Experimental Physiology

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1 540 Exp Physiol (2016) pp Research Paper Research Paper Rate of utilization of a given fraction of W (the curvature constant of the power duration relationship) does not affect fatigue during severe-intensity exercise Kristopher Mendes de Souza 1, Jeanne Dekerle 2, Paulo Cesar do Nascimento Salvador 1, Ricardo Dantas de Lucas 1, Luiz Guilherme Antonacci Guglielmo 1, Camila Coelho Greco 3 and Benedito Sérgio Denadai 3 1 Physical Effort Laboratory, Sports Center, Federal University of Santa Catarina, Florianópolis, Brazil 2 Centre for Sport and Exercise Science and Medicine, University of Brighton, Eastbourne, UK 3 São Paulo State University, Human Performance Laboratory, Rio Claro, Brazil Experimental Physiology New Findings What is the central question of this study? Does the rate of utilization of W (the curvature constant of the power duration relationship) affect fatigue during severe-intensity exercise? What is the main finding and its importance? The magnitude of fatigue after two severe-intensity exercises designed to deplete the same fraction of W (70%) at two different rates of utilization (fast versus slow) was similar after both exercises. Moreover, the magnitude of fatigue was related to critical power (CP), supporting the contention that CP is a key determinant in fatigue development during high-intensity exercise. Thus, the CP model is a suitable approach to investigate fatigue mechanisms during high-intensity exercise. The depletion of W (the curvature constant of the power duration relationship) seems to contribute to fatigue during severe-intensity exercise. Therefore, the aim of this study was to determine the effect of a fast versus a slow rate of utilization of W ontheoccurrenceoffatigue within the severe-intensity domain. Fifteen healthy male subjects performed tests to determine the critical power, W and peak torque in the control condition (T CON ) and immediately after two fatiguing work rates (THREE and TEN) set to deplete 70% W in either 3 (T THREE )or10min (T TEN ). The T THREE and T TEN were significantly reduced (F = 19.68, P = 0.01) in comparison to T CON. However, the magnitude of reduction in peak torque (T THREE = 19.8 ± 10.1% versus T TEN = 16.8 ± 13.3%) was the same in the two fatiguing exercises (t = 0.76, P = 0.46). There was a significant inverse relationship between the critical power and the reduction in peak torque during both THREE (r = 0.49, P = 0.03) and TEN (r = 0.62, P = 0.02). In contrast, the W was not significantly correlated with the reduction in peak torque during both THREE (r = 0.14, P = 0.33) and TEN (r = 0.30, P = 0.10). Thus, fatigue following severe-intensity exercises performed at different rates of utilization of W was similar when the same work was done above the critical power (i.e. same amount of W used). (Received 4 August 2015; accepted after revision 15 January 2016; first published online 21 January 2016) Corresponding author B. S. Denadai: Human Performance Laboratory, IB UNESP, Rio Claro, São Paulo, Brasil, Avenida 24 A, 1515, Bela Vista CEP bdenadai@rc.unesp.br DOI: /EP085451

2 Exp Physiol (2016) pp Fatigue and severe-intensity exercise 541 Introduction Critical power (CP), represented by the asymptote of the power duration hyperbolic relationship (Morton, 2006), corresponds to the highest sustainable rate of oxidative metabolism (Jones et al. 2010). The curvature constant (W ) of this hyperbola represents the total amount of work that can be performed above CP before exhaustion occurs (Morton, 2006). The W was first considered a finite energy store comprising intramuscular [ATP], phosphocreatine concentration ([PCr]), glycogen and stored O 2 (e.g. in blood and other tissues; Poole et al. 1988, 1990; Fukuba & Whipp, 1999). More recently, however, W has been related to the depletion of substrates (e.g. [PCr]) and the accumulation of metabolites (e.g. [P i ], [ADP], [H 2 PO 4 ] and [H + ]) to critical concentrations, beyond which the level of muscle fatigue is such that exhaustion occurs (Jones et al. 2008; Burnley et al. 2010; Vanhatalo et al. 2010). Fatigue is defined as an exercise-induced reduction in maximal muscle force or torque-generating capacity (Walsh, 2000; Gandevia, 2001; Enoka & Duchateau, 2008). Interestingly, during different knee-extension exercises performed to exhaustion above critical torque (CT; the asymptote of the isometric torque duration hyperbolic relationship), the reduction in peak torque seemed to be similar at the end of each exercise irrespective of intensity (and therefore when W,intheory,isequaltozero;Burnley et al. 2012). Therefore, the magnitude of fatigue seems to be independent of the rate of utilization of W when the same fraction of W is depleted. However, whether the rate of utilization of the same fraction of W can influence the magnitude of fatigue during whole-body exercise is not known. It has recently been suggested that fatigue during severe-intensity exercise is linked to the utilization of W and the slow component of oxygen uptake ( V O2 SC; Cannon et al. 2011; Murgatroyd et al. 2011; Grassi et al. 2015). Indeed, V O2 SC is correlated with both fatigue (decline in peak torque during an all-out isokinetic cycling exercise; Cannon et al. 2011) and W (Murgatroyd et al. 2011). The proposed mechanism for this relationship is the activation of a cascade of events involving depletion of substrates and accumulation of fatigue-related metabolites that directly influence the muscle force/torque production and efficiency (Murgatroyd et al. 2011). However, to our knowledge, the relationship between the rate of utilization of the same fraction of W, fatigue and V O2 SC during severe-intensity exercise has never been investigated. The main purpose of the present study, therefore, was to determine the effect of a fast versus slow rate of utilization of W on the magnitude of fatigue following two severe-intensity exercises set to complete the same amount of work above CP (70% W )ineithera3(three) or a 10 min (TEN) time period. We hypothesized that the magnitude of fatigue after THREE and TEN would not be significantly different. In addition, given that V O2 SC is both time and intensity dependent (Jones et al. 2011), we hypothesized that the V O2 SC would be different in each of the two conditions. Methods Ethical approval Written informed consent was obtained from each subject. The study was approved by the local Ethics Committee of the University and conducted according to the Declaration of Helsinki for human experimentation. Subjects Fifteen healthy male participants (mean ± SD: age, 26.0 ± 3.5 years; weight, 76.6 ± 10.4 kg; and height, 178 ± 7 cm) volunteered to participate in this study. The subjects participated in any exercise at a recreational level, but were not highly trained. They were familiar with cycle ergometry and the exercise testing procedures used in our laboratory. Overview The subjects were required to visit the laboratory on 10 occasions. Each subject performed the following testing stages: (i) a submaximal step incremental test (four to five work rates) to determine the lactate threshold (LT), followed by a maximal ramp incremental test for the measurement of peak oxygen uptake ( V O2 peak) andpeak power output (P peak );(ii)a5sall-outtestperformedinthe isokinetic mode at 120 r.p.m. to measure peak torque in controlled conditions (T CON ); (iii) four maximal constant work rate tests performed to exhaustion at 75, 85, 95 and 105% P peak for the determination of CP and W ;(iv)two fatiguing work rate protocols, each at a work rate set to deplete 70% W either at 3 (THREE) or at 10 min (TEN), immediately followed by a 5 s all-out test performed in theisokineticmodeat120r.p.m.foranothermeasure of peak torque (T THREE and T TEN ); and (v) two fatiguing work rate protocols performed identically to the previous stage (i.e. stage four) for modelling of the osygen uptake ( V O2 ) kinetics. With the exception of the tests conducted in the isokinetic mode, the pedal cadence was maintained constant at 70 r.p.m. The subjects were instructed to avoid any intake of caffeine or alcohol and strenuous exercise in the 24 h preceding a test session and to arrive at the laboratory in a rested and fully hydrated state, at least 3 h postprandial. All tests were performed at the same time of day in laboratory conditions with a controlled environment (19 22 C; 50 60% relative humidity) to

3 542 K. M. de Souza and others Exp Physiol (2016) pp minimize the effects of diurnal biological variation on the results (Atkinson & Reilly, 1996). With the exception of the submaximal and maximal incremental tests, which were performed on the same day, the subjects performed only one test on any given day, and the tests were each separated by h but completed within a period of 3 4 weeks. Equipment All tests were performed on an electromagnetically braked cycle ergometer equipped with pedal force/torque measurement (Excalibur Sport; Lode BV, Groningen, The Netherlands). The cycle ergometer was calibrated in accordance with the manufacturer s recommended procedures. Respiratory and pulmonary gas exchange variables were measured continuously using a breath-by-breath analyser (Quark PFTergo; Cosmed, Rome, Italy). Before each test, the O 2 and CO 2 analysis systems were calibrated using ambient air (20.94% O 2 and 0.03% CO 2 )andagasofaknowno 2 and CO 2 concentration (16.00% O 2 and 5.00% CO 2 ) according to the manufacturer s instructions. The gas analysis systems used have a response time of 120 ms for O 2 and 100 ms for CO 2, a sample range of 0 100% for O 2 and 0 10% for CO 2, and an accuracy of ±0.1% for both. Likewise, the turbine flowmeter was calibrated before each test using a 3 litre syringe (Quark PFTergo; Cosmed, Rome, Italy). The turbine flowmeter used has a resistance of <0.7 cmh 2 Ol 1 s 1 at a flow rate of 12 l s 1, aventilationrangeof0 300lmin 1 and an accuracy of ±2%. Breath-by-breath V O2 data were analysed throughout the tests (Data Management Software; Cosmed, Rome, Italy). Capillary blood samples (25 μl) were obtained from the earlobe of each subject, and the blood lactate concentration ([La]) was measured using an electrochemical analyser (YSL 2700 STAT; Yellow Springs Instruments, Yellow Springs, OH, USA). The analyser was calibrated in accordance with the manufacturer s recommended procedures. Incremental tests Initially, each subject performed a submaximal step incremental test to determine the LT. The test started at 60 W and was increased by 20 W every 3 min for four to five stages. Capillary blood samples were collected within the final 20 s of each stage for [La] determination. The LT was determined from the relationship between [La] and the work rate and was considered as the first sudden and sustained increase in [La] above the resting concentration (Carter et al. 2000). After 30 min of rest, the subjects performed a maximal ramp incremental test for the measurement of V O2 peak and P peak. The test started at 90% LT for 4 min and was then continuously increased byarateof25wmin 1 until volitional exhaustion. Each subject was verbally encouraged to undertake maximal effort. Breath-by-breath V O2 data were reduced to 15 s stationary averages, and the V O2 peak wasconsideredasthe highest average 15 s V O2 value recorded during the ramp incremental test. The P peak wasconsideredasthehighest power output attained in the ramp incremental test. All-out isokinetic test The protocol used in this study was similar to the protocol previously described by Cannon et al. (2011). Initially, the subjects performed a 5 min warm-up at LT, followed by two 5 s submaximal exercise bouts in an isokinetic mode with pedal cadence capped at 120 r.p.m., with a 2 min recovery in between. This was not used in the data analysis. After 5 min of rest, the subjects performed a 5 s all-out test in the isokinetic mode at 120 r.p.m. to measure peak torque on both left and right crank arms. The T CON wasthenconsideredastheaverageof the instantaneous peak torque of both left and right crank arms during the all-out isokinetic test. The subjects were given an auditory cue to begin the all-out effort in the seated position, and strong verbal encouragement was given throughout the 10 crank revolutions. The cycle ergometer was instrumented for crank arm torque measurement and calibrated in accordance with the manufacturer s recommended procedures. The isokinetic velocity was limited by the electromagnetic breaking system of the flywheel and computer controlled to the value programmed by the experimenter. Determination of CP and W The subjects performed four maximal constant work rate tests until exhaustion at 75, 85, 95 and 105% P peak (Vanhatalo et al. 2011a). These work rates led the subjects to exhaustion between 3 and 15 min, which is the range recommended for the determination of CP and W (Poole et al. 1988; Hill, 1993). Each test started with a 5 min warm-up at LT followed by 5 min of rest. Furthermore, after 3 min at 20 W, the power output was adjusted to one of the previously established work rates and the subjects were instructed to perform until they were unable to maintain the required work rate. Timing began when the pedal cadence reached 70 r.p.m. and stopped when the subject could not maintain a pedal cadence of >67 r.p.m. despite verbal encouragement (Caputo & Denadai, 2008). The time to exhaustion (t lim ) was measured to the nearest second. Individual CP and W estimates were derived from the four prediction trials by least-squares fitting of the following regression models, as follows.

4 Exp Physiol (2016) pp Fatigue and severe-intensity exercise 543 Non-linear power output (P) versus time to exhaustion (t lim ): t lim = W /(P CP) (1) Linear work (W) versus time to exhaustion (tlim ): W = (CP t lim ) + W (2) Linear power output (P) versus 1/time to exhaustion (t lim ): P = (W /t lim ) + CP (3) The three standard errors of the CP estimate (SEE) from eqns (1) (3) were compared in order to select the CP and W estimates from the best fit (Vanhatalo et al. 2010). Breath-by-breath V O2 data were recorded continuously during all the tests and were reduced to 15 s stationary averages. The peak V O2 wasconsideredasthe value obtained in the final 30 s of exercise. Fatiguing tests The subjects performed two fatiguing exercise protocols (Fig. 1). Each protocol started with a 5 min warm-up at the LT, followed by 5 min of rest. Furthermore, after 3 min at 20 W the power output was adjusted to deplete 70% W (the time integral of the work rate above CP using 70% W ) in either a 3 or a 10 min period (Dekerle et al. 2015). Immediately upon completion of the constant work rate exercise, the cycle ergometer was instantaneously switched to an isokinetic mode that limited the peak pedal cadence to 120 r.p.m. The subjects were given an auditory cue to begin a 5 s all-out effort in the seated position with strong verbal encouragement given throughout the 10 crank revolutions (i.e. 5 s at 120 r.p.m.). The T THREE and T TEN were then computed as the average of the instantaneous peak torque of both left and right crank arms during the all-out isokinetic test performed immediately after the 3 and 10 min fatiguing exercises, respectively. Breath-by-breath V O2 data were recorded continuously during both conditions and were reduced to 15 s stationary averages. The V O2 peak for the constant-load part of THREE and TEN was defined as the mean V O2 measured over the final 30 s of exercise. Additionally, capillary blood samples were collected over the final 20 s of baseline cycling and within the final 20 s of both fatiguing exercises to determine [La]. Furthermore, the subjects completed the first constant-load part of THREE and TEN only (no all-out effort) on separate occasions for the measurement of the V O2 kinetics. Modelling of V O2 kinetics Breath-by-breath data were filtered manually to remove outlying breaths, which were defined as breaths ±3 SD from the adjacent five breaths. For each fatiguing exercise, breath-by-breath data were linearly interpolated to 1 s intervals. The two identical transitions were then time aligned to the start of the exercise and averaged to enhance the underlying response characteristics (Lamarra et al. 1987). The single V O2 profile was further averaged in 5 s stationary bins for improvement of the V O2 kinetics parameters. The first 20 s of data after the onset of exercise (i.e. the phase I) was deleted, and a non-linear least-squares algorithm was used to fit the data thereafter (Ferguson et al. 2010). A single-exponential model was used to characterize the V O2 response to severe-intensity exercise, as described in the following equation: V O2 (t) = V O2 (b) + A p ( 1 e (t TDp)/τp ) (4) where V O2 (t) represents the absolute V O2 at a given time, t; V O2 (b) represents the baseline V O2 measured in the 60 s preceding the transition in work rate; A p is the amplitude; TD p is the time delay; and τ p is the time constant. The model fit was initially constrained to the first 60 s of exercise data (i.e s). The window was A 5 min Warm-up 5 min Rest 3 min 20 W 3 min Fatiguing Exercise 5 s All-out Isokinetic Exercise B 5 min Warm-up 5 min Rest 3 min 20 W 10 min Fatiguing Exercise 5 s All-out Isokinetic Exercise Figure 1. Schematic illustration of the fatiguing tests A, 5 s all-out isokinetic exercise performed immediately after the fatiguing exercise set to deplete 70% W (the curvature constant of the power duration relationship) in 3 min (THREE). B, 5 s all-out isokinetic exercise performed immediately after the fatiguing exercise set to deplete 70% W in 10 min (TEN).

5 544 K. M. de Souza and others Exp Physiol (2016) pp lengthened iteratively, until the exponential model fit demonstrated a discernible departure from the measured response profiles (as judged from the visual inspection of a plot of the residuals of the fit; Rossiter et al. 2001; Vanhatalo et al. 2011b). The magnitude of V O2 SC was expressed as the difference in V O2 between the amplitude of the fundamental ( V O2 (b) + A p ) and the end-exercise values (i.e. peak V O2 ). The functional gain of the entire response (i.e. end-exercise gain) was computed by dividing V O2 (i.e. peak V O2 V O2 (b))by work rate. In addition, a single-exponential model without a time delay and with a fitting window commencing at t = 0s(equivalenttothe mean response time; MRT) was used to characterize the kinetics of the overall V O2 response during the initial part of the two tests (i.e. 2 min), as described in the following equation: V O2 (t) = V O2 (b) + A ( 1 e t/mrt) (5) where V O2 (t) represents the absolute V O2 at a given time, t; V O2 (b) represents the baseline V O2 measured in the 60 s preceding the transition in work rate; and A and MRT represent the amplitude and mean response time, respectively, describing the overall increase in V O2 above the baseline (Bailey et al. 2011). Statistical analysis All the data throughout are expressed as mean values ± SD. The Shapiro Wilk test was applied to check for the Gaussian distribution of the data. One-way repeated-measures ANOVAs were performed and Bonferroni s corrected paired t test was used to identify differences where an overall difference had been found to be significant. Student s paired t test was used in two-set paired comparisons. The Pearson product moment correlation was used to test for bivariate relationship. All the analyses were carried out using the GraphPad Prism software package for Windows (version 5.0; GraphPad Prism Software Inc., San Diego, CA, USA). The level of significance was set at P < Results Incremental tests and predictive tests of CP and W The V O2 peak, P peak and LT were 3.71 ± 0.49 l min 1, 322 ± 26 W and 109 ± 15 W (34.1 ± 4.5% P peak ), respectively. The peak V O2 values recorded during the constant work rate tests performed at 75 (3.71 ± 0.45 l min 1 ), 85 (3.69 ± 0.39 l min 1 ), 95 (3.67 ± 0.50 l min 1 ) and 105% P peak (3.54 ± 0.42 l min 1 ) were not significantly different (F = 1.74; P = 0.21) from the V O2 peak measured during the ramp incremental test. The CP and W were 207 ± 17 W (64.3 ± 2.7% P peak ) and 21.3 ± 4.2 kj, respectively. The goodness of fit of the power duration relationship was r 2 = 0.99 ± The 95% confidence intervals associated with the estimated parameters of the power duration relationship were 2.9 to 4.1 W and 1.6 to 2.0 kj for CP and W, respectively. Fatiguing tests Peak torque. The fatiguing work rates set to deplete 70% W (14.9 ± 3.0 kj) in 3 and 10 min were 289 ± 25 (89.8 ± 2.9% P peak ) and 231 ± 19 W (71.9 ± 2.2% P peak ), respectively. The T THREE (108 ± 19 N m) and T TEN (112 ± 23 N m) were significantly reduced (F = 19.68, P = 0.01) in comparison to T CON (135 ± 20 N m; Fig. 2). However, these decreases ( 19.8 ± 10.1% for T THREE versus 16.8 ± 13.3% for T TEN ) were not significantly different (t = 0.76, P = 0.46) in both fatiguing exercises. The CP was inversely related to the change in peak torque so that participants with high CP showed smaller changes in peak torque for both THREE (r = 0.49, P = 0.03) and TEN (r = 0.62, P = 0.02). No relationship was found for W (THREE, r = 0.14, P = 0.33; and TEN, r = 0.30, P = 0.10). The decreases in peak torque in THREE and TEN were also not significantly correlated (r = 0.28; P = 0.30). Oxygen uptake kinetics. The V O2 kinetics parameters for both fatiguing exercises are presented in Table 1 and illustrated in Fig. 3 (representative subject). No significant differences between the two fatiguing exercises (THREE versus TEN) were found in V O2 (b) (t = 0.64, P = 0.52) and MRT (t = 0.29, P = 0.38). In contrast, the remaining parameters were significantly different between the two conditions. The τ p was shorter (t = 3.07, P < 0.01) for Torque (N m) * T CON T THREE T TEN Figure 2. The peak torque after the two fatiguing work rates set to deplete 70% W in either 3 or 10 min (T THREE and T TEN ) Determination of the peak torque in control conditions (T CON ) was performed without pre-existing fatigue. The fatiguing exercises resulted in a reduction in peak torque to about 80.2 ± 10.2 and 83.1 ± 13.3% of the T CON forthe3and10min work rates, respectively. P < 0.05 in relationship to T THREE and T TEN (one-way repeated-measures ANOVA).

6 Exp Physiol (2016) pp Fatigue and severe-intensity exercise 545 Table 1. Oxygen uptake kinetics parameters during THREE and TEN Parameter THREE TEN V O2 (b) (l min 1 ) 0.99 ± ± 0.16 A p (l min 1 ) 2.37 ± ± 0.27 Absolute V O2 (l min 1 ) 3.36 ± ± 0.26 τ p (s) 23.7 ± ± 8.4 V O2 SC (l min 1 ) 0.12 ± ± 0.17 Peak V O2 (l min 1 ) 3.54 ± ± 0.38 MRT (s) 48.1 ± ± 10.8 Data are the means ± SD. Abbreviations: absolute V O2, V O2 (b) + A p ; A p, amplitude of the V O2 primary component; MRT, mean response time; THREE and TEN, two fatiguing work rate protocols, each at a work rate set to deplete 70% W (the curvature constant of the power duration relationship) either at 3 (THREE) or at 10 min (TEN); τ p, time constant of the V O2 primary component; V O2, oxygen uptake; V O2 (b), baseline oxygen uptake; V O2 SC, slow component of oxygen uptake. P < 0.05 in relationship to THREE (student s paired t test). deplete 70% W at either a fast (3 min at 90% P peak ) or a slow rate (10 min at 72% P peak ) was similar in both conditions. This means fatigue is independent of the rate of utilization of W when the same amount of work is accumulated above CP. To the best our knowledge, this study is the first to report measures of fatigue above CP using different rates of utilization of the same fraction of W (i.e. 70%) in cycling exercise. Interestingly, previous studies have reported similar reductions in peak torque following exhaustive knee-extension exercises performed at various intensities above CT across the severe-intensity domain (i.e. when W was theoretically fully utilized at the end of each exercise; Burnley et al. 2010, 2012). Theextenttowhich theability to generate muscle torque is reduced during or after exercise performed above CP or CT is therefore proportional to the amount of W depleted, with greater muscle fatigue for a greater fraction of utilization of W [present study, average loss of 18% for 70% W utilized; Burnley et al. (2012), THREE compared with TEN. The A p (t = 3.54, P < 0.01) and absolute V O2 ( V O2 (b) + A p ; t = 3.62, P < 0.01) were significantly greater for THREE compared with TEN. However, the V O2 SC was significantly smaller (t = 3.01, P < 0.01) in THREE than in TEN. Both sets of V O2 SC values were significantly different from zero (THREE, t = 3.56, P < 0.01; and TEN, t = 10.9, P < 0.01). No relationship was found for V O2 SC and changes in peak torque for both THREE (r = 0.10, P = 0.37) and TEN (r = 0.32, P = 0.12). Physiological responses. The peak V O2 was not significantly different (t = 0.22, P = 0.83) between the two fatiguing exercises (Table 1). Thus, the end-exercise gain was significantly greater (t = 46.5, P < 0.01) for TEN (12.1 ± 1.0 ml min 1 W 1 )comparedwith THREE (9.5 ± 0.8 ml min 1 W 1 ). However, the durations of 3 and 10 min were not sufficient to allow for V O2 peak measured during the ramp incremental test to be attained at the end of exercise (F = 4.59, P = 0.02; 95.9 ± 5.3 and 96.4 ± 7.8% V O2 peak, for THREE and TEN, respectively). The baseline [La] was not significantly different (t = 0.65, P = 0.52) between THREE (1.29 ± 0.32 mmol l 1 ) and TEN (1.25 ± 0.38 mmol l 1 ), but the end [La] was significantly lower (t = 2.51, P = 0.02) for THREE (10.55 ± 1.68 mmol l 1 )comparedwithten (11.92 ± 1.95 mmol l 1 ). Discussion The main original finding of this study was that the magnitude of the reduction in peak torque measured immediately after two severe-intensity exercises set to A VO 2 (l min 1 ) B VO 2 (l min 1 ) VO 2peak VO 2peak Time (s) Time (s) Figure 3. Modelling of breath-by-breath oxygen uptake ( V O2 ) response (including the corresponding residual plots) during THREE (A) and TEN (B) for a representative subject The fundamental kinetics (i.e. V O2 primary component) was characterized with non-linear least-squares regression modelling (continuous line), with the fit extrapolated (dashed line) to the end of exercise. The dashed horizontal line represents the peak oxygen uptake ( V O2 peak) obtained during the ramp incremental test. The dotted vertical line indicates the onset of the slow component of V O2.

7 546 K. M. de Souza and others Exp Physiol (2016) pp average loss of 48% for 100% W utilized]. However, further studies examining the influence of rate and/or fraction of utilization of W on the magnitude of fatigue within the severe-intensity domain are required to confirm these findings, mainly during whole-body exercise (e.g. cycling). Central to our study was the use of an all-out isokinetic cycling protocol to quantify fatigue immediately after THREE and TEN. Given that muscle function recovers rapidly, within a few seconds after the cessation of exercise (Froyd et al. 2013; Coelho et al. 2015), it is crucial to measure fatigue immediately after the termination of exercise for the full magnitude of a change to be detected (Coelho et al. 2015). However, this is a complex task during cycling exercise because the techniques of strong validity that are commonly used to measure fatigue (e.g. muscle, nerve or motor cortex stimulation in conjunction with the performance of a maximal voluntary contraction) do not offer immediate measurements (Coelho et al. 2015). Moreover, these techniques have little in common with the dynamic muscle contractions characteristic of cycling exercise (Coelho et al. 2015). Unfortunately, our experimental design precluded identification of the origin of fatigue (i.e. central or peripheral). However, it has been reported that the development of fatigue within the severe-intensity domain is primarily related to peripheral mechanisms, with a small contribution of central mechanisms (Burnley et al. 2012). Peripheral fatigue is, in part, attributed to the depletion of [PCr] and the accumulation of [P i ], [ADP], [H 2 PO 4 ]and[h + ] (Fitts, 1994; Allen & Westerblad, 2001; Allen et al. 2008), all of these being associated with the utilization of W when within the CP framework (Jones et al. 2008; Burnley et al. 2010; Vanhatalo et al. 2010). Indeed, exercise performed above CP requires an additional energetic contribution from substrate-level phosphorylation that induces an inevitable accumulation of fatigue-related metabolites (Jones et al. 2008; Burnley et al. 2010; Vanhatalo et al. 2010). In the present study, thehigh[la]valuesrecordedattheendofboththree ( 11 mmol l 1 ) and TEN ( 12 mmol l 1 ) indicate that part of the total energy production was supplied by anaerobic glycolysis. Unexpectedly, the end [La] was significantly lower in THREE, suggesting a lower amount of energy supplied from anaerobic glycolysis in these conditions. This, alongside a lesser total amount of work produced in these conditions in order to deplete 70% W, can lead to the assumption that the accumulation of metabolites (e.g. [P i ], [ADP], [H 2 PO 4 ]and[h + ]) could possibly be lower in THREE. However, this may be seen as speculative, because the blood lactate does not completely mirror the muscle lactate metabolism during exercise (Gladden, 2004). In our experimental design, the power duration relationship was used to predict the individual work rate required to deplete the same fraction of W, reducing the interindividual variability of exercise tolerance during severe-intensity exercise (Murgatroyd et al. 2011). However, the magnitude of the reduction in peak torque was greatly variable between subjects in the present study (coefficient of variation >50%). The extent to which muscle fatigue developed during the two conditions was therefore both inter- and intraindividual dependent. The inverse relationship found between the CP and the change in peak torque (THREE, r = 0.49, P = 0.03; and TEN, r = 0.62, P = 0.02) could help to explain the large differences between subjects; participants with higher CP were less affected by the fatiguing exercise. This is in agreement with the findings of Coelho et al. (2015), who reported a significant negative relationship (P < 0.05) between the V O2 peak and the magnitude of fatigue measured immediately after a maximal ramp incremental cycling exercise. Thus, some physiological mechanisms associated with the aerobic nature of CP and V O2 peak seem to be involved with the ability to resist the development of muscle fatigue during high-intensity exercise. Individual distributions of muscle fibres may help to explain these findings. Participants with high aerobic capabilities (CP and V O2 peak) are thought to have a higher proportion of type I fibres (Murgatroyd et al. 2011; Rossiter, 2011) but are also likely to generate lower peak torque (Hamada et al. 2003). Type I fibres are also more fatigue resistant, so that individuals with a greater proportion of type I fibres are known to demonstrate lower reductions in peak torque after fatiguing exercise (Hamada et al. 2003). Beyond muscle fibre distributions, a faster and greater O 2 delivery, enhanced blood flow to and from the exercising muscles, and greater skeletal muscle capillary and mitochondrial densities can be expected in individuals with high CP (Murgatroyd et al. 2011; Rossiter, 2011); therefore, the accumulation of fatigue-related metabolites can be both slower and lower during exercise. A relationship between the rate of utilization of W and the V O2 SC has been shown and was associated with exercise tolerance during severe-intensity exercise (Murgatroyd et al. 2011). The V O2 SC represents a decreased efficiency of muscle contractions (i.e. the ratio of mechanical energy output to metabolic energy input) and seems to be preceded by and closely related to fatigue (Cannon et al. 2011; Grassi et al. 2015). In the present study, the peak V O2 values recorded at the end of THREE and TEN were not significantly different (Table 1). This led to greater end-exercise gain and V O2 SC for these conditions. Despite this, the magnitude of fatigue was similar in both conditions, with no correlation between the V O2 SC and the loss of peak torque (THREE, r = 0.10, P = 0.37; and TEN, r = 0.32, P = 0.12). Cannon et al. (2011) found that muscle fatigue occurred after only 3 min of cycling exercise performed above the LT (i.e. where a V O2 SC is

8 Exp Physiol (2016) pp Fatigue and severe-intensity exercise 547 present). However, there was no further development of muscle fatigue between 3 and 8 min of supra-lt exercise where the progression of muscle inefficiency (i.e. V O2 SC) was most evident (Cannon et al. 2011). Therefore, muscle fatigue and inefficiency during high-intensity exercise (e.g. above CP) may have different time courses. Further work would be needed to establish the relationship between both physiological phenomena. In summary, the present study demonstrated that the reduction in peak torque after two fatiguing exercises performed above CP set to deplete the same fraction of W (70%) at two different rates (fast versus slow) was similar in both conditions. Therefore, fatigue develops to the same extent irrespective of the rate of utilization of W when the same work is accumulated above CP. 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