VO 2 Kinetics During Heavy and Severe Exercise in ...

Training & Testing 1

IJSM/2414/22.3.2012/Macmillan IJSM/2414/22.3.2012/Macmillan

VO2 Kinetics During Heavy and Severe Exercise in Swimming

Authors Affiliations

Key words ▶ slow component ● ▶ exercise intensity domains ● ▶ front crawl ●

D. M. Pessoa Filho1, F. B. Alves2, J. F. Reis2, C. C. Greco1, B. S. Denadai3 1

Human Performance Laboratory, Physical Education, Rio Claro, Brazil Faculty Human Kinetics, Sport Sciences, Lisbon, Portugal 3 UNESP, Human Performance Laboratory, Rio Claro, Brazil 2

Abstract ▼ The purpose of this study was to describe the VO2 kinetics above and below respiratory compensation point (RCP) during swimming. After determination of the gas-exchange threshold (GET), RCP and VO2max, 9 well-trained swimmers (21.0 ± 7.1 year, VO2max = 57.9 ± 5.1 ml.kg − 1.min − 1), completed a series of “square-wave” swimming transitions to a speed corresponding to 2.5 % below (S − 2.5 %) and 2.5 % above (S+2.5 %) the speed observed at RCP for the determination of pulmonary VO2 kinetics. The trial below (~2.7 %) and above RCP (~2 %) was performed at 1.28 ± 0.05 m.s − 1 (76.5 ± 6.3 % VO2max) and 1.34 0.05 m.s − 1 (91.3 ± 4.0 % VO2max),

Introduction ▼

accepted after revision December 10, 2011 Bibliography DOI http://dx.doi.org/ 10.1055/s-0031-1299753 Int J Sports Med 2012; 33: 1–5 © Georg Thieme Verlag KG Stuttgart · New York ISSN 0172-4622 Correspondence Dr. Benedito Sérgio Denadai, PhD UNESP, Human Performance Laboratory Av. 24 A, 1515 13506-900 Rio Claro Brazil Tel.: +55/19/3526 4325 Fax: +55/19/3526 4321 [email protected]

Exercise intensity domains have been defined based on the pulmonary oxygen uptake (VO2) and blood lactate responses to constant work rate exercise [13]. During the transition from rest to constant work rate exercise of moderate intensity [i. e., below the lactate threshold (LT) or gas-exchange threshold (GET)], VO2 response approaches the new steady state with an exponential time course (the “primary” phase or phase II), after a short delay-like period (phase I or cardiodynamic phase) [37]. During the exercise performed in the heavy domain, which includes intensities above LT/GET, a slow component of the VO2 kinetics (VO2SC) is superimposed upon the rapid response, resulting in a delayed (15–20 min) submaximal steady state. Finally, in the severe intensity domain (i. e., above critical power – CP), a steady state is never achieved and VO2 increases progressively in a biexponential fashion, reaching its maximal values at the end of exercise [26]. It has been demonstrated that both the primary and slow components of the VO2 response reflect events occurring within the

respectively. The time constant of the primary component was not different between the trials below (17.8 ± 5.9 s) and above RCP (16.5 ± 5.1 s). The amplitude of the VO2 slow component was similar between the exercise intensities performed around RCP (S − 2.5 % = 329.2 ± 152.6 ml. min − 1 vs. S+2.5 % = 313.7 ± 285.2 ml.min − 1), but VO2max was attained only during trial performed above RCP (S-2.5 % = 91.4 ± 5.9 % VO2max vs. S+2.5 % = 103.0 ± 8.2 % VO2max). Thus, similar to the critical power during cycling exercise, the RCP appears to represent a physiological boundary that dictates whether VO2 kinetics is characteristic of heavy- or severe-intensity exercise during swimming.

exercising muscle [27]. Thus, pulmonary gasexchange kinetics is used to offer important noninvasive insights into muscle oxidative energy turnover in response to a transition to a higher metabolic rate (i. e., following the onset of exercise) [14, 15, 22]. The VO2 kinetic response to moderate and heavy/ severe constant load exercise has been well described in both cycling and running [5, 6]. However, few studies have analyzed the VO2 kinetic response in swimming, which imposes different biomechanical and physiological challenges when compared with land activities such as running and cycling. Demarie et al. [10] used a miniaturized telemetric metabolimeter with a mixing chamber to obtain VO2 values (average every 15 s) and determined the VO2SC during swimming at 50 %Δ (i. e., 50 % of the difference between critical speed and speed at VO2max). The VO2SC was calculated as the difference between the mean VO2 value between the 6th and 3rd min of the exercise, using only one transition. In a more recent study, Fernandes et al. [12] assessed pulmonary VO2 kinetics on a breath-by-breath basis and modeled the VO2 response during

■ Proof copy for correction only. All forms of publication, duplication or distribution prohibited under copyright law. ■ Filho DMP et al. VO2 Kinetics During Heavy … Int J Sports Med 2012; 33: 1–5

2 Training & Testing

swimming exercise. This improved temporal resolution would increase the confidence level in the analysis of VO2 kinetics. However, similar to Demarie et al. [10], the VO2SC was estimated using only one transition. It is important to note that the determination of the parameters of VO2 response in the heavy/severeintensity domain using only one transition can be associated with potential limitations (i. e., low confidence in response parameters) [11, 21]. The precision with which the parameters characterizing the non-steady state response can be estimated is influenced by the magnitude and distribution of breath-bybreath fluctuations (noise). Averaging of repeated bouts has been traditionally carried out to improve the signal-to-noise ratio of data [21]. Thus, both high temporal resolution and signal-to-noise ratio are essential to achieve confidence in characterizing the dynamics of VO2 [11]. Since VO2 kinetics differs within the various exercise intensity domains [7], it is important to understand the physiological boundaries that define the exercise intensity domains. The boundary that separates the heavy and severe exercise intensity domains is the critical power (CP) or critical speed (CS) [17]. However, measurement of the CP/CS is especially demanding in terms of both subject effort and testing time. Alternatively, other metabolic (blood lactate response) and cardio-respiratory (respiratory compensation point – RCP) indexes determined during the incremental test might be used to estimate the lower boundary between heavy and severe-intensity domains. Indeed, Dekerle et al. [8] have verified that RCP was similar to CP in cycling. However, it is unclear whether swimming at speeds just above and just below the RCP elicits VO2 dynamics that are characteristics of severe- and heavy-intensity exercise. Thus, the objective of this study was to characterize the VO2 kinetics around the RCP during swimming exercise. It was hypothesized that: 1) Phase II VO2 kinetics would be similar when swimming below and above the RCP; 2) the VO2max would be attained at a swimming speed above the RCP (indicative of severe-intensity exercise), but not at a swimming speed below the RCP (indicative of heavy-intensity exercise).

Material and Methods ▼ Subjects 9 well-trained male swimmers (21.0 ± 7.1 year, 178.2 ± 6.4 cm, 68.6 ± 9.1 kg, 13.0 ± 4.0 % body fat and VO2max = 3 999.2 ± 726.0 ml. min − 1) volunteered and gave written informed consent to participate in the present study, which was approved by the university’s ethics committee. Additionally, the experiment conformed to international ethical standards as required by the International Journal of Sports Medicine [16]. Participants competed at national championship level meets over middle to long distance (400–1 500 m) and had trained for at least 5 years, around 8 times a week. Their mean 400-m front crawl performance recorded during a training session was 257.6 ± 9.7 s at the time of the study (or 85.8 ± 3.2 % of the current world record velocity). Moreover, all the swimmers were frequent participants in experimental studies undertaken by our research group and, as such, were fully familiar with the test procedures and equipment employed in this study. The participants were instructed to refrain from intense training sessions at least 24 h before the experimental sessions. They were required to report to the laboratory in a rested state and to have refrained from food or bever-


ages containing caffeine, drugs, alcohol, cigarette smoking, or any form of nicotine intake 24 h before testing.

Experimental design Swimmers firstly performed an incremental test to exhaustion to determine the GET, RCP and VO2max. Participants then performed in randomized order the following protocols: a series of “square-wave” transitions of 7-min duration at 2.5 % below (S − 2.5 %) and above (S+2.5 %) the speed at RCP in order to determine the pulmonary VO2 kinetics. A standard warm-up was consistently performed before each test. All tests involved in-water starts and open turns without underwater gliding, and all took place in the same indoor 50 m pool (water temperature fixed at 29 °C). The swimmers adjusted their speed within each test to that prescribed by the first investigator, on the basis of acoustic (at every 25th m) and visual feedback. The swimmers were asked to maintain pace by keeping their feet at the level of the investigator walking on the deck. The difference between predicted and actual swim speed at every 50th meter was not higher than 0.01 m.s − 1 in all tests. Cardio-respiratory analysis of expired air was performed during all tests, using a breath-by-breath analyzer (K4b2, Cosmed, Italy). The K4b2 was calibrated prior to each test according to the manufacturer’s instructions before being connected to the swimmer by a previously validated [19] respiratory snorkel and valve system (Aquatrainer, Cosmed, Italy). The temperature of the flowmeter was also set to ambient temperature (24 ± 1 °C), in accordance with the manufacturer’s instructions. The swimmers performed 1 test per day with all tests being conducted within a 14-day period. At least 48 h separated the 2 experimental tests. Testing occurred at the same time of the day ( ± 2 h) to minimize the effect of circadian variation on performance.

Procedures The subjects performed a continuous incremental test (300-m stages) to voluntary exhaustion. The speed of the first 300-m stage was set at 70 % of maximal 400-m speed (S-400), and was subsequently increased by 5 % with the last stage completed at 100 % S-400. During the incremental test, the swimmers performed 6–8 stages. VO2max was calculated as the highest 30-s VO2 value reached during the incremental test. All participants fulfilled at least 2 of the following 3 criteria to ascertain VO2max was attained: (1) a respiratory exchange ratio (RER) greater than 1.1; (2) a peak HR at least equal to 90 % of the age-predicted maximal and; (3) the identification of a plateau of less than 150 ml.min − 1 between 2 subsequent stages The speed at VO2max (SVO2max) was defined as the minimal speed at which VO2max occurred [10]. GET and RCP were examined visually, using VE/ VCO2, VE/VO2, PETCO2 and PETO2 parameters [1, 35]. GET criteria were: increase in the VE/VO2 and in PETO2, without a concomitant change in VE/VCO2 and PETCO2, respectively. For RCP, the criterion used was the continuous increase in VE/VO2 and VE/VCO2 with a concomitant reduction in PETCO2. 2 independent investigators analyzed the plots of each index to determine GET and RCP. In the event that the investigators’ results did not agree, a third investigator was consulted. The VO2 at GET and RCP was determined using the average VO2 value of the last 30-s of the stage. The subjects subsequently performed 2 repetitions of “squarewave” transitions of 7-min duration at 2.5 % below (S − 2.5 %) and above (S+2.5 %) the speed at RCP. After a 3–4 min warm-up at a low




swimming speed (~ 80 % sVO2max) followed by 5 min of rest, the subjects were instructed to swim at the required speed. After a 1-h passive recovery period, the subjects performed an identical square-wave transition using the same exercise intensity of the first test [3]. The tests performed below and above RCP were conducted on different days.

VO2 kinetics modeling The breath-by-breath data from each exercise test were filtered manually to remove outlying breaths, defined as breaths ± 3SD from the adjacent 5 breaths. For each exercise transition, the breath-by-breath data were interpolated to give second-by-second values. The transitions for each exercise intensity were then time aligned to the start of exercise and averaged to enhance the underlying response characteristics. The first 20 s of data after onset of exercise (i. e., the cardiodynamic phase) [36] were deleted from the analyses. The individual “snorkel delay”, which corresponded to the difference between the onset of exercise and the time when the following breaths summed a tidal volume superior to the outlet tube volume, was then integrated into the time delay of the primary phase [29]. A nonlinear least-squares algorithm was used to fit the data, as described in the following equation: VO2(t) = VO2baseline + A1 × (1 – e–(t – TD)/τ)

(1)

Where VO2(t) represents the absolute VO2 at a given time t; VO2baseline represents the mean VO2 in the 30 s of the baseline period; A1 is the amplitude; TD is the time delay; and τ is the time constant. An iterative process was used to minimize the sum of the squared errors between the fitted function and the observed values, and the fitting window was constrained to the time point at which a departure from fundamental monoexponentiality occurred (as judged from visual inspection of a plot of the residuals of the fit) [3, 34]. The absolute primary amplitude (A’1) was defined as the sum of VO2baseline and A1. The end-exercise VO2 (EEVO2) was defined as the mean VO2 measured over the final 30 s of exercise. The VO2SC (A’2) was calculated as the difference between the absolute primary amplitude and the EEVO2. The relative contribution of slow component to net increase in VO2 at end exercise (A’2 %EEVO2) was also calculated [A’2/(A1+ A’2) × 100]. To provide a description of the overall kinetic response [i. e., mean response time (MRT)], the above equation with TD constrained to 0 s (i. e., no delay term) was fit from exercise onset to the end of exercise. The VO2 was assumed to have essentially reached its maximal value (i. e., VO2max) when the value of (1 – e-(t/τ)) from Eq. 1 was 0.99, (i. e., when t = 4.6 × τ) [4].

VO2max) of GET, RCP, and VO2max. The RCP corresponded to the midpoint between GET and VO2max. There was no significant difference between predicted and actual swim speed for both below and above RCP conditions ▶ Table 2). (● The VO2 kinetics parameters of the “square wave” transitions are ▶ Table 3. The TD and τ were not significantly difdescribed in ● ferent between the intensities around RCP (p > 0.05). The MRT was significantly longer during swimming below RCP (p < 0.01). The values of A’1, EEVO2, and % VO2max were significantly higher while swimming above RCP rather than below RCP. However, both A’2 and % A’2EEVO2 were not significantly different between the intensities around RCP (p > 0.05). The time to achieve VO2max ▶ Fig. 1 shows during swimming above RCP was 152.2+32.9 s. ● the VO2 kinetic response for one representative subject exercising at 2.5 % below (S − 2.5 %) and above (S+2.5 %) the speed at RCP.

Table 1 Mean ( ± SD) values of gas-exchange threshold (GET), respiratory compensation point (RCP) and maximal oxygen uptake (VO2max) obtained during the incremental test. N = 9.

VO2max GET RCP

VO2

VO2max

Speed

Delta

(ml.kg − 1.min − 1)

( %)

(m.s − 1)

( %∆)

57.9 (5.1) 41.1 (7.2) 49.8 (5.5)

100 70.5 (8.0) 85.9 (4.8)

1.40 (0.03) 1.21 (0.06) 1.32 (0.05)

100 0 50.9 (16.5)

%∆, percent difference between VO2max and the GET

Table 2 Reference and performed values for exercise transitions around respiratory compensation point (RCP). Values are Mean ( ± SD). N = 9. Trial

Speed (m.s − 1)

Relative to RCP ( %)

Delta ( %∆)

1 2

1.35 (0.05) 1.29 (0.05)

2.5 − 2.5

71.0 (11.3) 20.1 (7.0)

1 2

1.34 (0.05) 1.28 (0.05)

2.0 (1.2) − 2.7 (0.7)

69.9 (13.3) 20.5 (8.6)

Reference

Performed

%∆, percent difference between maximal oxygen uptake and the gas-exchange threshold

Table 3 Parameters of VO2 on-kinetics around respiratory compensation point (RCP) N = 9. Parameters −1

Statistical analysis Data are presented as mean ± SD. Normality of the distribution was checked by the Shapiro-Wilk’s W test. The effects of exercise intensity on VO2 kinetics responses were analyzed using Paired t-test. Significance was set at p ≤ 0.05.

Results ▼ ▶ Table 1 presents mean ± SD values of VO (absolute and rela● 2 tive), speed and %Δ (i. e., % of the difference between GET and

VO2Baseline (ml.min ) TD (s) τ (s) A’1 (ml.min − 1) A’2 (ml.min − 1) A’2 %EEVO2 EEVO2 (ml.min − 1) %VO2max attained MTR (s)

2.5 % below RCP

2.5 % above RCP

474.90 ± 91.24 17.9 ± 3.5 17.8 ± 5.9 3313.4 ± 599.7 329.2 ± 152.6 10.4 ± 4.3 3642.6 ± 660.8 91.4 ± 5.9 40.3 ± 10.9

523.78 ± 79.08 14.5 ± 6.1 16.5 ± 5.1 3796.1 ± 677.0† 313.7 ± 285.2 8.3 ± 6.1 4109.9 ± 808.0† 103.0 ± 8.2† 33.1 ± 7.2†

Values are means ± SD. τ, the time constant; TD, time delay; A’1, absolute primary amplitude; A’2 and % A’2 EEVO2, amplitude of the slow component expressed as absolute and relative values, respectively; EEVO2, end-exercise VO2; MTR, mean response time. † P < 0.05 in relation to 2.5 % below RCP


4 Training & Testing


VO2 (L.min –1)

3.0 2.5

VO2max

2.0

RCP GET

1.5 1.0

2.5% < RCP 2.5% > RCP

0.5 0.0 –100

0

100

200 Time (s)

300

400

500

Fig. 1 Pulmonary oxygen uptake (VO2) around respiratory compensation point (RCP) for one representative subject. The baseline for VO2 at each intensity is illustrated in the 30 s prior to the beginning of the exercise (time zero). GET, gas-exchange threshold; VO2max, maximal oxygen uptake.

Discussion ▼ The objective of this study was to describe the VO2 kinetics during swimming exercise performed above and below the RCP. In accordance with studies investigating running exercise [7], the primary phase of the VO2 response was similar above and below the RCP. Since the VO2max was only attained when exercising above RCP, these data suggest that the RCP may be used to discriminate between the heavy and severe exercise intensity domains during swimming exercise. It is acknowledged that the main parameter which discriminates between swimming speeds that reside in the heavy and severe exercise intensity domains (i. e., CS) was not determined. However, the exercise intensity at RCP expressed as % sVO2max (94 %) is similar to CS reported by Demarie et al. [10] (91.4 % sVO2max) and Dekerle et al. [9] (92 % S-400) during swimming exercise. Moreover, RCP corresponded to 51 %Δ, or half-way through between GET and VO2max. Accordingly, in running [32] and cycling exercise [8, 33], the boundary between the heavy and severe-intensity domains, has been associated with 50–60 %∆. Collectively, these data support that RCP can be used to demarcate the heavy- and severe-intensity exercise domains in swimming. Our time constant values were similar to those recently reported by Reis et al. [30] for highly trained swimmers during both heavy (15.8 s) and severe (15.8 s) exercise domains. During supine exercise, where the musculature is at or above heart level, the gravitational assistance to blood flow is abolished, and O2 delivery is slowed [23]. In these conditions, slower VO2 kinetics has been attributed to insufficient muscle O2 availability [23]. In swimming, the acute adaptations (i. e., increase in intrathoracic blood volume, cardiac output, and muscular blood flow) imposed by constant hydrostatic pressure from the micro-gravitational environment [25], might counterbalance the lack of gravitational assistance to blood flow during supine exercise. Thus, it is not surprising that the time constant for the primary component in severe/heavy exercise for the present study (~17 s), is similar to values reported previously in trained individuals during cycling [20] and running [2] performed at similar exercise intensities as those investigated here. Moreover, similar to that found during both swimming [30] and running [7], the primary phase (τ) of the VO2 response was invariant at different exercise intensities performed around RCP. Thus, in swimming it is likely that both O2 delivery, its utilization, and their interac-

tion are not modified during the exercise performed below and above RCP. Similar to previous studies [10, 12, 30, 31], our VO2 kinetics data confirm the existence of a VO2SC during swimming performed above moderate exercise intensity domain (i. e., > GET). However, caution is required to compare these data, since the experimental conditions (i. e., exercise intensity and confidence in response parameters) were not similar among studies. There is a substantial body of evidence showing that the VO2SC originates primarily from within the exercising muscles [27, 28]. Additionally, different experimental designs (electromyogram, glycogen depletion, and selective slow-twitch fiber neural blockade), have supported involvement of muscle fiber activation patterns in the development of the VO2SC (see Jones et al. [18], for a review). During constant-work-rate exercise, VO2SC has been associated with the progressive recruitment of additional higher-order (type II) muscle fibers, which have been considered to have poor efficiency compared with type I fibers. Recent studies, however, performed in animal models (electrically stimulated canine gastrocnemius muscle contracting in situ) [38] as well in humans (maximal voluntary exercise, i. e., 3-min “all-out” test) [34] have demonstrated that progressive fiber recruitment is not requisite for development of the VO2SC. In land activities, the VO2SC is higher for cycling than running during heavy [5] and severe exercise domain [5, 24]. At similar exercise intensities (i. e., %Δ), our VO2SC values were apparently lower than those measured for cycling and within the range of those reported for running [5]. These data may be related to the biomechanical differences between exercise modes. Differences in muscle efficiency and contraction type (i. e., isometric vs. concentric vs. eccentric) may determine different muscle fiber activation patterns for the same exercise intensity and duration, and consequently, modifies the VO2SC. Finally, exercise at S+2.5 % RCP resulted in the attainment of VO2max. During the S − 2.5 % RCP trial VO2 increased to only~92 %VO2max, and appeared to reach a plateau. Thus, the RCP seems to be close to the lower boundary of the severe-intensity domain, at least during swimming exercise. However, since the S − 2.5 % RCP trial was not performed until exhaustion, it is possible that the VO2max might have been attained in this trial, had the exercise duration been extended. In the severe intensity domain, the time to attain VO2max is dependent on the exercise intensity and aerobic status training [4]. In our study, the time to attain VO2max (~150 s) was similar to that found in well-trained athletes during cycling (~160 s) performed in the severe intensity domain (95 % vVO2max) [4]. In this regard, 7-min severe swim exercise may be considered as a possible alternative to incremental test for the determination of VO2max. However, a great individual variability of end-exercise VO2 has been found (93–110 % VO2max), suggesting that not all subjects were able to achieve a true VO2max during constant swim speed. Whether this may be attributed to day-to-day biological variation in VO2 or individual characteristics (e. g., those with a large anaerobic capacity and/or rapid VO2 kinetics) requires further work. In summary, we have demonstrated that, similar to land activities such as running and cycling, the primary phase of the VO2 response was similar at different exercise intensities performed around RCP. Although a VO2SC does indeed develop in both exercise intensities, VO2max was only attained when exercising above RCP. Thus, expected VO2 kinetics was evident when exercising below and above the RCP, respectively, during swimming exercise in humans.



Acknowledgements ▼ We thank the subjects for participation in this study, FAPESP, FUNDUNESP and CNPq for financial support. These data were originally presented and published in the following publication: Pessoa Filho DM, Reis JF, Alves FB, Denadai BS. In: Kjendlie PL, Stallman RK, Cabri J (eds). Oxygen uptake kinetics around the respiratory compensation point in swimming. Proceedings of the XIth International Symposium for Biomechanics and Medicine in Swimming, Oslo: Norwegian School of Sport Science, 2010; 215–217.

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