instructed to exert maximal effort during all repetitions. A short rest period (45 seconds) was allotted between each repetition. Maximal force (torque), work, and relative force (N/m/kg FFM) were obtained for both extension and flexion.
Statistical analysis
Data are reported as mean (SD) and were analysed using GraphPad Prism version 3.0 (San Diego, California, USA). One way analysis of variance with repeated measures was used to examine differences in gas exchange variables between the three o2max tests. One way analysis of variance was used to locate differences in o2max, %BF, and muscular force between groups. If a significant F ratio was obtained, Tukey’s post hoc test was used to identify differences between means. Pearson’s correlation coefficient and stepwise regression were used to identify relationship between Δo2 at o2max and FFM, o2max, and muscular force. Linear regression was used to determine the slope of the o2 versus time relationship at the end of incremental exercise. With a sample size equal to 10, and mean o2max equal to 3.45 (0.73) l/min, statistical power was 0.13. Statistical significance was set at 0.05.
RESULTS
o2max testing
Mean o2max was not significantly different (p = 0.07) from trial 1 (3.5 (0.7) l/min) to trial 2 (3.6 (0.8) l/min) and trial 3 (3.5 (0.7) l/min) in our 30 subjects. o2max was significantly higher (p<0.05) in the ET group (52.1 (4.3) ml/kg/min) than the Rec (45.9 (3.8) ml/kg/min) and STR (45.7 (3.2) ml/kg/min) groups. Mean Δo2 at o2max was similar (p>0.05) in the ET (33.0 (27.3) ml/min), Rec (30.3 (24.1) ml/min), and STR (44.4 (23.8) ml/min) groups. Using the criterion of Δo2 ⩽50 ml/min at o2max, a plateau in o2 at o2max was found in a majority of subjects in the ET (66%), Rec (82%), and STR (60%) groups. Figure 1A shows regression lines of the o2 versus time relationship for three subjects in each group during the last 30 seconds of incremental treadmill exercise. In these subjects, o2max was 40–60 ml/kg/min. All slopes of o2 versus time were not significantly different from zero, indicating attainment of a plateau in o2 at o2max for all subjects. However, when the last 60 seconds of gas exchange data were analysed (fig 1B), the slopes for four subjects were significantly different from zero; therefore the o2 plateau occurred in only five of nine subjects. For example, the slopes for subjects ET2 and STR1 were significantly different from zero (p<0.05): 0.043 (0.016) and 0.037 (0.023) ml/kg/min/s respectively.
Regression lines describing the change in o2 versus time during the last 30 seconds (A) and 60 seconds (B) of incremental exercise for three subjects in the endurance trained (ET), recreationally active (Rec), and strength/sprint trained (STR) groups.
Assessment of body composition
Mean %BF was lower (p<0.05) in STR compared to ET and Rec. Percent body fat (mean±SD) was equal to 11.0 (4.0), 15.0 (5.1), and 17.7 (5.8)% in STR, ET, and Rec, respectively.
Assessment of muscular force
Relative muscular force of the knee extensors was equal to 3.4 (0.6), 2.9 (0.6), and 3.0 (0.8) N.m/kg FFM respectively in the STR, ET, and Rec groups. Knee extensor force did not differ (p = 0.19) among the groups. Similarly, the maximal force of the knee flexors was comparable (p = 0.16) among the groups: STR (1.8 (0.3) N.m/kg FFM), ET (1.6 (0.2) N.m/kg FFM), and Rec (1.6 (0.3) N.m/kg FFM).
Correlation and regression analyses
A significant positive relationship (r = 0.81, p<0.05) was exhibited between maximal force of the knee extensors and FFM. However, no significant relationship (p>0.05) was evident between Δo2 at o2max and FFM, maximal force of the knee extensors and knee flexors, and o2max (fig 2A–D).
Relationship between Δo2 and (A) fat-free mass (FFM), (B) knee extensor force, (C) knee flexor force, and (D) o2max in 30 subjects.
Three-predictor regression models were developed to ascertain significant predictors of Δo2 at o2max. Only three predictors were used to maintain a case to variable ratio of at least 10:1.13 A model including age, FFM, and knee flexor yielded the largest R2 value (0.213), with age (p = 0.018) as the only significant predictor of Δo2. Age by itself accounted for 16% of the variance in Δo2 at o2max. A model comprising maximal gas exchange and cardiovascular data (heart rate, o2max, and respiratory exchange ratio) yielded an R2 value of 0.007.
DISCUSSION
The primary aim of this study was to elucidate incidence, and identify determinants of, the plateau in o2 at o2max in subjects participating in endurance, power, and recreationally based activities. Subjects completed multiple o2max tests and a separate test of muscular strength using isokinetic dynamometry. The results show a high incidence of a plateau in o2 at o2max, illustrated by a mean Δo2 at o2max ⩽50 ml/min as well as incidence of slopes of o2 versus time not different from zero in the later stages of incremental exercise. The only significant predictor of the Δo2 at o2max was age, suggesting that body composition, training status, and/or training history do not affect incidence of the o2 plateau. It is suggested that sampling interval is the primary factor determining incidence of a plateau in o2 at o2max in most subjects exercising to volitional fatigue. Sampling interval and Δo2 at o2max must be carefully selected by researchers to increase incidence of a plateau in o2 at o2max, and the slope of o2 versus time should also be used as an objective criterion to ascertain attainment of a plateau in o2 at o2max.
Our moderate to high incidence of a plateau in o2 at o2max is dissimilar to recent data in young, healthy subjects. Research in 50 elite endurance athletes completing maximal treadmill exercise found a plateau in o2 at o2max in only 39% of men and 25% of women.14 However, o2max was represented as the highest o2 attained for 60 seconds, and their plateau criterion was rather large (2.1 ml/kg/min). In 21 physically active men,6 a plateau in o2 (final o2 ⩽50% of the average change in o2 per elevation stage during testing) occurred only 23.8% of the time. Data from Myers et al3 revealed a plateau in o2 in only two of six subjects when gas exchange data were obtained breath by breath, yet these data are characterised by dramatic variability. In contrast, work from our laboratory15 in 37 active men and women revealed 87% and 76% incidence of a plateau in o2 at o2max with shorter sampling intervals (breath by breath and 15 seconds), and lower incidence when gas exchange data were obtained every 30 seconds (57%) and 60 seconds (11%). In 20 men and women,16 a plateau in o2 was observed in 80% of subjects during maximal cycle ergometry in which biomechanical signal processing techniques were used to identify incidence of the o2 plateau. Consequently, it is plausible that different sampling intervals and magnitude of the plateau criterion used may explain these disparate findings regarding o2 plateau incidence.
It is not known if multiple, identical o2max tests completed by a single subject increase the incidence of the o2 plateau at o2max. Previous data in five men17 completing up to three maximal treadmill tests a day showed a lower plateau incidence when incremental exercise was preceded by two earlier maximal trials. In our study, there was greater variability in the mean Δo2 at o2max in trial 1 (38.8 (47.6) ml/min) than in subsequent trials (34.4 (22.6) ml/min and 35.8 (25.4) ml/min). However, all mean values were within our plateau criterion (Δo2 at o2max ⩽50 ml/min). Furthermore, a plateau in o2 at o2max was found in 80%, 80%, and 70% of subjects in trials 1 and 2 versus 3. These results indicate that completion of multiple bouts of incremental exercise on separate days does not affect the magnitude or incidence of the o2 plateau at o2max. Similarly, there was a high incidence of maximal respiratory exchange ratio (⩾1.10; 77%, 80%, and 83%) and heart rate criteria (within 10 beats/min of 220 – age; 70%, 77%, and 64%) across trials, similar to recent data in 50 elite athletes.14 Overall, our subjects of varying fitness backgrounds were able to attain o2max, and our data emphasise that a o2 plateau is a real phenomenon in most subjects exercising to volitional fatigue.
So, what experimental conditions maximise incidence of the plateau in o2 at o2max? Exercise mode is not a factor, as a plateau in o2 at o2max has been shown in response to incremental treadmill, cycle ergometer,15,16 and rowing exercise.18 Data from Duncan et al19 revealed that plateau incidence is similar with continuous (50%) and discontinuous (60%) protocols. However, researchers must seriously consider the exact sampling interval and magnitude of plateau criterion to reveal high incidence of a plateau in o2 at o2max. It appears that shorter sampling intervals (breath by breath and 15 seconds) increase the incidence of the plateau in o2 at o2max compared with the traditionally accepted 60 second interval. This is demonstrated in fig 3, gas exchange data from incremental cycle ergometry in an active male. It shows plateau incidence (Δo2 at o2max ⩽50 ml/min) when gas exchange data are obtained breath by breath or every 15 seconds, yet no plateau in o2 at o2max with sampling intervals of 30 or 60 seconds.4 Longer sampling intervals also induce greater variability in Δo2 at o2max and significantly reduce o2max.15 The widespread availability of modern breath by breath gas exchange systems enables acquisition of gas exchange data with the greatest precision and temporal resolution. Through its ability to acquire data at shorter sampling intervals, this technology may increase the incidence of a plateau in o2 at o2max irrespective of the type of exerciser, protocol, or mode of exercise. This technology also allows the researcher to select more stringent plateau criteria (Δo2 at o2max ⩽50 ml/min) rather than traditionally accepted values (Δo2 at o2max ⩽150 ml/min) obtained with Douglas bags.1
Alterations in o2 plateau incidence with changes in sampling interval in response to maximal cycle ergometry in a recreationally active man.4 (A) Breath by breath; (B) every 15 seconds; (C) every 30 seconds; (D) every 60 seconds.
What is already known on this topic
A plateau in o2 at o2max is recognised as a criterion to confirm attainment of o2max. For years, researchers have used the criterion of Δo2 ⩽150 ml/min at exercise termination to confirm o2max attainment, despite the fact that it was developed from discontinuous treadmill testing. In the 1990s, the research that developed the o2max concept, the fundamental measure of exercise physiology, was rightfully criticised, and yet the debate has ebbed.
What this study adds
This study elucidates physiological and methodological factors that may alter the incidence of the o2 plateau at o2max. The data show that a plateau in o2 at o2max is a real phenomenon, and its occurrence is primarily a methodological issue. Objective and accurate measurement techniques that will increase plateau incidence are recommended. These data may modify current practice of using the obsolete criterion to confirm o2max, and encourage the use of more objective and accurate plateau criteria. At worst, they may spur energetic debate of a very fundamental issue in exercise physiology.
CONCLUSIONS
Data demonstrated that Δo2 at o2max was similar across ET, Rec, and STR athletes. Approximately 80% of fit, young subjects demonstrated a plateau in o2 at o2max for all trials, and when the slope of o2 versus time was analysed in nine subjects, all subjects revealed a plateau in o2 at o2max with shorter sampling intervals. Multiple regression analysis identified age as the only significant predictor of Δo2 at o2max, emphasising that muscular strength, FFM, and o2max do not alter incidence of the o2 plateau. The exact magnitude of the o2 plateau criterion selected as well as frequency of data sampling are better determinants of the plateau in o2 at o2max than subject factors. It is recommended that scientists acquire breath by breath gas exchange data, analyse the slope of o2 versus time in the later stages of incremental exercise, and select a more stringent plateau criterion to maximise plateau attainment and obtain a more precise pattern of the o2 response to incremental exercise. Incidence of a plateau in o2 at o2max may be a primarily methodological, and not physiological, issue.
REFERENCES
Commentary
For much of the past 50 years, exercise scientists and clinicians have assessed both baseline cardiorespiratory performance and the changes induced by aerobic training in terms of an individual’s maximal oxygen intake (o2max). Methods of determining o2max have been evaluated extensively by a number of well respected physiologists,1–3 and an international working party sponsored by the International Biological Programme subsequently standardised the test protocol.4 Testing in this era was based on the use of multiple Douglas bags, with the subject making maximal efforts on each of several days. None of these early investigators found difficulty in bringing most subjects (whether young or old) to a plateau of oxygen consumption as defined by Taylor and associates.1 Nevertheless, it is fair to comment that subjects were pushed to extreme exhaustion, and some more recent researchers may have been reluctant to stress their subjects as severely. Wagner5 has argued that a “high pain and fatigue tolerance” is needed to develop an oxygen consumption plateau. The low prevalence of plateaus in athletes who were preparing themselves for Olympic competition6 does not negate Wagner’s view. In my experience, international competitors are often reluctant to make a supreme effort merely for the delight of a laboratory scientist. In support of my contention, some of the subjects tested by Doherty et al6 had a peak respiratory gas exchange ratio of less than 0.90—hardly a strong anaerobic effort! A further issue raised by this paper is the impact of breath by breath analysis on the demonstration of an oxygen consumption plateau. If a plateau is seen when using data from the last 15 seconds of two successive exercise stages, but disappears if data are averaged over the last 30 or 60 seconds, it may be that the current generation of subjects are reluctant to maintain peak effort as long as 60 seconds.
Footnotes
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Competing interests: none declared