Oxygen Cost of Ventilation During Incremental Exercise
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Oxygen Cost of Ventilation During Incremental Exercise 1 Journal of Exercise Physiologyonline (JEPonline)
Volume 8 Number 5 October 2005
Managing Editor Systems Physiology - Cardiopulmonary Robert Robergs, Ph.D. Editor-in-Chief OXYGEN COST OF VENTILATION AND ITS EFFECT ON Robert Robergs, Ph.D. Review Board THE VO2 PLATEAU Todd Astorino, Ph.D. Julien Baker, Ph.D. DEREK W MARKS1, ROBERT A ROBERGS2, JEFF NELSON2, Tommy Boone, Ph.D. CHANTAL VELLA2, JENNA BELL-WILSON2, MARC APKARIAN2. Lance Dalleck, Ph.D. Dan Drury, DPE. 1 Hermann Engals, Ph.D. California State University, Stanislaus, Department of Physical Eric Goulet, M.Sc. Education and Health Robert Gotshall, Ph.D. 2The University of New Mexico, Exercise Physiology Laboratories Len Kravitz, Ph.D. James Laskin, Ph.D. ABSTRACT Jon Linderman, Ph.D. Derek Marks, Ph.D. Cristine Mermier, Ph.D. Marks DE, Robergs RA, Nelson J, Vella C, Bell-Wilson J, Apkarian Daryl Parker, Ph.D. M. Oxygen Cost Of Ventilation And Its Effect On The VO2 Plateau. Robert Robergs, Ph.D. JEPonline 2005;8(5):1-13. Evidence of significant oxygen requirements Brent Ruby, Ph.D. for ventilation during moderate to intense exercise prompted the Jason Siegler, Ph.D. Greg Tardie, Ph.D. investigation into whether the oxygen cost of ventilation effects the Ben Zhou, Ph.D. presence of the VO2 plateau. The purpose was to compare whole-body VO2 (wb-VO2) to locomotor-muscle VO2 [VO2LOC; calculated by subtracting the VO2 of the muscles used to ventilate during exercise Official Research Journal of (VO ) from wb-VO ] during maximal exercise for the presence of the The American Society of 2RM 2 Exercise Physiologists VO2-plateau. Twenty-two subjects performed a VO2max test on a cycle (ASEP) ergometer to determine the range of VE for each subject. On a separate occasion, the VO2RM was measured at nine different VE using isocapnic ISSN 1097-9751 hyperpnea trails. Maximal VO2RM equaled 18.14.4% of wb-VO2max. VO2RM increased exponentially with increasing VE. The increase in wb- VO2 during maximal exercise was greater than that for wb-VO2 when compared to VO2LOC (p < 0.05), and therefore the incidence of a VO2 plateau was greater for VO2LOC (15 of 22 subjects) when compared to wb-VO2 (1 of 22 subjects, p < 0.05). The VO2RM significantly reduces the wb-VO2 slope during maximal exercise, and can be accurately predicted [0.007614 x VE based on the regression equation {VO2RM (mL/min) = 346.9 (L/min)] }. The data indicate that the use of wb-VO2 to model and understand body and muscle energetics during exercise can be questioned. Correcting wb-VO2 for the VO2RM provides a more valid representation of the oxygen uptake kinetics of the exercising muscles.
Key Words: Hyperventilation, Maximal Exercise, Maximal Oxygen Uptake, Work of Breathing, Incremental Exercise, Plateau
INTRODUCTION Oxygen Cost of Ventilation During Incremental Exercise 2
During a maximal incremental exercise test the presence of a VO2 plateau has been interpreted as a limitation in the amount of oxygen delivered to the working muscles, or as an inability of the working muscles to consume and utilize oxygen at an increasing rate despite increasing oxygen demand (7, 14,19). The VO2 plateau does not appear in all subjects during maximal exercise tests and remains a controversial variable in the argument over the limiting factors to maximal oxygen consumption (VO2max) (2-5). The ability to detect this plateau may be hindered when using whole-body VO2 (wb- VO2) models, such as the Fick Equation, or expired gas analysis indirect calorimetry, because using these methods allows for the inclusion of the oxygen uptake of active tissues other than the contracting skeletal muscles supporting the mode of exercise, such as the respiratory muscles, to influence the slope of the VO2 - intensity relationship.
Knight et al. (6) and Poole et al. (7) each conducted research in attempt to determine if wb-VO2 measured via indirect calorimetry would accurately reflect the increase in leg VO2 during incremental cycling exercise. In both studies wb-VO2 was compared to leg VO2 over a range of power ranging in intensity between 20 – 115% of VO2max. It was found that the slope of VO2 as a function of power measured from expired gases is not significantly different than that measured in the exercising legs during submaximal and maximal exercise intensities. The authors concluded that their findings “… lend credence to extrapolating body VO2 to that of the exercising legs…(6).”
Despite the findings and interpretations of Knight et al. (6) and Poole et al. (7), others suggest that the VO2 and cardiac output utilized by the respiratory muscles during high-intensity exercise is large enough to account for a significant portion of whole-body VO2 (8-12). As exercise intensity increases so does the need to ventilate the lungs and thus the work of breathing increases. Because the distribution of cardiac output is generally proportional to the metabolic activity of the tissue, as the work of breathing increases so does the amount of blood flow and VO2 devoted to the respiratory muscles (12).
In a series of studies conducted by Harms et al. (9-12), it was found that during high intensity exercise the respiratory muscle loading reduced leg blood flow and leg VO2. Decreasing the work of the respiratory muscles had the opposite effect. Additionally, the authors concluded that 14-16% of total cardiac output is directed to the respiratory muscles to support their metabolic requirements during intense exercise. Their findings suggest that the respiratory muscles compete with limb muscles for total cardiac output during exercise. They also imply that increases in wb-VO2 during high intensity exercise are due to increases in VO2 by both respiratory and locomotor muscles.
For nearly 50 years, attempts have been made to measure the oxygen cost of ventilation (1,13,15- 17). Research to date reveals that the oxygen cost of ventilation increases out of proportion to increases in VE, thus resulting in an exponential relationship between these variables as VE increases (1,13,15-19). In most of this early research data was acquired at rest without any association to exercise. Despite these differences, the results from the early studies are remarkably consistent with later studies. In 1993, Coast et al. (20) were able to accurately determine the oxygen cost of ventilation during exercise conditions from altered ventilation at rest. They concluded that when the duration of inspiration and expiration, and respiratory rate (RR) during resting hyperpnea were matched to those experienced during exercise there was no difference in the measured work of breathing. In their study, subjects performed resting hyperpnea trials in the exercise position, using a metronome to control the RR and the duration of inspiration and expiration.
Because the respiratory muscles demand a consequential portion of cardiac output and O2 during high-intensity exercise, wb-VO2 models may not accurately represent the VO2 of the locomotor Oxygen Cost of Ventilation During Incremental Exercise 3 muscles. This is especially true during high intensity exercise when trying to detect the presence of a VO2 plateau in these muscles. A novel approach to address this issue would be to separate the wb- VO2 into two components; one that represents the oxygen uptake required for ventilation (VORM), and one that represents the oxygen uptake of the locomotor muscles (VO2LOC). This could be done by subtracting the VO2RM from the wb-VO2, which would result in a VO2 that would more accurately represent the metabolic changes occurring in the locomotor muscles (VO2LOC). The VO2LOC could then be examined for the presence of the VO2 plateau phenomenon. Refer to Table 1 for definitions of the unconventional abbreviations used in this study.
Although there is rationale in applying the VO2RM to the invalidity of a diverse number of procedures that estimate muscle energetics from whole body VO2 measures, such as the slow component of VO2 during non-steady state exercise and the accumulated oxygen deficit, we wanted to evaluate the influence of the VO2RM on the incidence of the VO2 plateau at VO2max. Consequently, the purpose of this study was to determine the VO2RM across a range of VE representing an incremental exercise test to exhaustion. We hypothesized that the VO2RM to VE relationship would be curvilinear, and consequently that adjusting wb-VO2 by the VO2RM would significantly lower the VO2 response, as well as change the VO2-power slope near maximal exercise intensities. Such a response should in turn influence the incidence of the VO2 plateau, which is arguably the best criterion to establish the attainment of a true VO2max. A secondary purpose was to develop a regression equation to predict the VO2RM from ventilation.
Table 1. Definitions of unconventional abbreviations used in this study. Abbreviation Definition
VO2RM The oxygen uptake required to ventilate a given volume of air, which includes the respiratory muscles and postural muscles. Measured as whole-body VO2 while hyperventilating in the sitting position. Individual VO2RM The oxygen uptake required to ventilate a given volume of air. Values are derived from the subject’s own oxygen uptake response applied to their own exercise ventilation data. Composite VO2RM The oxygen uptake required to ventilate a given volume of air. Values are derived by applying the oxygen uptake response of all subjects to the individual subject’s exercise ventilation data. VO2LOC The estimated oxygen uptake of the locomotor muscles. Calculated by subtracting the VO2RM from wb-VO2. Individual VO2LOC The estimated oxygen uptake of the locomotor muscles. Calculated by subtracting the Individual VO2RM from wb-VO2. Composite VO2LOC The estimated oxygen uptake of the locomotor muscles. Calculated by subtracting the Composite VO2RM from wb-VO2. wb-VO2 Whole-body VO2; VO2 measured via expired gas analysis.
METHODS Subjects Twenty-two healthy non-smoking volunteers (13 men, and 9 women) served as subjects for this study. Prior to participation, each subject read and signed an informed consent form approved by The University of New Mexico’s Institutional Review Board. All subjects were familiar with cycle ergometry testing, and had performed an incremental maximal exercise test within the past year.
Testing was performed at an altitude of 1572 meters (PB ~ 635 mmHg), and all subjects were residents at this altitude for at least one-year. Expired Gas and Heart Rate Measurement During all tests VO2, carbon dioxide production (VCO2), VE, and RR were measured breath-by-breath using a fast response turbine flow transducer (K.L. Engineering Model S-430, Van Nuys, CA) and AEI oxygen and carbon dioxide electronic gas analyzers (AEI Technologies, Model S-3A and Model CD- 3H, Pittsburgh, PA). Ventilation volumes were calibrated using a 3 L syringe, and the gas analyzers Oxygen Cost of Ventilation During Incremental Exercise 4 were calibrated with 3 gases of known concentrations. Raw signals were acquired through a junction box and integrated with a data acquisition card (National Instruments, Austin, Texas) to a computer. Data were acquired and processed using custom written software (LabVIEW, National Instruments, Austin, TX). All VO2 and VCO2 values were converted to STPD and all VE were reported in BTPS. Heart rate was monitored with a Polar heart rate monitor (Polar Electro Oy, Finland), and recorded each minute. Exercise Test During their first visit to the laboratory subjects performed an exercise test for the determination of VO2max on a constant load cycle ergometer (Excalibur Sport, Corval Lode B.V., Lode Medical Technology, Groningen, The Netherlands). After resting expired gases were collected for 2 min, a 2- min warm-up was performed at a workload of 50 Watts, which was immediately followed by the exercise protocol. A ramp protocol was used where power increased by 20-30 Watts/min. The criterion for termination of the exercise tests was failure of the participant to maintain 40 rev/min on the cycle ergometer or volitional fatigue.
Measurement of the VO2RM After a minimum of 48 hours following the exercise test, subjects returned to the lab for the measurement of the VO2RM. To obtain this data, subjects were asked to mimic nine different VE volumes that represented the range of VE experienced during the exercise test. These volumes included the lowest and highest exercise VE, as well as seven evenly distributed VE in between. The VE and RR results from the exercise test were used to determine the levels of VE and the corresponding RR used in the mimicking trials. All 9 measurements were performed in random order during one visit to the lab.
The mimicking trials were performed at rest, sitting on the cycle ergometer in the exercise position. This was done so that body position differences would not affect the VO2RM between the exercise and resting conditions, and so that any use of the arms during exercise for a given VE would be mimicked. Before each trial, 2 min of baseline gas measurements were collected. The subjects then performed a practice attempt lasting several seconds at the target VE and then continued to breathe at the target VE for 3-5 min. A minimum of 5 min separated each mimicking trial. To accurately simulate the target VE, RR, and tidal volume, subjects breathed in rhythm to a metronome, which was matched to the RR experienced during exercise for that VE. Verbal feedback and computer displayed VE (updated every breath) were provided to the subjects to help achieve and maintain the target VE. Between 3-5% carbon dioxide was added to the inspirate, which was verified by sampling through the CO2 analyzer, to maintain end tidal CO2 levels and avoid hypocapnea (20). Calculation of the VO2RM and VO2LOC From each VE mimicking trial, the final 30 s of steady state VO2 data with the corresponding VE data was averaged and their values were graphed (Prism® graphing software package, GraphPadTM Software, Inc. version 3.0, San Diego, CA). Steady state was defined as an increase in VO2 less than 25 mL O2/min. Failure to achieve steady state or failure to reach and maintain the target VE within 10% resulted in the omission of that trial from the subsequent data analyses. A best-fit curve was then applied to the plotted data points in order to create an equation defining the Individual VO2RM equation (Figure 1). Also, all VE mimicking trial data across all subjects were plotted together and a best-fit curve was applied to this data for the determination of the Composite VO2RM equation (Figure 2). For each subject, the equation representing the Individual VO2RM and Composite VO2RM were applied to each of the breath-by-breath VE data points collected during the maximal exercise test. The resulting data sets represented the VO2RM of each breath throughout the maximal exercise test based on the individual and composite equations (Figure 3). Subtracting the Individual and Composite VO2RM values from each subject’s corresponding wb-VO2 data created the Individual VO2LOC and Composite VO2LOC data sets (Figure 4). The purpose of performing both the individual Oxygen Cost of Ventilation During Incremental Exercise 5 and composite VO2RM equations on each subject’s data was for later determination of the accuracy of using the composite equation to predict individual VO2RM. Whole-Body VO2 and VO2LOCSlope Analyses The wb-VO2, Individual VO2LOC, and Composite VO2LOC data were analyzed by three techniques for the presence of a VO2 plateau. For all techniques, an increase in VO2 < 50 mL O2/min was considered a plateau (21). Because no standard exists for how to analyze VO2 data for the presence of a plateau, two traditional techniques in addition to a new computer generated technique were employed. For all analyses, the breath-by-breath data was smoothed with a seven breath moving average. The first technique fit a linear regression equation to the final minute of VO2 data in all 3 VO2 data sets and used the slope identified by the equation to determine the magnitude of change in VO2 (Figure 5a). The second technique used the exact same procedure, but only the final 30 s of data were analyzed (Figure 5b). The third and final technique fit a third-order polynomial curve to the final 5 min of VO2 data (Prism® graphing software package, GraphPadTM Software, Inc. version 3.0, San Diego, CA), and then the final 30 s of the curve was analyzed for the presence of a plateau (Figure 5c). This final approach was used for its ability to detect the multiple slope changes in the VO2-Watts measurements. Additionally, the three VO2-Watts data slopes within each technique were compared to each other to determine if there was a difference in their magnitude. Statistical Procedures Standard descriptive statistics, using the Statistical Package of the Social Science (SPSS, version 10.0, Chicago, IL), were calculated to evaluate the mean characteristics of the participants in this study.
A repeated measures ANOVA (slope detection techniques) was applied to the data in order to determine if there was a difference in the magnitude of the slopes within each technique. When significant, Tukey’s HSD post hoc test was used to identify where the differences existed. Significance was set at p < 0.05. All data are reported as meanstandard deviation.
RESULTS
Descriptive characteristics of the subjects are provided in Table 2. Subjects were of moderate to high cardiorespiratory and muscular endurance fitness, and had a wide range of maximal VEmax.
Table 2. Descriptive data of the subjects. Age Height Weight (kg) HRmax VO2max VO2max VEmax BTPS (years) (cm) (beats/min) (mL/min) (mL/kg/min) (L/min) Men (N = 13) 31.57.7 179.07.1 77.08.7 181.08.0 446879 58.01.0 168.019.3 Women (N = 9) 24.26.3 161.57.7 56.59.3 181.55.9 282692 50.01.6 105.826.3 All Subjects (N = 22) 28.97.2 172.310.2 69.711.7 181.16.8 3907102 56.11.5 142.538.2
HR max, maximal exercise heart rate; VO2max, maximal oxygen uptake; VEmax BTPS, maximal exercise ventilation in BTPS.
Subjects were able to closely match the target VE volumes during the mimicking trials. Of 198 trials, 14 (7.1%) were not used due to lack of steady state or inability to match the target volume within 10%. To ensure the repeatability of the VE and corresponding VO2 measurements, four subjects repeated all testing procedures. The mean difference in the mimicking of the 9 target VE between the two trials was 1.42.87%, while the difference in the VO2 was 3.83.3%. Oxygen Cost of Ventilation During Incremental Exercise 6
Figure 1 presents the data for VO2 across a range of VE for a representative subject. Most 1500 [0.007994 (VE L/min)] VO2RM (ml/min) = 301.6 subjects had a similar exponential increase in R2 = 0.975 VO2 with increasing VE. A composite plot of all 1250 SEE = 50.7 ml O2/min subjects’ VO2 response to increasing VE is presented in Figure 2, with complete )
n 1000 i
presentation of the data in Table 3. The m / individual curves fit to the VO2RM data for each L m 750 2 (
subject was non-linear with a mean R =0.974 2 O with a standard deviation of 0.019. The V 500 composite regression equation (VO2RM (mL/min) = 346.9 [0.007614 x VE (L/min)]), including all 2 250 184 VE mimicking trials, produced an R of 0.911 with a standard error of estimate of 73.0 mL O2/min (Figure 2). Clearly, the VO2RM 0 0 25 50 75 100 125 150 175 200 increased considerably at VE values V (L/min) experienced during moderate to intense E-BTPS exercise, as occurs during the final minutes of Figure 1. Oxygen consumption (VO2) measured at different VE during hyperpnea trials for a representative a test to VO2max. subject. Data represent the oxygen consumption of the respiratory muscles (VO2RM). In order to clearly show the different data processing procedures, data from representative subjects are presented in Figures 3-5. Figure 3 presents the raw VO2RM data compared to the data calculated from the composite regression curve (Composite VO2RM). Figure 4 presents a comparison of Wb-VO2 and Wb-VO2 adjusted for each of the subject’s VO2RM and Composite VO2RM. Figure 5 provides the results of the 3 different slope detection procedures used to identify the presence of a VO2 plateau.
[0.007614 x VE (L/min)] 1500 VO2RM (mL/min) = 346.9 R2 = 0.911 SEE = 73 mL O2/min 1250 ) n i
m 1000 / L m (
750 2 O V 500
250
0 0 25 50 75 100 125 150 175 200
VE-BTPS (L/min)
Figure 2. Composite data representing all VO2RM mimicking trials for all subjects. Oxygen Cost of Ventilation During Incremental Exercise 7
1750
1500 ) n i 1250 m / L m
( 1000
2 O V 750
Indiv. VO2RM 500 Comp. VO2RM
250 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Time (min)
Figure 3. The breath-by-breath Individual VO2RM and Composite VO2RM data during an exercise test of a representative subject. Individual VO2RM, breath-by-breath VO2 data determined from equation defining the individual relationship between VE and VO2; Composite VO2RM, breath-by-breath VO2 data determined from equation defining the composite relationship between VE and VO2. Data are presented commencing at 3 min to avoid the 2 min rest and initial min of increment at 50 Watts.
3500 wb-VO2
3000 )
n 2500 i Indiv. VO2LOC m /
L 2000 m (
2 1500
O Comp. VO2LOC V 1000
500
0 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Time (min)
Figure 4. The three breath-by-breath VO2 data sets of a representative subject. Wb-VO2, whole body VO2; Individual VO2LOC, wb-VO2 minus the Individual VO2RM; Composite VO2LOC, wb-VO2 minus the Composite VO2RM. Data are presented commencing at 3 min to avoid the 2 min rest and initial min of increment to 50 Watts.
Table 3. Ventilation volumes with corresponding VO2RM data for the nine mimicking trials.
Mimicking VE BTPS VO2RM VO2RM Trial (L/min) (mL/min) (mL/kg/ min) N 1 32.15.0 44571 6.41.0 22 2 45.67.8 49880 7.11.2 22 3 58.512.1 54590 7.81.3 22 4 73.116.1 623112 8.91.6 20 5 84.319.7 669153 9.62.1 20 6 100.423.4 761181 10.92.5 20 7 113.030.5 835210 12.03.0 19 8 127.128.8 916214 13.12.9 18 9 137.834.4 1011280 14.54.0 21
VE BTPS, ventilation expressed as BTPS; VO2RM, oxygen uptake for ventilation; N, number of completed trials. Oxygen Cost of Ventilation During Incremental Exercise 8
wb-VO2 Indiv. VO2LOC Comp. VO2LOC a 4000 )
n 3000 i m / L m
( 2000
2 O V 1000
0 14.5 15.0 15.5
b 4000 ) n i 3000 m / L m
( 2000
2 O V 1000
0 15.00 15.25 15.50 15.75
c 4000 ) n i 3000 m / L m
( 2000
2 O V 1000
0 10 11 12 13 14 15 16 Time (minutes)
Figure 5. Example of the three slope detection techniques on a representative subject’s data. a) 1-min technique; Change in VO2 defined by linear regression applied to final 60 s of breath-by-breath exercise data. b) 30-s technique; Change in VO2 defined by linear regression applied to final 30 s of breath-by-breath exercise data. c) 5-min technique; Change in VO2 defined by change in the third-order polynomial curve over final 30-s of breath-by-breath exercise data (inside box). Oxygen Cost of Ventilation During Incremental Exercise 9
Maximal oxygen consumption of the respiratory muscles, adjusted for resting VO2, equaled 5.00 18.1±4.4 % of the wb-VO2. The changes in the 4.75 oxygen cost per liter VE between ~30 to ~60 ) E V
L/min remained fairly constant at around 4 mL L
/ 4.50 O2 per L/VE in this study. Above ~60 L/min the L m ( oxygen cost per liter VE rose sharply to over 5 2 O 4.25 V mL O2/L VE at ~140 L/min (Figure 6). The high correlation between VO2RM and VE indicated 4.00 that the VO2RM can be predicted based on exercise VE data alone, with 95% of the 3.75 25 50 75 100 125 150 subjects falling within 146 mL O2/min of the VE BTPS (L/min) predicted VO2RM. Figure 7 presents the measured VO2RM from the hyperpnea trials and Figure 6. The mean data for the VO2RM per liter of VE for all subjects. the predicted VO2RM applied to the VE data from the hyperpnea trials. The meanstandard deviation of residuals from the prediction were –0.3373.04 mL O2/min. b a 0.2 1.6 ) n 1.4 i m /
L 0.1 (
M R ) 1.2 2 n i m / O L ( V
1.0 -0.0 M d R e r 2 P
O -
V 0.8 d
e d r e u r s P -0.1 0.6 a e M
0.4 -0.2 0.2 0.25 0.50 0.75 1.00 1.25 1.50 1.75 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 Measured VO2RM (L/min) Measured VO 2RM (L/min)
Figure 7. a) The predicted versus measured VO2RM data. Diagonal line represents the line of best fit. b) Residuals plot of the difference between the measured minus predicted VO2RM and the measured VO2RM. Solid line represents the mean residuals, and the dotted lines represent 2 SD from the mean.
The means, standard deviations, and VO2 plateau data are presented in Table 4. The results of the repeated measures ANOVA show that there was a significant main effect between the three VO2 data slopes (F = 43.78, df 2/42, p < 0.05), indicating that there was a difference in the magnitude of slopes between wb-VO2, Individual VO2LOC, and Composite VO2LOC. Specifically, the change in wb-VO2 was greater than VO2LOC during the final minute or 30 s of the incremental exercise test. There was not a significant interaction (F = 2.24, df 4/84, p > 0.05) between the slope detection technique and the type of VO2 data. The application of Tukey’s HSD post hoc test determined that within each slope detection technique the change in the VO2 slope during maximal exercise for Individual VO2LOC and Composite VO2LOC were significantly lower than the wb-VO2 slope, yet were not different from each other. The lack of a difference between the Individual VO2LOC and Composite VO2LOC slopes indicates Oxygen Cost of Ventilation During Incremental Exercise 10 that using the composite regression equation to determine the VO2RM provided similar values as the Individual VO2RM. The incidence of the VO2 plateau was much greater in the ventilation adjusted slopes, especially for the 5-min technique where only one plateau was detected in the wb-VO2, and 15 were present after the individual and Composite VO2RM adjustments were made (Table 4). This provides evidence that regardless of the slope detection technique used, adjusting wb-VO2 data by the VO2RM results in a greater incidence of the VO2 plateau.
Table 4. Oxygen uptake (VO2) plateau data reported as change in VO2 (L/min) resulting from the three slope detection techniques on the three VO2 data sets. Change in VO2 (mL/min) Technique wb-VO2 Individual VO2LOC Composite VO2LOC 1-min 297.3241.0 (3/22) 56.7237.1* (9/22) 59.0234.3* (9/22) 30-s 292.8383.1 (6/22) -1.4407.1* (14/22) -15.7377.2* (13/22) 5-min curve 303.8194.3 (1/22) 22.8233.5* (15/22) 25.9220.1* (15/22)
Data in parentheses=number of VO2 plateaus (increase in VO2 < 50 mL/min) detected / total number of VO2 data sets. wb-VO2 = change in whole-body VO2; Individual VO2LOC = change in ventilation adjusted VO2 data slope using the Individual VO2RM data; Composite VO2LOC = change in ventilation adjusted VO2 data slope using the Composite VO2RM data; 1-min = linear regression slope fit to final minute of data; 30-s = linear regression slope fit to final 30 s of data; 5-min curve = best-fit curve fit to final 5 min of data and final 30 s analyzed for plateau. *Different than wb-VO2, p < 0.05.
DISCUSSION
The significant findings of this study were; that the VO2RM could be accurately predicted by use of the regression equation created from combining all VO2RM trials among the subjects in this study, and that the incidence of a VO2 plateau was greater when wb-VO2 was adjusted for the Individual or Composite VO2RM.
The results of this study are in agreement with previous studies regarding the magnitude of maximal VO2RM and the exponential relationship between VO2RM and increasing VE (1,18-20). In addition, the changes in the oxygen cost per liter VE with increases in exercise intensity were similar to previously reported O2 cost data of near maximal and maximal exercise VE (1,8,12,15,18).
The high correlation between VE and VO2RM from the composite regression equation suggests that the VO2RM can be predicted based on exercise VE data alone. This finding was confirmed by the lack of differences between the Individual VO2LOC and Composite VO2LOC data sets (Figure 3). It is therefore possible to predict the VO2RM without performing resting hyperpnea VO2 measurements for subsequent wb-VO2 adjustments for the VO2RM. This regression equation (Figure 2) would therefore be appropriate to apply to VE data obtained during cycling exercise to accurately predict the VO2RM among subjects with similar characteristics as in this study.
In contrast to the findings of Knight (6), and Poole et al. (6) the present study suggests that using wb- VO2 data to represent the VO2 of the locomotor muscles is inaccurate, especially during high intensity exercise. Therefore, modeling and interpreting wb-VO2max data as a representation of the locomotor muscles is problematic because that data does not accurately represent the oxygen uptake of those muscles. Our findings suggest that the changes in the VO2 of the locomotor muscles are significantly lower than that presented by wb-VO2 during maximal exercise intensity. This study found that as exercise intensity increases the VO2 devoted to VE increases exponentially. The present data supports that of Aaron et al. (18,19) and Harms et al. (9,10), who found that the blood-flow and VO2 required to ventilate during high intensity and maximal exercise is potentially large enough to occupy a significant portion of wb-VO2 data. Oxygen Cost of Ventilation During Incremental Exercise 11
As we did not measure cardiac output or limb blood flow it is difficult to interpret the VO2 plateau as being either a central or peripheral induced phenomenon. However, what is clear is that when comparing the VO2LOC to wb-VO2 there is a more pronounced VO2 plateau in the VO2LOC. It is tempting to hypothesize that the proportion of the cardiac output that has to feed the respiratory muscles is reducing limb blood flow and oxygen delivery to the working muscles, as has been postulated by Harms et al. (9,10). Nevertheless, the only data that would more clearly prove such a central limitation is to show that cardiac output or limb blood flow also demonstrated a plateau response similar to the VO2LOC. Conversely, if blood flow to the working muscles continued to increase despite a plateau in VO2LOC, then there is a limitation that occurs in the working muscle despite a continued increase in blood flow. This latter scenario would be clear evidence of a peripheral limitation.
Our reporting of significant differences between wb-VO2 and VO2LOC differ to the findings of Knight et al. (6). These differences can be explained by dissimilarity of the methods between the two studies. Knight et al (6) used seven subjects who exhibited a wb-VO2 plateau during cycling exercise. The wb-VO2 and leg VO2 were compared at the exercise intensity at which the plateau first became apparent and one exercise intensity above that (either 108% or 115% of that intensity). The wb-VO2 data was averaged over 45 s during which leg blood flow was measured for the determination of leg VO2. Knight et al. (6) successfully established that the asymptotic behavior of wb-VO2 is also present in leg VO2 during near maximal and maximal exercise intensities. However, their data were not sensitive enough to determine if the rate of change in VO2 between whole-body and legs were different than each other. In the present study, using breath-by-breath data during incremental exercise, simultaneous changes in the kinetics of wb-VO2 and the VO2LOC are confirmed as exercise intensity reaches maximum. It is the finding that several subjects who did not attain a wb-VO2 plateau did attain a VO2LOC plateau that proves the assumption that wb-VO2 responses are similar to leg VO2 is incorrect.
The finding that the incidence of the VO2 plateau was greater when wb-VO2 was adjusted for the VO2RM is important because the plateau is a primary criterion for the identification of VO2 max (6). As discussed by Howley et al. (4,6), the use of secondary criteria, such as a respiratory exchange ratio > 1.15, being within 10% of age-predicted maximum heart rate, and a maximum blood lactate > 8 mmol/L should not be the sole measures in confirming the attainment of VO2max. The applicability of this study therefore comes in the ability to confirm the attainment of VO2max through identification of a VO2LOC plateau. Furthermore, our results indicate that one of the main reasons for the low incidence of the VO2 plateau in past research may be that the VO2RM masks what might be a true VO2 plateau response. In other words, the use of wb-VO2 may lead to a high probability of false-negative detection of the VO2 plateau.
Not all subjects in this study attained a plateau in VO2 even after the adjustment for the VO2RM. Past research has reported between 30% and 100% of subjects tested attained a plateau (6). Most of the variability in these studies could be explained by differences in protocol, fitness and age of subjects, the criteria used to establish a plateau, and the data collection and analysis procedures used (5). Problems with the identification of a VO2 plateau could be the result of using discontinuous or step protocols; clinical, senior, pediatric, or sedentary subjects; plateau criteria as conservative as an increase 150 mL O2/min or 3 mL O2/kg/min; data averaging of up to 90 s; or comparing consecutive breath-by-breath data points (4-6). While the criterion used in this study was somewhat conservative compared to past research, it is an accepted means for plateau identification (4). The data collection technique and slope detection methods used in this study made use of advanced equipment and analytical tools, which arguably would detect a plateau, were one to exist. The inconsistent presence of the VO2 plateau could be a result of the heterogeneity of the population used. A somewhat large subject age and fitness range was tested, and despite previous experience performing maximal cycle Oxygen Cost of Ventilation During Incremental Exercise 12 ergometry, an inability to cycle to a workload eliciting the presence of a plateau in oxygen consumption existed.
All three of the slope detection techniques used in this study were sensitive to the changes caused by the VO2RM adjustment. The lack of a standardized technique to use for the detection of a plateau in VO2 may stem from the lack of a clear definition of a VO2 plateau. While the use of breath-by-breath data collection is becoming more common, being able to analyze the changes occurring amidst the variability between breaths becomes more important. The 30-second slope detection techniques used in this study was very sensitive to the high amount of variation in the breath-by-breath VO2 measurements. The breath-by-breath data, even after the 7-breath averaging, increased in a wave- like pattern with increasing exercise intensity, and a 30-s portion of this slope analyzed at any time during exercise could have shown either increasing or decreasing VO2. Use of the 1-min technique eliminated some of the variability in the breath-by-breath VO2 seen with the 30-second technique, however the extended duration of this analysis may have captured some of the increases in VO2 prior to the plateau, which would have limited its ability to detect a plateau if one existed. The use of the 5- min technique virtually eliminated the variability of the breath-by-breath data while not altering the trend in oxygen uptake. The subsequent analysis of the final 30 s from the 5-minute slope provided an accurate representation of the change in VO2 during maximal exercise. For these reasons, the 5- min technique was best at determining the changes occurring in VO2 and its use is recommended for future studies investigating the presence of a VO2 plateau.
Based on the results of this study, it can be concluded that changes in wb-VO2 during maximal cycling exercise are significantly influenced by the VO2RM. The results demonstrate that when wb-VO2 is adjusted for the VO2RM the VO2 slope becomes significantly smaller, which is an indication that the O2 uptake of the exercising muscles is misrepresented by wb-VO2 data. From these findings, identification of the VO2 plateau is more accurate when using VO2LOC instead of wb-VO2, because of the significant changes occurring in the VO2LOC that are not represented by the wb-VO2 during incremental exercise to exhaustion. Also, the VO2RM can be accurately predicted from cycling VE data among subjects with similar characteristics as in the present study. Use of the prediction equation developed here, which would allow for the determination of the VO2LOC and subsequent analysis of cycling exercise data for the presence of a VO2 plateau. Additional research is needed to validate the accuracy of the VO2RM prediction equation, especially at different altitudes and across different subject populations, and to explore the causes of the presence or absence of the VO2 plateau phenomenon.
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