Eur J Appl Physiol (2012) 112:1637–1644 DOI 10.1007/s00421-011-2123-9
ORIGINAL ARTICLE
Similarity in physiological and perceived exertion responses to exercise at continuous and intermittent critical power Lu´cio Fla´vio Soares-Caldeira • Nilo Massaru Okuno • Marcelo Magalha˜es Sales • Carmen Sı´lvia Grubert Campbell Herbert Gustavo Simo˜es • Fa´bio Yuzo Nakamura
•
Received: 5 April 2011 / Accepted: 9 August 2011 / Published online: 28 August 2011 Ó Springer-Verlag 2011
Abstract The purpose of this study was to compare the physiological responses [oxygen uptake (VO2), heart rate (HR) and blood lactate concentrations ([BLa])] and the rating of perceived exertion (RPE) response until exhaustion (TTE) at the continuous (CPc) and intermittent (CPi) critical power workloads. Ten moderately active men (25.5 ± 4.2 years, 74.1 ± 8.0 kg, 177.6 ± 4.9 cm) participated in this study. The incremental test was applied to determine the highest values of oxygen uptake (VO2max), heart rate (HRmax), blood lactate concentrations ([BLamax]), and maximal aerobic power (MAP). Continuous and intermittent exhaustive predictive trials were performed randomly. The hyperbolic relation between power and time was used to estimate CPc and CPi. CPi was derived from predictive trial results at an effort and recovery ratio of 30:30 s. Exercise at CPc and CPi as well as the physiological and RPE responses were measured until exhaustion. The values of physiological variables during CPc and CPi did not differ in either TTE test and were lower than the VO2max, HRmax and [BLamax] values. RPE was maximal at Communicated by David C. Poole. L. F. Soares-Caldeira N. M. Okuno F. Y. Nakamura Grupo de Estudo das Adaptac¸o˜es Fisiolo´gicas ao Treinamento (GEAFIT). Centro de Educac¸a˜o Fı´sica e Esporte, Universidade Estadual de Londrina, Londrina, PR, Brazil L. F. Soares-Caldeira (&) Departamento de Educac¸a˜o Fı´sica. Centro de Cieˆncias Biolo´gicas e da Sau´de, Universidade Norte do Parana´, Avenida Paris, 675, Jardim Piza, Londrina CEP 86041-120, Brazil e-mail:
[email protected] M. Magalha˜es Sales C. S. G. Campbell H. G. Simo˜es Grupo de Estudos do Desempenho Humano e das Respostas Fisiolo´gicas ao Exercı´cio, Universidade Cato´lica de Brası´lia, Brası´lia, DF, Brazil
the end of exercise at CPc and CPi. There was a high correlation between VO2max (L min-1) and CPc and CPi intensities (r C 0.90) and between MAP, CPc and CPi (r C 0.95). Similar physiological and RPE responses were found at CPc and CPi for the times analyzed. Keywords Critical power Physiological responses Perceived exertion Time to exhaustion Intermittent effort
Introduction There is a hyperbolic relationship between power output and time to exhaustion (TTE) during continuous and intermittent exercise. The power asymptote corresponds to the theoretical intensity that can be maintained without inducing fatigue and is called critical power (CP), while the hyperbolic curvature is thought to represent a finite energy store (W0 ) that is used at intensities above CP (Monod and Scherrer 1965). Since Moritani et al. (1981) were the first to apply the critical power model to cycling ergometry using only continuous predictive trials, the derived parameters will be referred to in the present study as continuous CP (CPc) and W0 (W0 c). Some studies have suggested that CPc corresponds to the highest power output associated with the blood lactate steady-state profile (i.e., maximal lactate steady-state) (Poole et al. 1988, 1990), and exercise above this intensity may elicit maximal oxygen consumption (VO2max) (Hill and Smith 1999; Jones et al. 2010) and entail a progressive rise in blood lactate concentration ([BLa]) (Poole et al. 1988, 1990). On the other hand, recent investigations have suggested that CPc represents the highest intensity that can be maintained for a relatively prolonged time (20–40 min) without causing VO2max, even in the absence of a metabolic or physiological steady-state (Brickley
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et al. 2002; Dekerle et al. 2009; Williams et al. 2008). This means, therefore, that CP slightly overestimates the maximal lactate steady-state intensity (Pringle and Jones 2002; Smith and Jones 2001; Simo˜es et al. 2005). However, little is known about the application of this model to intermittent exercise. Its application would be important because intermittent loads are commonly used with high-level athletes and in fitness programs (Laursen and Jenkins 2002). Dupont et al. (2002) and Berthoin et al. (2006) applied the critical power model to exhaustive running trials performed intermittently, using a 15:15 s exercise to rest ratio in both adults and children, respectively. Berthoin et al. (2006) showed high goodness of fit values (R2 [ 0.99) for the linear distance– time equation, which is mathematically equivalent to the aforementioned hyperbolic relationship. The estimated intermittent critical velocity was highly correlated with continuous velocity (r = 0.86), although the former was significantly higher than the latter. This finding was recently confirmed while comparing intermittent CP (CPi) derived from 30:30 s protocols with CPc in cycle ergometry (Okuno et al. 2011). Furthermore, Dupont et al. (2002) showed that VO2max can be maintained for different durations in an intensity-dependent manner at several workloads above intermittent critical velocity (i.e., 110–140% of maximal aerobic speed). Nevertheless, there are no studies that describe the physiological and perceptual responses and TTE of exercise at CPi and compare these variables with the physiological responses to exercise at CPc. This knowledge could further contribute to our understanding of the physiological meaning of CPi and help physical trainers plan individualized intermittent training sessions based on an intensity that can be maintained for a prolonged time without reaching VO2max. In theory, any intensity above CPi would induce VO2max. Hence, training designed to improve this important fitness component should be planned according to the individual power–time relationship and the time that VO2max is to be maintained at each intensity above CPi (Billat et al. 2000; Dupont et al. 2002). The objective of this study was to examine the physiological responses, the perceived exertion response and the TTE associated with exercise at CPi and to compare them with those obtained at CPc. Our hypothesis was that despite differences in power output, the associated physiological and perceptual responses would be similar, evidencing the usefulness of CPi for providing the upper limit for prolonged intermittent exercise without reaching VO2max.
Methods Subjects The sample size was calculated based on the assumption that 1.2 ± 2.2 mM of difference in [BLa] at the steady
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phase of prolonged exercise at CPi and maximal lactate steady-state is meaningful (Okuno et al. 2011). We used Morepower 4.9 software (Campbell and Thompson 2002) to determine that a sample size of ten subjects was the minimum needed to provide a power of 80% with an alpha of 0.05 for the analysis. Hence, ten men (25.5 ± 4.2 years, 74.1 ± 8.0 kg, 177.6 ± 4.9 cm) were recruited. All subjects were informed about the procedures and risks associated with participating in the study and gave their written informed consent. The research was approved by the institutional research ethics committee. The subjects were required to keep their usual dietary habits and avoid intense exercise and caffeinated or alcoholic beverages in the 24 h prior to the tests. Moreover, the subjects were instructed not to fast, but to have a light meal *3 h prior to testing. All of the subjects were familiar with the testing procedures since they had already taken part in studies involving similar procedures within the past 6 months. Experimental design Testing took place in three phases: (1) maximal incremental test; (2) four continuous and four intermittent predictive trials; and (3) one exhaustive test at CPc and one at CPi. In phase 2 the order of the tests was randomized, alternating between continuous and intermittent predictive trials. Subsequently, the randomization was also applied to the testing order at CPc and CPi. All tests were performed on different days at approximately the same time of day, at least 3 h postprandial, and with a room temperature ranging from 20 to 24°C. The study was completed in a 5-week period. Equipment All tests were performed on an electromagnetically braked cycle ergometer (LODEÒ, Excalibur Sport, Groningen, The Netherlands). Heart rate (HR) was measured by means of a telemetric device (PolarÒ S810i Electro Oy, Kempele, Finland) with RR intervals recorded beat-by-beat throughout the tests. The HR data were analyzed using Polar Precision Performance 4.03 software, which was equipped with a filter to remove occasional ectopic beats. The oxygen uptake analyses (VO2) were performed in the breath-by-breath mode using an open circuit system (MetaLyzer 3B, Cortex, Leipzig, Germany). The equipment was calibrated prior to each test using ambient air and gas of known O2 (16%) and CO2 (5%) concentrations. The turbine flow-meter was calibrated using a 3-L syringe. These procedures followed the manufacturer’s instructions. For blood lactate analyses, 25 ll of blood were sampled from the earlobe with heparinized capillary tubes. After each collection, blood samples were immediately
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transferred to Eppendorf tubes containing 50 ll of NaF (1%), and the mixture was stored on ice until the [BLa] was determined with an electrochemical device (YSI 2300 Select, Ohio, USA). Incremental exercise testing After a short warm-up period (3 min at 30 W), the subjects remained seated and resting for 3 min for baseline measurements. Subsequently, the test began at 30 W (70 rpm) with increments of 10 W min-1 until the subjects reached voluntary exhaustion or until failure to maintain the minimum cadency of 65 rpm for a period longer than 5 s despite verbal encouragement. Strong verbal encouragement was provided throughout the test. Gas exchanges were recorded in breath-by-breath mode (MetaLyzer 3B, Cortex, Leipzig, Germany) while HR was recorded at each 5-s interval (PolarÒS810i Electro Oy, Kempele, Finland). HRmax was determined as the average of the final 10 s of the recordings. The VO2max was considered as the average of the final 30 s of the recordings of the incremental test. No secondary criteria for a true VO2max (e.g., age predicted HRmax ± 10 bpm or [BLa] C 8 mM) value was used due to evidence showing that such criteria do not necessarily indicate maximal exertion levels (Poole et al. 2008). Maximal aerobic power (MAP) was considered the power generated during the complete stage prior to exhaustion. At the end of the incremental test, a blood sample was collected to analyze the blood lactate concentration ([BLamax]). Predictive trials The subjects performed, in random order, four continuous and four intermittent predictive trials until exhaustion. The power outputs led to test durations between 2 and 15 min (Dekerle et al. 2006) in both continuous and intermittent trials. At the end of each trial, oxygen consumption (VO2end), blood lactate concentration ([BLaend]), heart rate (HRend), and rating of perceived exertion (RPEend) on the 15-point Borg scale (Borg 1982) were registered. The warm-up and baseline measurement procedures were the same as those performed during incremental testing. The subjects were provided with no information about their power output or exercise duration. The continuous predictive trials were performed at 86 ± 5% (Wc1), 95 ± 5% (Wc2), 107 ± 8% (Wc3) and 121 ± 6% (Wc4) MAP, while the intermittent bouts were performed at 110 ± 7% (Wi1), 120 ± 7% (Wi2), 132 ± 8% (Wi3), and 144 ± 7% (Wi4) MAP. The intermittent predictive trials were performed with 30 s of exercise interrupted by 30 s of active recovery (50% of MAP), but TTE was computed excluding the
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recovery periods for modeling purposes (Dupont et al. 2002; Berthoin et al. 2006). The performance data were fitted to the two-parameter hyperbolic power–time equation (Hill 1993): Time ¼ W 0 =ðpower CPÞ: The CP and W0 parameters were estimated for both continuous (CPc and W0 c) and intermittent (CPi and W0 i) trials. Continuous and intermittent critical power trials After the continuous and intermittent power–time modeling, the subjects performed two trials in random order, one at CPc and one at CPi intensity until either exhaustion or 60 min elapsed. The exercise and recovery ratio (30:30 s), and active recovery at 50% of MAP during the CPi trial were the same as in the intermittent predictive trials. The physiological responses ([BLa], VO2 and HR) at CPc and CPi were analyzed at the following points: baseline, after 2.5 min of exercise, after 5 min of exercise, and every 5 min from the 5th min until the 30th min, as well as at exhaustion (TTE) using a 5-s average for VO2 and HR. Blood sampling took *30 s to be completed. Using the 15-point Borg scale (Borg 1982), the RPE was also recorded simultaneously with VO2, HR and [BLa] measures by interpolation, since subjects were required to report when they felt that the perception of effort was changing from one point to the next (Nakamura et al. 2008), which did not necessarily coincide with the points when the physiological data were recorded. Statistical analyses The Gaussian distribution of data was verified by the Kolmogorov–Smirnov test (with Lilliefor’s correction). Student’s t test was used to compare the W0 c with W0 i, TTE and the physiological variables ([BLaend], VO2end and HRend) at the end of exercise at both CPc and CPi. The Wilcoxon (Z) test was applied to detect differences between RPEend at CPc and RPEend at CPi. Repeated measures ANOVA was used to compare the [BLaend], VO2end and HRend of the predictive trials and to compare them with the physiological responses of incremental testing ([BLamax], VO2max and HRmax). The Friedman’s test was used to compare the RPEend of the predictive trials, since all participants reported an RPE of 20 at the moment of exhaustion, precluding a normal distribution of data. Two-way repeated measures ANOVA was adopted to compare [BLa], VO2 and HR responses at baseline and at 2.5, 5, 10, 15, 20, 25, and 30 min of exercise, as well as TTE at CPc and CPi. Sphericity was checked using
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Continuous predictive rials Intermittent predictive trials Continuous power-time relationship Intermittent power-time relationship
900 800
CPc
CPi
700
time (s)
Mauchly’s test, and whenever the test was violated we performed the necessary technical corrections by applying the Greenhouse–Geisser test. Pearson’s correlation was performed to assess the relationship between CPc and CPi and the relationship between CPc and CPi with VO2max and MAP. Pearson’s correlation was also used to assess the relationship between VO2max and TTE at CPc and CPi, TTE between CPc and CPi, and W0 c and W0 i. The significance level was set at 5% (P \ 0.05). Data are presented as mean ± standard deviation (SD).
600 500 400 300 200 100 0 100
MAP 150
200
250
300
350
400
450
500
550
Power (W)
Fig. 1 Hyperbolic power–time relationship using the group means of predictive trials for the continuous and intermittent parameters calculation
Results The performance and physiological variables (VO2max, HRmax, [BLamax], MAP and recovery power output) derived from the incremental tests are shown in Table 1. The power–time equation was successfully fitted to performance data of both continuous and intermittent predictive trials (Fig. 1). The power outputs and % MAP were significantly different among predictive trials in both continuous and intermittent exercise modes (F = 197.0; P \ 0.001) as well as between exercise modes (i.e., continuous 9 intermittent) (F = 218.6; P \ 0.001). TTE differed in the continuous predictive trials and in the intermittent predictive trials (F = 87.4; P \ 0.001). For [BLaend], a significant difference was found only between Wc4 and Wi4. Additionally, Wc1 and Wc2 presented different responses in continuous trials than Wc4 for [BLaend] and HRend (P \ 0.001). For VO2end (F = 1.7; P = 0.13) and RPEend (Z = 5.4; P = 0.61), there were no differences among the predictive trials. For RPEend, the median for all continuous and intermittent trials corresponded to the highest value of the Borg scale (RPE = 20). These comparisons are all shown in Table 2. The comparisons of power output, TTE, and the physiological and perceptual variables (VO2, HR, [BLa] and
Table 1 Mean ± standard deviation (SD), and minimum and maximum values of physiological variables, and performance reached in the incremental test Mean ± SD VO2max (L min-1) VO2max (mL kg-1 min-1) HRmax (bpm) [BLamax] (mM)
Minimum
Maximum 3.9
3.3 ± 0.4
2.8
47.3 ± 5.9
39.0
55.5
190.1 ± 7.8 9.8 ± 1.1
178 8.3
203 11.7
MAP (W)
258.3 ± 37.5
195
320
P50%rec (W)
129.2 ± 18.7
98
160
VO2max maximum oxygen consumption, HRmax maximal heart rate, [BLamax] maximal blood lactate concentration, MAP maximal aerobic power, P50%rec 50% of the maximal aerobic power. N = 10
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RPE) at the end of the CPc and CPi tests are shown in Table 3. The paired t test showed a higher absolute power output (W) at CPi than at CPc. The values for percentage of MAP (% MAP) were different between CPc and CPi (P \ 0.05). Nevertheless, there were no differences (P [ 0.05) between the physiological (VO2end, HRend and [BLaend]) and RPE responses measured at exhaustion in CPc and CPi (Table 3). Furthermore, no significant difference was observed between TTE at CPc (47.1 ± 10.8 min) and CPi (48.6 ± 9.9 min) (t = 0.44, P = 0.67). W0 i (23,034 ± 4,806 J) was significantly different from 0 W c (19,207 ± 3,677 J) (P \ 0.05) and they were poorly correlated (r = 0.44, P = 0.20). In contrast, although CPc was lower than CPi, they were strongly correlated (r = 0.96, P \ 0.001). High correlations were found between VO2max (L min-1) and power output (W) at CPc (r = 0.91, P \ 0.05) and between VO2max (L min-1) and CPi (r = 0.90, P \ 0.05). Furthermore, significant correlations were found between MAP and CPc (r = 0.98, P \ 0.01) and between MAP and CPi (r = 0.95, P \ 0.01). However, no significant correlations were found between VO2max (L min-1) and TTE at CPc (r = -0.50, P [ 0.05) or CPi (r = -0.52, P [ 0.05). The physiological ([BLa], VO2 and HR) and RPE responses over the course of cycling at CPc and CPi are presented in Fig. 2. Variables were compared at baseline, 2.5, 5, 10, 15, 20, 25, and 30 min of exercise and at TTE. There were no differences between CPc and CPi responses at any point for [BLa] (F = 0.03, P = 0.86), VO2 (F = 0.73, P = 0.52), HR (F = 0.82, P = 0.46) and RPE (F = 1.43, P = 0.26). However, there was a ‘time effect’ for all variables ([BLa], VO2 and HR; P \ 0.001) at CPc and CPi. Additionally, the values expressed as percentage of the maximal values obtained at the end of exercise in the incremental test at CPc and CPi corresponded to 81.8 ± 28.3% and 81.8 ± 21.6% of [BLamax],
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Table 2 Performance (Power, TTE and %MAP) and physiological variables (VO2end, [BLaend] and HRend) at continuous (Wc1–Wc4) and intermittent trials (Wi1–Wi4) Continuous predictive trials Wc1
Wc2
Wc3
Wc4
Power (W)
223.0 ± 39.1*§
245.0 ± 41.5*§
275.8 ± 41.7*§
312.0 ± 40.0*§
TTE (s)
640.5 ± 112.7*
377.0 ± 57.9*
236.7 ± 46.2*
149.5 ± 27.0*
% MAP
86.1 ± 4.6*§
94.7 ± 4.8*§
107.0 ± 8.2*§
121.0 ± 5.7*§ 3.2 ± 0.4
-1
VO2end (L min ) [BLaend] (mM) HRend (bpm)
3.4 ± 0.5
3.4 ± 0.5
3.4 ± 0.4
11.1 ± 1.3
11.7 ± 1.6
10.0 ± 2.1
180.3 ± 8.9
181.8 ± 8.0
176.7 ± 10.5
8.8 ± 1.6#§ 171.2 ± 10.5#
Intermittent predictive trials Wi1
Wi2
Wi3
310.4 ± 51.7*
340.8 ± 51.8*
371.6 ± 53.5*§
TTE (s)
582.7 ± 80.4*
356.5 ± 54.5*
246.0 ± 37.9*
187.3 ± 28.9*
% MAP VO2end (L min-1)
110.5 ± 6.8*§ 3.3 ± 0.4
120.0 ± 6.7*§ 3.4 ± 0.4
132.0 ± 7.9*§ 3.4 ± 0.4
144.1 ± 7.0*§ 3.3 ± 0.4
11.3 ± 1.7
11.8 ± 1.7
11.6 ± 1.2
11.9 ± 1.8§
181.1 ± 11.2
184.0 ± 10.3
180.9 ± 11.6
[BLaend] (mM) HRend (bpm)
§
Wi4
285.9 ± 49.5*
Power (W)
§
§
178.7 ± 9.8
Values are means ± SD on the continuous and intermittent predictive trials for power (W), time to exhaustion (TTE), percentage of maximal aerobic power (% MAP), and values of oxygen uptake end (VO2end), blood lactate end ([BLaend]), and heart rate end (HRend). Comparisons with one-way ANOVA between the factor exercise (continuous and intermittent exercise), and among predictive trials within the continuous (Wc1–Wc4) and intermittent (Wi1–Wi4) * Significantly different within the continuous predictive trials or within the intermittent predictive trials (P \ 0.05) §
Significantly different between continuous and intermittent predictive trials (P \ 0.05)
#
Significantly different of the Wc1 and Wc2 (P \ 0.05)
82.5 ± 4.5% and 85.2 ± 5.0% of VO2max, and 93.3 ± 2.6% and 92.5 ± 4.8% of HRmax, respectively.
Discussion This study showed that the work rates corresponding to CPc and CPi were different despite the similarity of the shape of the power–time relationship. The data derived from the intermittent predictive trials were rightwardshifted compared with those from the continuous trials (Fig. 1). In both cases, a high goodness-of-fit (R2 = 0.97 ± 0.03) between power output and TTE was observed, suggesting that the hyperbolic equation adequately described the relationship assumed by the critical power model. This was true even using active recovery periods in the intermittent predictive trials, which contrasts with the passive recovery used by Dupont et al. (2002) and Berthoin et al. (2006). Furthermore, the physiological ([BLa], VO2 and HR) and RPE responses during exercise at CPc and CPi were similar. Although CPi was significantly higher than CPc, they were highly correlated (r = 0.96). This is consistent with the findings of Berthoin et al. (2006), who found a 0.93 correlation between intermittent critical velocity derived
from 15 s runs interspersed with 15 s of passive rest and continuous critical velocity. Moreover, the mean differences between continuous and intermittent parameters were of the same magnitude (26.6% in Berthoin et al. 2006 and 27.9% in our study) irrespective of protocol differences. Hence, there are many similarities between our results and those of Berthoin et al. (2006). The reasons for the difference between CPi and CPc have not been addressed yet, although we can point out that partial W0 restoration takes place during intermittent test recovery periods, which allows greater exercise tolerance (TTE) than during continuous exercise at the same power output. As a consequence, CPi is sustained by a combination of aerobic and anaerobic energy sources, without necessarily inducing net lactate accumulation (Okuno et al. 2011). In our study, the aerobic nature of CPc and CPi was evidenced by their high correlations with VO2max (r = 0.91 and 0.90, respectively). However, these results are not definite evidence of the validity of CPc and CPi as measures of aerobic capacity since VO2max corresponds to aerobic power and not capacity. VO2max is more dependent on central O2 transport than metabolic thresholds, which depend more heavily on peripheral O2 extraction and utilization (Di Prampero 1985; Sjo¨din et al. 1981).
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Table 3 Mean, standard deviation (SD), minimum and maximum values of performance, TTE, physiological and perceptual measures at continuous critical power (CPc), and intermittent critical power (CPi) Continuous critical power Mean ± SD Power (W) Percentage MAP (%) TTE (s) VO2end (L min-1) HRend (bpm) [BLaend] (mM) RPEend (borg 6–20)#
Minimum
Intermittent critical power Maximum
Mean ± SD
Minimum
Maximum
191.9 ± 32.7*
137
243
245.5 ± 47.9
178
317
74.1 ± 3.2*
69
78
94.6 ± 6.7
83
105
2,830.1 ± 643.6 2.6 ± 0.2
1,836 2.2
3,600 2.9
2,916.0 ± 606.6 2.6 ± 0.2
2,130 2.4
3,600 3.0
177.3 ± 7.3
166.2
183.7
176.1 ± 14.3
162.1
195.2
7.9 ± 2.3
4.3
10.5
7.8 ± 1.7
4.3
9.5
18.4 ± 1.1
17
20
19.4 ± 2.2
17
20
* Significantly different to CPi (P \ 0.05) #
Comparison performed with Wilcoxon test (Z = -1.63, P = 0.10) Legends
Accordingly, it has been previously shown that CPc and CPi are highly correlated with continuous (McLellan and Cheung 1992; Wakayoshi et al. 1993) and intermittent (Okuno et al. 2011) maximal lactate steady-states, respectively, although some controversy exists (Dekerle et al. 2005; Pringle and Jones 2002). A finite energy reserve from anaerobic sources and myoglobin O2 stores has been suggested as a basis for W0 (Miura et al. 2000; Monod and Scherrer 1965; Moritani et al. 1981). Although there is evidence supporting W0 as an indirect measure of anaerobic capacity (i.e., maximal accumulated oxygen deficit) (Chatagnon et al. 2005; Hill and Smith 1994), there is no consensus about this (Zagatto et al. 2008) and a poor test–retest coefficient is often reported (Gaesser and Wilson 1988; Taylor and Batterham 2002) which has resulted in doubts about its validity (Dekerle et al. 2006). In our study, a low and non-significant correlation was found between W0 c and W0 i (r = 0.44; P = 0.20). A strong association was also lacking in Berthoin et al. (2006). Consequently, the application of a critical power model to both continuous and intermittent exercise appears to produce inconsistent anaerobic capacity estimates, so caution should be taken when using it in testing routines. Another source of inconsistency comes from the difference observed between W0 i and W0 c. This is in contrast to Berthoin et al. (2006), who reported similar anaerobic capacity estimates. To date, there is no consensus about whether differences exist between W0 i and W0 c or about the reasons for the differences found in our study. This issue should be addressed in future studies, but it is possible that W0 is not a valid measure of anaerobic capacity. During the continuous and intermittent predictive trials, the end values of [BLa], VO2 and HR were not significantly different from those found at the end of the incremental test. For RPE, all subjects reported values [19 on the
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15-point Borg scale (Borg 1982). Similar results were found with several continuous and intermittent intensities that elicited VO2max (Demariel et al. 2000; Dupont et al. 2002). Thus, it is possible to obtain maximal physiological responses of [BLamax], VO2max and HRmax while exercising both continuously and intermittently at high intensities (Demariel et al. 2000). Furthermore, if the exercise is maintained until exhaustion, maximal RPE values are expected. During CPc and CPi trials [BLa], VO2 and HR at TTE were significantly lower than the [BLamax], VO2max and HRmax values obtained at the end of the incremental test (P \ 0.05). [BLa] and VO2 at the end of CPc and CPi trials were between 81.8 and 85.2% of their maximal values during the incremental test. Thus it is clear that these physiological responses were submaximal in spite of the RPE C 19 in all subjects but one subject who stopped at the 60th min. These results corroborate the existence of a ‘‘critical metabolic rate’’ (Barker et al. 2006), since irrespective of exercise mode and power outputs (continuous or intermittent), similar physiological responses were obtained at CP. In Barker et al. (2006), *79% of VO2max was elicited during CPc derived from predictive trials using different pedaling frequencies (60 and 100 rpm), which led to significantly different power outputs at CPc (207 and 189 W, respectively). Poole et al. (1990) also found a steady VO2 of *79% of VO2max during CPc, with a [BLa] of 5.6 mM, which was lower than ours (*7.9 mM). The [BLa] value found in our study was close to that reported by Brickley et al. (2002) (7.3 mM). A possible explanation for this discrepancy may be related to the features of the protocol. In our study, subjects cycled until exhaustion, whereas Poole et al. (1990) stopped the exercise at the 24th min and Brickley et al. (2002), although interrupting exercise at 20 min, found unsteady physiological responses. Hence, it
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A
&
10
b Fig. 2 Physiological responses ([BLa], VO2, and HR) and RPE
[BLa end ] = 9.8 mM
during continuous (CPc) and intermittent (CPi) critical power for panels A, B, C, and D, respectively. *Difference significant in comparison with the baseline for VO2, HR, and [BLa] at CPc and with the time of 2.5 min for RPE within CPc (P \ 0.05). &Difference significant in comparison with the baseline for VO2, HR, and [BLa] at CPi and with the time of 2.5 min for RPE within CPi (P \ 0.05). § Difference significant in comparison with the measure from TTE for VO2, HR, [BLa], and RPE within CPc (P \ 0.05). #Difference significant in comparison with the measure from TTE for VO2, HR, [BLa], and RPE within CPi (P \ 0.05)
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[BLa] mM
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6 CPc CPi
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*
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15
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* *
2
# §
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Baseline 5
30
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time (min)
B
VO2 max = 47.3 ml.kg-1.min-1
50
VO2 ml.kg-1.min -1
45
&
40
&
&
&
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*
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*
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35 30 25 20
* §
15
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CPc CPi
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10 5
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0 Baseline 5
15
25
35
45
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time (min)
C
190 * * §
170
*
*
*
& #
& #
&
HRmax = 190.1 bpm *
§
§
Conclusions
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150
HR (bpm)
*
*
130
& & # #
110
& #
& #
&
CPc CPi
§
90 70
#
50 30
Baseline 5
10
15
20
25
30
35
40
45
50
55
60
65
time (min)
D
&
RPE (borg 6-20)
20 # &
18
# &
# &
&
#
16
#
*
#
14 §
12
§
§
*
*
CPi
§
CPc
§
10 8 6 0
5
10
15
2005). Nevertheless, Okuno et al. (2011) showed that CPi was not significantly different from the intermittent maximal lactate steady-state. In our study, the submaximal nature of exercise at CPc and CPi was also confirmed by the HR responses, which reached *93% of HRmax. In summary, the only variable in our study that rose to maximal values during exercise at CPc and CPi was RPE, suggesting that this may be the key determinant of exercise intolerance (Marcora and Staiano 2010; Nakamura 2010) at intensities encompassing heavy and severe domains (according to the nomenclature of Gaesser and Poole 1996).
20
25
30
35
40
45
50
55
60
65
Time (min)
is possible that Brickley et al. (2002) overestimated the maximal sustainable power output associated with physiological steady-state when calculating CPc. This finding is not uncommon in the literature (Dekerle et al. 2003,
Consistent with our hypotheses, although CPi derived from a 30:30 s exercise and active recovery regimen was higher than CPc, the physiological (VO2, HR, [BLa]) and perceptual (RPE) responses were not different between exercise modes at the same points in each protocol. In addition, exercise tolerance, as measured by TTE, was also similar between CPi and CPc. These results imply that the use of critical power modeling to analyze intermittent performance data can produce CP estimates that appear physiologically equivalent to those obtained by traditional continuous data modeling. Hence, the power–duration relationship can be used in training practice to set a metabolic rate (CPi) that can be sustained for prolonged time without inducing maximal VO2, HR or [BLa]. The same power–duration relationship can be useful for setting a range of power outputs above CPi with predictable TTE, which can be used in training to improve VO2max. However, since W0 i was not equivalent to W0 c, and the correlation between these anaerobic measures was weak, the validity of the critical power model for intermittent exercise can be questioned. Acknowledgments The authors thank CAPES for scholarship funding and the subjects that participated during data collection. Conflict of interest The authors declare that they have no conflicts of interest.
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