incremental test, SV, stroke volume, cardiac output, heart rate, maximal value, diff, attainment, oxygen uptake, dynamic exercise, Sports Exerc, exercise tests, Sports Med, muscle mass, J. Appl, J. Physiol, incremental exercise test, Maximal values, exercise intensity, exercise duration, Incremental exercise, continuous increase, exponential functions, exercise test, Arterial oxygen saturation, Measurements, gas analyzers, Medical Graphic Corporation, ECG, exercise testing, skeletal muscle, skeletal muscle perfusion, progressive exercises, exercise intensities, BILLAT, J. C. RENOUX, R. RICHARD, J. P. KORALSZTEIN, cycling exercise, American College of Sports Medicine, V. L. BILLAT
Effect of Exercise Intensity on Relationship between V O2max and Cardiac Output PIERRE-MARIE LEPRETRE1, JEAN-PIERRE KORALSZTEIN2, and VERONIQUE L. BILLAT1,2 1LIGE, Department of Sciences and Technology in Sports and Physical Activities (STAPS), University of Evry Val d'Essonne, Evry, FRANCE; and 2Sport Medical Center of the CCAS, Paris, France
ABSTRACT LEPRETRE, P.-M., J.-P. KORALSZTEIN, and V. L. BILLAT. Effect of Exercise Intensity on Relationship between V O2max and Cardiac Output. Med. Sci. Sports Exerc., Vol. 36, No. 8, pp. 13571363, 2004. Purpose: The purpose of this study was to determine whether the maximal oxygen uptake (V O2max) is attained with the same central and peripheral factors according to the exercise intensity. Methods: Nine well-trained males performed an incremental exercise test on a cycle ergometer to determine the maximal power associated with V O2max (pV O2max) and maximal cardiac output (Q max). Two days later, they performed two continuous cycling exercises at 100% (tlim100 5 min 12 s 2 min 25 s) and at an intermediate work rate between the lactate threshold and pV O2max (tlim50 12 min 6 s 3 min 5 s). Heart rate and stroke volume (SV) were measured (by impedance) continuously during all tests. Cardiac output (Q ) and arterial-venous O2 difference (a-vO2 diff) were calculated using standard equations. Results: Repeated measures ANOVA indicated that: 1) maximal heart rate, V E, blood lactate, and V O2 (V O2max) were not different between the three exercises but Q was lower in tlim50 than in the incremental test (24.4 3.6 L·min1 vs 28.4 4.1 L·min1; P 0.05) due to a lower SV (143 27 mL·beat1 vs 179 34 mL·beat1; P 0.05), and 2) maximal values of a-vO2 diff were not significantly different between all the exercise protocol
s but reduced later in tlim50 compared with tlim100 (6 min 58 s 4 min 29 s vs 3 min 6 s 1 min 3 s, P 0.05). This reduction in a-vO2 diff was correlated with the arterial oxygen desaturation (SaO2 15.3 3.9%) in tlim50 (r 0.74, P 0.05). Conclusion: V O2max was not attained with the same central and peripheral factors in exhaustive exercises, and tlim50 did not elicit the maximal Q . This might be taken into account if the training aim is to enhance the central factors of V O2max using exercise intensities eliciting V O2max but not necessarily Q max. Key Words: STROKE VOLUME, ARTERIAL-VENOUS DIFFERENCE, CYCLING, HYPOXEMIA
Exercise workloads between 85 and 100% of the power associated with V O2max in an incremental test (pV O2max) will elicit V O2max, provided they are performed for a sufficient time duration (515min according to the exercise intensity) (6). However, the respective contribution of central (the cardiac output: Q ) and peripheral factors (the arterial-venous oxygen difference: a-vO2 diff) in the attainment of V O2max according to the workload is not clear. McCole et al. (21) have reported that an incremental test of a total duration of 12 min (with four stages of 3-min duration) elicited V O2max with a lower cardiac output (due to lower value of stroke volume) compared with an incremental protocol lasting 6 min only (including six stages of 1 min). The authors hypothesized that, in contrast with the longer duration test, the temperature necessary to elicit cutaneous vasodilatation was not reached during the shorter Address for correspondence: Pierre-Marie Lepretre, department of STAPS, University of Evry Val d'Essonne, Sport Medical Center of the CCAS, 2 avenue Richerand, Paris, F-75010, France. E-mail: [email protected]
Submitted for publication October 2003. Accepted for publication March 2004. 0195-9131/04/3608-1357 MEDICINE & SCIENCE IN SPORTS & EXERCISE® Copyright © 2004 by the American College of Sports Medicine
test, preventing the increased competition for the distribution of Q (21). However, the 1-min stage duration protocol allowed reaching almost a twice higher treadmill slope than the 3-min stage duration (14% vs 8%). This could induce a larger muscular mass recruitment during shorter than longer progressive exercises, and it is well known than the stroke volume is influenced by the muscle mass recruited which itself influences the muscle pump and hence the venous return (20). The large difference in power output (as 2% represents 1 km·h1 of speed increment, i.e., 1 MET: 3.5 mL·kg1·min1) and the absence of V O2 steady states makes it difficult to make conclusions about the disassociation between maximal V O2 and Q during exercise. Furthermore, it has been reported that the time course of the stroke volume was affected by the exercise mode (22) and that cardiac output declines more during cycling versus running exercise. This is due to the development of high intramuscular forces in cycling that would compress or even partially occlude venous blood return from the leg (22). A notable difference was also reported with the time course of oxygen uptake between running and cycling at the same relative supra-threshold intensity in well-trained triathletes (5). Indeed, few studies have reported the time course of these central and peripheral factors during supra-lactate threshold exercises and have examined subthreshold exercises. Cheatham et al. (11) have shown that the increase in a-vO2 diff was associated with that of V O2 during 40 min of an exercise performed at 64.5% of V O2max. They interpreted
this as a manifestation of the BoЁhr effect rather than a further decrease in end capillary oxygen pressure and the exercise lactic acidosis might cause different responses in tissue oxygen extraction (11,17). However, if the BoЁhr effect contributes to enhancing a-vO2 diff by decreasing venous O2 concentration (CvO2) in supra-threshold exercises, these intensive exercises may induce hypoxemia, especially in highly fit endurance subjects. Indeed, Billat et al. (8) have reported a hypoxemia in exhaustive constant speed runs performed at 90, 100, and 105% of the pV O2max, and the time to exhaustion at 90% of V O2max was inversely correlated with the degree of hypoxemia: a longer exercise duration was associated with a more pronounced hypoxemia, that is, a larger decrease in arterial oxygen de-saturation (SaO2). Therefore, the drop in CvO2 could be balanced out by the decrease in CaO2 (15). Given that the oxygen pulse (SV a-vO2 diff) has been reported to be constant in exhaustive running (10 min) at 95% of pV O2max (18), this raises the question of the balance between the peripheral and the central factors of V O2 during exhaustive supra-threshold exercises eliciting V O2max. Because very heavy exercise is currently and mostly used during interval training with the goal of improving V O2max (14), it is of interest to check whether exercise performed at the low and high range of this heavy-intensity domain elicits the same value of central and peripheral factors of V O2max. However, behind this (similar) maximal V O2 elicited, we hypothesized that the maximal value of Q is not systematically reached and sustained. Therefore, the purpose of this study was to determine in endurance trained subjects and in cycling exercise whether the maximal oxygen uptake (V O2max) is attained with the same central and peripheral factors in exhaustive constant load exercises performed at 100% of the power associated with V O2max in an incremental test (pV O2max) and at p50, which was defined as the intensity midway between the power associated with the lactate threshold (pLT) and pV O2max. METHODS Nine healthy male triathletes participated in this study after giving their written voluntary informed consent in accordance with the guidelines of the University of Evry Val d'Essonne. All subjects were free of cardiac and pulmonary disease. Their Physical Character
istics are reported in Table 1. Protocol. Three tests (one incremental and two constant work exercises) were performed at 1-wk intervals, at the same time of the day and 23 h after a light breakfast in an air-conditioned room (Fig. 1). All tests were performed with the subjects in the upright position on an electronically braked ergometer (ERGOLINE 900, Hellige, Markett). Seat
and handlebar heights were set for each subject and kept constant for all the tests. The pedaling frequency selected by each subject between 70 and 110 rev·min1. Incremental exercise. After a 3-min warm-up at 80 W, each subject performed a 3-min stage incremental exer- cise test to exhaustion with 40 W work increment for total exercise duration not exceeding 20 min. In this incremental protocol, V O2max was defined as the highest 30-s oxygen uptake value reached during exercise with a respiratory exchange ratio greater than 1.1, blood lactate greater than 8 mM, and a peak heart rate at least equal to 90% of the age-predicted maximum. A plateau of V O2 was identified if the V O2 of the least stage was not greater than the previous one by 1.75 mL·kg1·min1. The pV O2max was determined as the lowest power eliciting V O2max (7). If during the last stage, a subject achieved V O2max without completing the 3-min stage, pV O2max was calculated as following:
pV O2max pF t/180 40
where pF was the power of the last complete stage (W), t was the time the last workload was maintained (s), and 40 W was the power output increment between the last of two stages. Constant-load exercises. In randomized order, each subject completed two severe constant workload exercises until exhaustion separated by at least 48 h. Time limit (tlim) was the time at which the subject was no longer able to pedal at the given power output. After a 15-min warm-up at 30% of pV O2max (which was below the lactate threshold for all subjects) and 5 min at rest, the work rate was increased within 30 s to p50 or pV O2max. The power p50 was defined as the intensity midway between the power associated with the lactate threshold (pLT) and the work requiring 100% of pV O2max. Measurements of gas exchange. The minute ventilation (V E) and gas exchange parameters (V O2, V CO2) were measured breath-by-breath by an open-circuit metabolic cart with rapid O2 and CO2 analyzers (CPX, MedGraphics Cardiorespiratory system
s, Medical Graphic Corporation, Wisconsin). Before each individual exercise test, the pneumotachograph was calibrated using a 3-L calibration syringe (Hans Rudolph), and the gas analyzers were calibrated using reference gases with known O2 (16.0%) and CO2 (5.0%) concentrations. During all tests, the breath-bybreath data were smoothed and averaged every 5 s. Measurement of blood lactates. During both exercise tests, 10-L ear blood samples were collected and immediately analyzed for blood lactate concentration (YSI 1500, Sport L-lactate Analyzer, Bioanalytical Products, Yellow Springs, OH). The samples were taken at rest, every 3 min during the all-out exercise, at the end of cycling, and at 2 and 4 min during recovery after the exercise test. For the incremental test, the lactate threshold (LT) was defined as
TABLE 1. Physical characteristics of the subjects.
Subjects (N 9)
Mass (kg) 71 4
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Height (cm) 176 4
Body Fat (%) 11.4 2.4
Training Time (hwk1) 13 4 http://www.acsm-msse.org
FIGURE 1--Experimental design. the V O2 corresponding to the starting point of an accelerated lactate accumulation of 4 mM and expressed in percent of V O2max (2). Arterial oxygen saturation measurements. SaO2 was recorded pulse-to-pulse using a noninvasive external device (Ohmeda 3800 oximeter, Datex-Ohmeda, Louisville, KY
) and then averaged every 5 s. The ear pulse oxymeter was calibrated using an internal protocol before each test, according to the manufacturer's instructions. The pulse oximeter used an exclusive algorithm (Trutrak®, Oxymetry technology, Datex-Ohmeda), which identified, quantified, and corrected the clinical movement of the subject. Pulse oximetry has been reported to be accurate across a broad range of SaO2 (from 57.2 to 97.6%) and provides an arterial O2 saturation estimate with mean error of 2% (3). According to Dempsey and Wagner (15), arterial hypoxemia induced by exercise was defined as an arterial O2 saturation 88% (or SaO2rest end of exercise greater than 10%). Measurements of cardiac parameters (heart rate, stroke volume) and calculation of Q , a-vO2 diff. We used a bioimpedance determination for stroke volume and heart rate (Physioflow, Manatec Type PF05L1, France). The theoretical basis for this technique and its application and validity for exercise testing have been previously described (4,10). The physioflow device measures impedance changes (dZ) in responses to a small-administered electrical current. Two sets of electrodes (Ag/AgCl, Hewlett Packard 40493 E), one electrode transmitting and the other sensing, respectively, were applied above the supra-clavicular fossa at the left base of the neck and along the xiphoid. A further set of two electrodes was used to monitor a single ECG (CM5 position). With this impedance device, a first evaluation of stroke volume index (SVi) is calculated during a calibration procedure based on 24 consecutive heartbeats recorded in the resting condition (SVical). This evaluation keeps the largest impedance variation during the systole (Zmax Zmin) and the largest rate of variation of the impedance signal (dZ/dtmax, called the contractility index). The SVi calculation also depends on the thoracic flow inversion time (TFIT, in m·s1) measured on the first mathematical derivative of the impedance signal. TFIT is the time interval between the first zero value after the beginning of the cardiac cycle (beginning of the ECG's QRS complex) V O2MAX-Q DISSOCIATION DURING HEAVY EXERCISE
and the first nadir after the peak of the ejection velocity
(dZ/dtmax). Afterward, TFIT is weighted using a specific algorithm. During the data acquisition
phase, the variation of parameters were analyzed and compared with those obtained during the calibration procedure. So, the stroke volume (SV) was the product between the electrical physical volume of thorax (VEPT), the ventricular ejection time (VET), and the maximum rate of the impedance change during the systolic upstroke [(dZ/t)max] divided by the basal thoracic impedance (Z0). Each displayed stroke volume (SV) value represents the mean of 15 successive artifact-free beats. For this experiment, SV and heart rate were measured continuously during each test with beat-to-beat data smoothed by a 5-s moving averaging algorithm. Q calculation by the device is based on the following formula:
Q HR SVi BSA
where Q is expressed in liters per minute, HR is the heart rate based on the R-R interval measurement, determined on the ECG first derivative, dECG/dt, which provides a more stable signal than the ECG signal itself, SVi (mL·m2), and BSA was the body surface area
calculated according to the formula of Haycock (BSA 0.024265 BM0.5378 H0.3964, where BM is body mass in kilograms and H is height in centimeters). In a previous study, Richard et al. (24) showed that the mean difference between the Q ob- tained using direct Fick and the impedance method was not significant at rest (0.07 L·min1, i.e., 1%) and during steady state exercise (0.26 L·min1, i.e., 2%) (10). In addition, they also reported that the direct Fick method was highly corre- lated with the impedance method during steady state exercise (r2 0.79, P 0.05, N 40) (10) and during incremental testing (r 0.94, P 0.01, N 50) (24). Previously, highly significant correlations were obtained in the stroke volume (r 0.84, P 0.001) and the cardiac output values
(r 0.98, P 0.001) between the direct Fick and impedance cardiography methods in six healthy male subjects during maximal cycling exercise tests (28). There- fore, it seems that the impedance cardiography provides accurate Q measurements during exercise. When associated with V O2 measurements, it allows the estimation of the arterial-venous difference by the Fick equation:
a-vO2diff V O2/Q .
Data kinetics modeling. The time course of a-vO2 diff was described by exponential function
s fitted to the data with nonlinear regression
techniques (Sigma Plot2000, Jandel, Chicago, IL). The calculation of the best-fit parameters was chosen by the program so as to minimize the sum of the squared differences between the fitted function and the observed response. Two exponential models were used to describe a-vO2 diff for each work rate. According to the both following equations, oxygen uptake kinetics fit by either mono-exponential  or mono-exponential plus drop  function:
a-vO2difft a-vO2diffb A1*1 e tTD1/1 Medicine & Science in Sports & Exercise
a-vO2difft a-vO2diffb A1*1 etTD1/1 A2*t TD2/2  where a-vO2 diff(t) represents arterial venous difference at time (t), a-vO2 diff(b) is the baseline of the value at the end of the warm-up, A1 was the asymptotic amplitude for the second exponential of oxygen uptake kinetics, 1 was the time constants, and TD1 was the time delay of each exponential. In the second model to describe the a-vO2 diff kinetics, A2 was the slope of the linear regression, 2 was the time constant of the drop, and TD2 was the time delay for the linear regression. Each curve fitting used an iterative least-squares approach. The rationale for selection of the appropriate model was based on analysis of the residuals around the line of best fit (8). statistical method
s. Descriptive statistics are expressed as mean and SD. According to the data, the normality distribution of the population was analyzed by variance comparison using of the Fisher Snedecor test. Statistical comparisons over time of the physiological variables (V O2, Q , HR, SV, and a-vO2 diff) were made using a one-way repeated measures ANOVA. Paired t-tests were used to compare a-vO2 diff kinetics responses. Scheffe post hoc tests were carried out when appropriate. All significant difference
s are at the P 0.05 level unless stated otherwise. RESULTS Maximal values of the peripheral and central factors of V O2max during the incremental exercise. The maximal values measured during the incremental test are in Table 2. All subjects reached a plateau of V O2. Heart rate reached its maximal value at V O2max (Fig. 2). Stroke volume increased until 93.0 6.0% of V O2max corresponding to 89.2 5.3% of pV O2max, then SVmax remained constant until the attainment of V O2max and pV O2max. Therefore, Q max was reached with the attainment of maximal heart rate. The arterial-venous O2 difference increased up to 61.2 23.3% of pV O2max corresponding to 63.8 26.4% of V O2max. So, V O2max was reached with Q max and a-vO2 diffmax in the incremental test. This was not the case for the constant-load exercises at work rates of p50 (88.0 1.5% of pV O2max) or 100% of pV O2max. Comparison between the maximal values of the peripheral and central factors of V O2max between constant-load and incremental exercises. Figures 35 show an example of the responses of cardiorespiratory parameters (V O2, Q , HR, SV, and a-vO2 diff) at both dif-
FIGURE 2--Example of the evolution of heart rate (HR), stroke volume (SV), and cardiac output (Q ) according to the percentage of maximal oxygen uptake (%V O2max) during the incremental test in subject 7. Squares represent heart rate kinetics during tlim100; empty lozenges represent Q kinetics during tlim100; full triangles represent SV kinetics during tlim100. ferent intensities and time to fatigue (tlim100 and tlim50, respectively). In all subjects, the time to exhaustion was significantly longer at p50 than at pV O2max (726 195 s vs 312 145 s, P 0.01). Peak lactate was not signifi- cantly different when comparing the three tests (Table 2). V E, V O2max, and HRmax were reached in the constant-load exercises whatever their intensities (Table 3). During the incremental test, SV reached its maximal value at higher work rates (89.2 5.3% of pV O2max) compared with p50 (88.0 1.5% of pV O2max, P 0.01). Furthermore, SV was lower in the tlim50 test (143 27 mL·beat1) compared with the incremental test (179 34 mL·beat1; P 0.05). Therefore, Q was also lower in tlim50 compared with the incremental test (24.4 3.6 L·min1 vs 28.4 4.1 L·min1; P 0.04). Therefore, maximal value of a-vO2 diff was not significantly different when comparing all the ex-
TABLE 2. Time to exhaustion, maximal power output and maximal blood lactate determined during incremental, tlim50 (p50), and tlim100 (pV O2max) exercises.
Subjects (N 9)
Time to fatigue (s) Power (W) Power (% of pV O2max) Maximal blood lactate (mmolL1)
1240 153 344 37 100.0 0.0 10.7 1.9
726 195* 303 38 88.0 1.5 11.6 2.9
312 145*§ 344 37 100.0 0.0 11.2 2.8
* P 0.05, level of significance between incremental and constant work rate exercises. § P 0.05, level of significance between time to exhaustion at pVO2max and p50.
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FIGURE 3--Example of modeling kinetics of heart rate (HR), stroke volume (SV), cardiac output (Q ), and oxygen uptake (V O2) responses during the workload at pV O2max (tlim100) in subject 7. Circles V O2 kinetics during tlim100; squares represent heart rate kinetics during tlim100; empty lozenges represent Q kinetics during tlim100; full triangles represent SV kinetics during tlim100. http://www.acsm-msse.org
FIGURE 4 --Example of modeling kinetics of heart rate (HR), stroke volume (SV), cardiac output (Q ), and oxygen uptake (V O2) responses during the workload at p50 (tlim50) in subject 7. Circles represent V O2 kinetics during tlim100; squares represent heart rate kinetics during tlim100; empty lozenges represent Q kinetics during tlim100; full triangles represent SV kinetics during tlim100. ercises (Table 3 and Fig. 5), but it decreased later in tlim50 compared with tlim100 (6 min 58 s 4 min 29 s vs 3 min 6 s 1 min 3 s, P 0.05, i.e., 43.0 22.5% and 48.3 18.2% of the time limit, NS). This narrowing in a-vO2 diff was correlated with the arterial oxygen desaturation that appeared in tlim50 (SaO2rest end of exercise 15.3 3.9%) (r 0.74, P 0.05) but not in tlim100 (SaO2rest end of exercise 11.9 2.8%) or in the incremental test (SaO2 8.8 2.8%, P 0.05). DISCUSSION The focus of this study was to examine whether the maximal oxygen uptake (V O2max) was reached with the same central and peripheral factors in exhaustive constant load exercises performed at pV O2max and p50 compared FIGURE 5--Example of arterial-venous difference kinetic (a-vO2 diff) during exercise at pV O2max (tlim100) and at p50 (tlim50) in subject 7. Squares and dashed line represent a-vO2 diff values and its mathematical model during time to exhaustion at pV O2max (tlim100); circles and solid line represent a-vO2 diff values and its mathematical model during time to exhaustion at p50 (tlim50). V O2MAX-Q DISSOCIATION DURING HEAVY EXERCISE
TABLE 3. Maximal values during incremental exercise and constant workload at p50 (tlim50) and pV O2max (tlim100).
Subjects (N 9)
V O2max (mLmin1)
diff (mL) (Lmin1)
4463 452 171 14 179 34 28.4 4.8 13.2 1.4 163.4 26.3 8.8 2.8
4363 453 175 13 143 27* 24.4 3.6* 13.1 2.1 167.1 27.7 15.3 3.9
4545 500 173 14 153 27 25.8 4.1 13.3 0.9 161.6 35.2 11.9 4.8
* P 0.05, level of significance between incremental and constant work rate exercises. § P 0.05, level of significance between time to exhaustion at pV O2max and p50.
with the reference values of a classic incremental test (with 3-min stages). The main finding from this study was that V O2max was attained with a lower value of cardiac output due a lower SV in tlim50 than in the incremental test. The maximal arterial-venous O2 difference was not significantly different according the exercise intensity but decreased more slowly in the tlim50 compared with tlim100 test. First, we are going to discuss the value of power output at which the stroke volume reaches its maximum. Maximal values of the peripheral and central fac- tors of V O2max during the incremental exercise. In agreement with recent studies performed with trained sub- jects (9,16), SV reached its maximal value almost until V O2max (93 6% of V O2max). Indeed, SV did not plateau at 40 60% of V O2max as reported in nonactive subjects (1). This continuous increase in SV until exhaustion could be explained by a greater left ventricular filling in well-trained males (16,30). In the present study, the heart rate increase until the attainment of pV O2max was responsible of the attainment Q max at the end of the incremental exercise test. In agreement with a previous investigation by Grassi et al. (17), who observed an increase in muscle oxygenation ([O2Hb HHb]) until 60 65% of V O2peak, our results also showed an increase in a-vO2 diff until 63.8 26.4% of V O2max. Therefore, V O2max was reached with the maximal value of Q and a-vO2 diff during the incremental test. Q -V O2 responses during two severe exercises eliciting V O2max. Despite the fact than tlim50 elicited V O2max (6,14), the maximal stroke volume was not reached. In a pilot study, Billat et al. (9) have reported that during a constant load exercise at 90% of pV O2max (11 min 42 s 4 min 7 s), both SV and V O2 increased until exhaustion reaching the maximal value measured in an incremental test. The SV increase with time in constant supra-threshold ex- ercise was not observed in the present study. This is prob- ably due to the fact that the work rate was not sufficient because the intensity that elicited SVmax in the increment test was higher than p50 (89.2 5.3 vs 88.0 1.5% of pV O2max, P 0.01). Also, the exercise duration was put forward as being a major factor for the achievement of V O2max without Q max attainment. This "Q V O2 disassociation" has been attributed to the lower stroke volume in an incremental test lasting 12 min versus 6 min (21). In spite of similar maximal values of oxygen uptake and heart rate, maximal stroke volume and maximal cardiac output values were lower in the 12-min than 6-min test. A longer exercise Medicine & Science in Sports & Exercise 1361
duration could induce a vasodilatation and an increased vascular conductance via the muscle pump in skeletal muscle
considered as a determinants of sustained exercise hyperemia in skeletal muscle (13). However, the duration of exercise, even in exercise eliciting V O2max (so-called "heavy exercise," (29)) is not sufficient to elicit SVmax. It seems that in trained subjects, 90% of pV O2max is an intensity threshold for the attainment of maximal SV both in an incremental test (with 3-min stages) and constant-load exercise. This threshold intensity to reach the maximal stroke volume attainment could depend on the quadriceps muscle mass involved in the cycling exercise, which might be higher in tlim100 compared with tlim50. At the onset of dynamic exercise, the mechanical effects
of the muscle pump increased the arterial-venous pressure gradient (12). Therefore, the pumping action of contracting skeletal muscle appeared to form an important regulator of increasing blood flow. The rate of increase in blood flow during exercise is closely coupled to motor unit recruitment with dilation beginning at the first contraction (26). Indeed, Nassis and Geladas (22) have recently demonstrated that the decline in stroke volume during an exercise at 60% of pV O2max was higher in cycling than in running. They showed a moderate correlation between the one-leg quadriceps muscle mass and SV decline in cycling, implying that individuals with high thigh muscle mass would present a smaller SV decline in cycling compared with running (22). In the present study, we have one exercise mode (cycling) but two intensities that have a difference of 40 W, i.e., 4.44 kgF (at a rate of pedaling equal to 90 rpm). This difference was sufficient to reach or not reach the maximal stroke volume at the onset of the dynamic exercise. The quicker recruitment of a large muscle mass at the onset of exercise at the higher intensity was responsible to the attainment of maximal value of SV. Alternatively, the progressive recruitment of new unit motors associated with respiratory muscles (19,23,25) could be responsible for the attainment of V O2max without the Q max during tlim50. Arterial-venous O2 difference responses during two severe exercises eliciting V O2max. Our results showed that a-vO2 diff was not significantly different between the three exercises. However, a-vO2 diff narrows less quickly in the lower work rate test (tlim50). McCole et al. (21) postulated that the lower stroke volume during 12-min versus the 6-min incremental test could be in part compensated by the increased of arterial-venous O2 difference (a-vO2 diff). The time course of a-vO2 diff may be the result REFERENCES 1. ASTRAND, P. O., T. E. CUDDY, B. SALTIN, and J. STENBERG. Cardiac output during submaximal and maximal work. J. Appl
. Physiol. 19:268 274, 1964. 2. AUNOLA, S., and H. RUSKO. Reproducibility of aerobic and anaerobic thresholds in 20 50 years old men. Eur. J. Appl. Physiol. 53:260 266, 1984. 3. BENOIT, H., F. COSTES, L. FEASSON, et al. Accuracy of pulse oximetry during intense exercise under severe hypoxic conditions. Eur. J. Appl. Physiol. Occup. Physiol. 76:260 263, 1997.
of a rise of lactic acidosis and a subsequent decrease in pH above lactate threshold (27). In our study, the blood lactate level was not different between the three exercises. Previously, Demarie et al. (14) found a correlation between the vastus lateralis deoxygenation changes with the oxygen uptake changes and the blood lactate increase between the third minute of exercise and the time to fatigue. Furthermore, the decrease in a-vO2 diff was associated with a hypoxemia induced by exercise (SaO2rest end of exercise), which was also more pronounced during tlim50 than in incremental test. Thus, muscle de-oxygenation from 6065% of V O2peak could be attributed to capillary-venular hemoglobin desaturation, as a consequence of a rightward shift of the oxy-hemoglobin dissociation curve determined by the onset of lactic acidosis (17). On the other hand, during severe exercise below pV O2max, the right shift of the oxy-hemoglobin dissociation curve on tissue oxygenation would be responsible of the attainment of maximal value of V O2. On the contrary, the longer duration of the exercise would be responsible for a more marked hypoxemia which would counterbalance the BoЁhr effect and would be responsible for ending the exercise. Hence, we can define pV O2max as the work rate allowing matching Q max with maximal value of a-vO2 diff, both in incremental and constant work rate. This work rate (pV O2max) has as also been reported as being the work rate eliciting V O2max for the longest duration in constant load exercise (6). CONCLUSION Despite there being no difference in the maximal value of V O2, the V O2max reached during the incremental test resulted in a greater maximal Q compared with the constant p50 workload exercise. The lower SV seems to be the main contributor of the nonachievement of Q max during time to exhaustion at p50 whereas V O2 reached its maximal value in the both cases. So, V O2max was not reached with the same central and peripheral factors in exhaustive exercises performed at 88.0 1.5 or 100% of pV O2max. This might be taken into account for training at V O2max using different work rates (90 100% pV O2max). The authors thank Dr. Jean Slawinski for contributions to the presentation of this manuscript. This study was supported by grants from Caisse Centrale des Activiteґ s Sociales d'Electriciteґ et de Gaz de France, la Fondation Gaz de France, Geґ nopole®, and le Conseil Reґ gional d'Ile de France. 4. BERSTEIN, D. P. A new stroke volume equation for thoracic bioimpedance: theory and rationale. Crit. Care Med. 14:904 909, 1986. 5. BILLAT, V. L., V. BINSSE, and J. P. KORALSZTEIN. High level runners are able to maintain a VO2 steady-state below VO2max in an all-out run over their critical velocity. Arch. Physiol. Biochem. 106:38 45, 1998. 6. BILLAT, V. L., N. BLONDEL, and S. BERTHOIN. Determination of the velocity associated with the longest time to exhaustion at maximal oxygen uptake. Eur. J. Appl. Physiol. 80:159 161, 1999.
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V O2MAX-Q DISSOCIATION DURING HEAVY EXERCISE
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P Lepretre, J Koralsztein