It has been postulated that a diffusion limitation of ATP from sites of production (e.g., cytoplasm and mitochondria) to subcellular sites of ATP utilization (e.g., myofibrillar acto-myosin ATPase) can be overcome by phosphorylcreatine/creatine (PCr/Cr) shuttling (38). This process is governed mostly by the function of creatine kinase (CK) and its high-energy product PCr. A linear model of respiratory control postulates that the speed in which muscle PCr falls exponentially after the onset of exercise is a function of both mitochondrial resistance and the metabolic capacitance (28,3028,30). In theory, an increase in mitochondrial volume (e.g., after exercise training) would produce a lower mitochondrial resistance and faster PCr kinetics (17,2817,28). However, if mitochondrial resistance remains constant, then an increase in the metabolic capacitance, due to an increase in the total muscle Cr content, would result in slower PCr kinetics (38). Supporting this prediction, Jones et al. (28) reported that a 5-d Cr supplementation (20 g·d−1) resulted in an increased total muscle Cr content and slower muscle PCr kinetics following the onset of 6 min of either moderate- or heavy-intensity, constant-load, knee-extensor exercise.
There is a direct and inverse proportionality between the products of PCr splitting and the muscle or pulmonary V˙O2 response (39). This synchronism guarantees an efficient diffusion of ATP from the ATP-generating sites to ATP-consuming sites. Jones et al. (26) reported that Cr supplementation (20 g·d−1 for 5 d followed by 5 g·d−1 until the end of the testing sessions) did not change time-based parameters of the V˙O2 response during 6 min of moderate- or heavy-intensity, constant-load exercise, although the V˙O2 response was approximately 10% and 4% slower after supplementation, respectively. The authors subsequently argued that intersubject variability regarding the changes in muscle Cr content after Cr supplementation precluded the attainment of statistical significance (26). Importantly, the amplitude of the primary component and end exercise V˙O2 were reduced in heavy exercise, and the magnitude of this reduction was associated with the percentage of type II fibers in the vastus lateralis (VL). These results suggest that an increase in the metabolic capacitance (via an increase in muscle Cr content) might limit the increase in V˙O2 during heavy exercise by either altering muscle efficiency or changing motor unit recruitment. A further prediction of this model is that a slower V˙O2 response to exercise after Cr loading will be counterbalanced by an increased accumulated oxygen deficit (AOD), perhaps reflecting a higher anaerobic contribution. To date, however, these hypotheses have not been tested during short-duration, all-out exercise.
Self-paced time trials, performed at supra V˙O2max intensities, have recently been used to investigate the effects of different interventions on V˙O2 and AOD responses to all-out exercise (3,373,37). In this exercise model, athletes are free to vary the exercise intensity throughout the trial, providing a closer simulation of a “real-world” competition. Furthermore, performance during such tasks has been reported to be influenced by motor unit recruitment (as inferred from integrated electromyography [iEMG] signal), the rate of PCr breakdown, and the V˙O2 response (3,343,34). To date, however, no study has examined the V˙O2 and AOD response to an all-out, self-paced exercise after a Cr-loading period. While Cr loading appears to slow PCr breakdown, delaying the V˙O2 response, and to alter muscle efficiency and/or motor unit recruitment during constant-load exercise performed at heavy intensities (26,2826,28), the effects of Cr loading on V˙O2, AOD, and muscle recruitment during all-out exercise is unknown. If the V˙O2 response is altered by an increase in the metabolic capacitance (i.e., an increase in the total muscle Cr content) after Cr loading, this would provide important insights about how intracellular metabolism and respiratory control are regulated during self-paced, high-intensity exercise. In particular, this would provide useful information regarding how a change in the CK reaction equilibrium could affect the regulation of oxidative phosphorylation and performance during all-out exercise.
Therefore, the purpose of this study was to investigate the effect of dietary Cr loading on the V˙O2, AOD, energy system contributions, and muscle recruitment pattern during an all-out, self-paced cycling exercise (1-km time trial). It was hypothesized that Cr loading would reduce iEMG and/or slow the V˙O2 response, and increase the AOD during the time trial.
Twenty-four amateur male cyclists volunteered for this study, but five did not complete all of the trials (due to non–study-related injuries) and were excluded from data analysis. Therefore, 19 participants (average weekly training volume: 298.9 ± 134.4 km·wk−1) were analyzed. The participants were randomly divided into two groups: Cr (n = 10) and placebo (n = 9). Exclusion criteria included (a) risks associated with cardiovascular, pulmonary, or metabolic disease factors; (b) intake of Cr and/or beta-alanine within 180 d before the beginning of the study; and (c) adoption of a vegetarian diet. Participants were informed about the risks and benefits of participating in the study before signing a consent form. This study was approved by the Maceio Academic Center Ethics Committee. Table 1 shows the participants’ main characteristics.
During the first visit, body mass, height, and percentage body fat were measured. Thereafter, a maximal incremental test was performed to determine V˙O2peak and peak power output (PPO). During the second and third visits (separated by 24 h), participants performed two 1-km time trial familiarization sessions. Forty-eight hours after the second familiarization session, participants performed the presupplementation time trial (the fourth visit). Over the next 5 d, participants consumed either Cr monohydrate plus dextrose or a placebo (dextrose). Afterwards, the participants performed the postsupplementation time trial (the fifth visit). Athletes were asked to record their food and drink intake during the 24 h before the first visit and to replicate this at every subsequent visit. The participants were instructed to refrain from any exercise 24 h before each test and not to consume any nutritional supplements during the experimental period. A diet and exercise diary was applied to check compliance with the recommendations.
Body composition and incremental test
Body density was estimated based on three skin folds (pectoral, abdomen, and thigh) (25) and then converted to a body fat percentage using the Siri equation (35). Fat-free mass was estimated by subtracting body fat mass from the total body mass. Participants performed a 3-min warm-up at a power output (PO) of 100 W and then workload was increased 30 W every 3 min in a square-wave manner until volitional exhaustion. The test was performed on a cycle simulator (RacerMate, Computrainer, Seattle, USA), which was calibrated before each test in accordance with the manufacturer’s recommendations. An automatic gas analyzer was used to measure V˙O2 breath-by-breath during the test (Cortex Metamax 3B, Cortex Biophysik, Leipzig, Germany). The volume of carbon dioxide production (V˙CO2) and ventilation (V˙E) were also measured. Before each experiment, a 3-L syringe, ambient air and a cylinder of known concentrations of O2 (12%) and CO2 (5%) were used to calibrate the metabolic cart, according to the manufacturer’s recommendations. Heart rate (HR) was measured by an HR monitor coupled to the gas analyzer (Polar Electro Oy, Kempele, Finland). The V˙O2peak was determined from the average V˙O2 during the last 30 s of the test. Maximal HR was defined as the highest HR value obtained during the test. The PPO was determined as the highest PO achieved during the last completed stage. When the participants could not maintain the PO during the entire stage (i.e., <3 min), the PPO was calculated using the fractional time completed in the last stage multiplied by the increment rate (i.e., 30 W).
1-km cycling time trial
The pre- and postsupplementation trials were conducted at the same time of the day for a given participant to avoid any circadian rhythm interference (12). The same cycle simulator and calibration procedures used in the incremental test were used for the time trials. First, a 5-min rest was employed to measure baseline V˙O2. Then, a 5-min warm-up was performed at 150 W followed by a 5-min rest and an extra 5-min warm-up at 70% PPO. The first warm-up was used to estimate gross mechanical efficiency and then mechanical aerobic and anaerobic POs during the time trial (Pae and Pan, respectively) (9,159,15). The second warm-up was used to normalize the EMG signal (1). After a 5-min rest, participants were instructed to perform a 1-km time trial as quickly as possible. The distance covered was conveyed to the participants every 200 m. The total time to complete the trials was revealed to participants only after the conclusion of all experimental tests.
V˙O2, V˙CO2, V˙E, respiratory exchange ratio (RER) and HR were continuously measured during the entire test using the same equipment used for the incremental test. PO was measured every second via software connected to the cycle simulator (RacerMate One, Seattle, Washington, USA). EMG signals of the VL muscle of the right leg were recorded via bipolar Ag-AgCl surface electrodes at an inter-electrode distance of 20 mm. The reference electrode was placed over the fibular head surface. The skin preparation, placement, and location of the electrodes were in accordance with the recommendations of Hermens et al. (21). The EMG signal was continually measured with a sampling frequency of 2000 Hz (Telemyo 900, Noraxon, Scottsdale, AZ, USA). Blood samples (25 μL) were collected from the earlobe before the trial, and immediately, 3, 5, and 7 min after the trial. Blood samples were immediately placed in microtubes containing 1% of sodium fluoride, and centrifuged at 3000 rpm for 5 min to separate the plasma. Plasma lactate concentration ([La]) was determined with commercial kits (Biotecnica, Varginha, Brazil) using a spectrophotometer (Eon, Biotek, Winooski, USA).
Creatine supplementation protocol
Each group received Cr monohydrate (Creapure, Alzchem, Germany; 4 × 5 g·d−1, total 20 g·d−1) or a placebo (dextrose; the same supplementation regimen) for 5 d. Dextrose (5 g per dose) was also added to Cr to mask its flavor. The participants were advised to consume their supplements along with meals (e.g., breakfast, lunch, afternoon snack, and dinner). The supplement packages were coded so that neither the investigators nor the participants were aware of the contents until the completion of the analyses. According to the manufacturer, Creapure shows typical analytical values of Cr monohydrate > 99.9%, creatinine < 67 ppm, dicyandiamide < 30 ppm, and undetectable traces of dihydrotriazine.
Determination of aerobic and anaerobic mechanical PO
Pae and Pan were calculated second-by-second during the trial. First, the metabolic PO (Pmet) was calculated after achieving steady state during a warm-up at 150 W using the follow equation (15):
where RER is the respiratory exchange ratio and V˙O2 is the oxygen uptake.
Then, gross mechanical efficiency was determined by dividing the value of the warm-up PO (150 W) for Pmet. During the time trial, Pmet was estimated using equation 1 and adopting an RER equal to 1 (24). Pae was calculated by multiplying the Pmet by the gross mechanical efficiency. Pan was calculated by subtracting the Pae from the external total PO. As the time to complete the time trial differed for all participants, for further analysis Pae, Pan and PO were plotted against time from the 1st to the 75th second (last second in which there are data for all participants). The total external work was calculated from the integral of the external PO versus time.
V˙O2 response analysis
To characterize the V˙O2 response, breath-by-breath records were initially examined to exclude errant breaths (values lying more than 4 SD from the local mean) as described elsewhere (2). Then, breath-by-breath data were linearly interpolated to provide second-by-second values. Given that the participants only completed one trial in each condition, and the measured POs throughout the trial were above PPO, we used a monoexponential model to characterize the response of the overall V˙O2 response, as previously suggested (2,322,32):
where V˙O2(t) is the oxygen uptake at a given time t, V˙O2baseline is the mean V˙O2 during the last 60 s of the rest period, A is the exponential amplitude above the baseline, and τ the time constant.
The data were adjusted using a nonlinear least square algorithm (2,322,32). Because we were more interested in characterizing the overall V˙O2 response during the trials and calculating a corresponding AOD, a single-exponential model without time delay, with the fitting window commencing at t = 0 s (equivalent to the mean response time [MRT]) was performed as recommended (2,402,40). The AOD was calculated by multiplying the MRT by the V˙O2 amplitude (i.e., A) (2). The total O2 consumed (in liters) was also computed from the integral of the V˙O2 versus time.
Analysis of the electromyographic signal
The raw EMG signal was full-wave rectified and filtered with a second-order, Butterworth, band-pass filter with cutoff frequencies set at 10 and 400 Hz. An envelope representing the muscle activation was determined using a moving RMS filter with a window of 50 ms. The period of activation during a burst was determined as the period where the signal was above a threshold of 15% of the maximum activity during the trial for at least 100 ms. These parameters were selected based on the signal–noise relationship of the EMG data and were visually verified to correctly identify periods of muscle activation (8,33,348,33,348,33,34). For each burst of EMG activation, we calculated the iEMG, defined as the area under the EMG versus time curve divided by the period of activation. Mean iEMG was calculated for 5-s intervals throughout the time trial, and these values were normalized to the mean iEMG during the last minute of the warm-up at 70% of PPO (1). The procedures were performed using Noraxon’s Myoresearch software (version 1.08).
The data were analyzed to determine whether any significant differences existed between the pre- and postloading period. Data normality was assessed using the Shapiro–Wilk test. Since all variables were normally distributed, differences between the pre- and postloading period for dependent variables were examined using paired t-tests. The influence of loading on the V˙O2, PO, Paer, Pan, and iEMG was explored with a two-way (pre- and postloading vs time) repeated-measures ANOVA followed by Bonferroni adjustment to localize such differences. Significance was accepted when P < 0.05. Values are reported as means ± SD unless otherwise noted. All statistical analyses were conducted using the SPSS statistical package (SPSS, version 13.0).
Compliance with supplementation and exercise, and body mass and fat-free mass
All participants recorded in their dietary-exercise diary that they strictly adhered to the dietary guidelines and did not perform any exercise during the 24 h before the trials. Only two participants in the Cr group and two in the placebo group were able to correctly distinguish which supplement they had consumed. Dietary Cr supplementation resulted in a significant increase in both the body mass (69.9 ± 6.8 kg vs 70.7 ± 7.5 kg, P = 0.03) and fat-free mass (62.1 ± 4.6 kg vs 62.8 ± 5.1 kg, P = 0.04). No differences between pre- and postsupplementation were found in the placebo group for body mass (72.8 ± 5.9 kg vs 73.0 ± 5.7 kg, P = 0.12) or fat-free mass (63.2 ± 6.9 kg vs 63.2 ± 7.1 kg, P = 0.69).
Power output, aerobic and anaerobic contributions, and V˙O2 response
The PO, Paer, and Pan profiles are shown in Figure 1, while V˙O2 during the time trial is shown in Figure 2. For both groups (Cr and placebo), participants adopted an “all-out” pacing strategy reaching a peak power ∼ 7 s after the trial had started, followed by a gradual decline in PO until the end of the trial (Fig. 1A and B). There was no main effect (P > 0.05) of supplementation or a supplementation–time interaction for PO in both groups (Fig. 1A and B). Although there was no main effect of supplementation for V˙O2, Paer, and Pan (P > 0.05), there was a supplementation–time interaction in the Cr group (P < 0.05), but not in the placebo group (P > 0.05). Cr loading reduced V˙O2 in the interval between 12 and 23 s (Fig. 2A), and this reduction was accompanied by a reduction in Paer during the interval between 15 and 25 s (Fig. 1C). Otherwise, Cr loading increased Pan during the interval between 17 s and 28 s compared to preloading (Fig. 1E). There was no alteration in the placebo group for any of these variables (Fig. 1B, D, and F).
The MRT was lower (∼18%) after Cr loading, compared to preloading (P < 0.05; Table 2). However, the MRT was not significantly altered in the placebo group. End V˙O2 was not altered in either group, but the total O2 consumed over the trial was reduced after Cr loading, without an alteration in the placebo group (Table 2). The oxygen deficit was also increased by 19% after Cr loading (P < 0.05), while it remained unaltered in the placebo group (Table 2). The peak plasma lactate concentration was similar pre- versus postsupplementation for both groups (Table 2).
The iEMG of the VL is shown in Figure 3. There were no significant changes from pre- to postsupplementation in iEMG for either group.
Performance and total mechanical work
The mean PO and time to complete the 1-km time trial were similar between pre- and postloading for both the Cr and the placebo group (Table 2, P > 0.05). Similarly, the total mechanical work was not altered from pre- to postsupplementation in either group (Table 2, P > 0.05). The estimated efficiency at 150 W was also not altered from pre- to postsupplementation in either group (Cr pre: 20.5% ± 1.3% vs Cr post: 20.6% ± 1.3% and placebo pre: 20.0% ± 1.2% vs placebo post: 20.6% ± 1.2%, P > 0.05).
Consistent with our hypothesis, an increase in metabolic capacitance (via Cr loading) led to a significantly slower V˙O2 response and a lower total O2 consumption during a self-paced, 1-km cycling time trial performed at a supra-V˙O2peak intensity. This slower V˙O2 response was accompanied by a reduction in the mechanical aerobic contribution and an increase in both the mechanical anaerobic contribution and the AOD. To the best of our knowledge, this is the first study showing that after 5 d of Cr loading, there is a slower V˙O2 response, which is “compensated” by a greater anaerobic contribution, during a self-paced, 1-km cycling time trial performed at supra-V˙O2max intensities.
The Cr loading protocol used in the present study has been demonstrated to increase total muscle Cr content by 15%–20% and the muscle PCr content by 10%–20%, although this increase occurs in most, but not all, individuals (18,19,2318,19,2318,19,23). This increase would lead to an increase in the metabolic capacitance, as predicted by the linear model of respiratory control (30). Based on this model, it can be hypothesized that an increased metabolic capacitance, due to higher total muscle PCr content, would slow the V˙O2 kinetics (and PCr kinetics) during the onset of exercise.
It is important to underline the main differences between constant-load and all-out exercises before discussing the main mechanisms by which Cr loading may have caused a slowing of the V˙O2 response in the present study. High-intensity, constant-load exercises show a time-dependent increase in iEMG amplitude, which in turn reflects a progressive recruitment of additional fast-twitch motor units and/or an increase in the firing frequency of motor units that have already been recruited (37). These patterns of recruitment have been associated with a continued increase in V˙O2 over the time, the so-called V˙O2 slow component (27). This slowing of the V˙O2 response theoretically reflects the recruitment of less efficient type II muscle fibers (i.e., fast-twitch motor units) (27). However, adoption of an all-out exercise is expected to activate all muscle fibers types from exercise onset, “anticipating” the recruitment of type II muscle fibers and the start of the V˙O2 slow component (37).
While it is not yet possible to accurately measure fast- and slow-twitch motor unit recruitment separately during whole-body exercise, the iEMG amplitude has been recognized as an important tool to reflect muscle recruitment (4,6,164,6,164,6,16). Our results showing similar iEMG in the Cr and placebo groups throughout each trial suggest that Cr loading did not alter muscle recruitment patterns. Nonetheless, total O2 consumed was reduced, suggesting that Cr loading influences intracellular oxygen uptake without altering the muscle recruitment profile during an all-out time trial. It is interesting to note that a reduction in the V˙O2 amplitude and end V˙O2 has been reported during constant-load exercise performed at a heavy (which requires additional activation of type II muscle fibers) rather than a moderate intensity (which recruits predominantly type I muscle fibers) after Cr loading (26). In addition, individuals with a higher proportion of type II muscle fibers appear to have a greater reduction in V˙O2 during heavy exercise after Cr loading (26). In the present study, we used a 1-km time trial, performed above PPO and requiring an all-out pacing strategy, which would have required the recruitment of type II muscle fibers.
While the V˙O2 response was slowed and total O2 consumed reduced, pacing profile was not altered. A reduced V˙O2 in the second part of the trial without a corresponding PO reduction would indicate either an increased muscle efficiency or a compensatory increase in the anaerobic contribution. Although muscle efficiency may differ between low- and high-intensity exercises (10), we found no difference in the estimated efficiency from pre- to postsupplementation in either group. On the other hand, we found an increase in the Pan in the second part of the trial, suggesting that a reduced V˙O2 might have been compensated by an increased anaerobic contribution. It is unlikely that the greater anaerobic contribution can be attributed to an increase in anaerobic glycolysis. Plasma lactate at the end of the trial was similar between pre and post conditions in both groups. In addition, PCr hydrolysis may inhibit glycolysis, and therefore, an increased PCr availability resulting from Cr loading should reduce, not increase, the rate of anaerobic glycolysis (38). It might be suggested that there was a greater contribution of PCr breakdown to ATP turnover after Cr loading, which would serve as a temporal energy buffer at the beginning of the trial, sparing O2 demand and slowing the V˙O2 response. This supposition would be consistent with other previous studies that indicated a critical role for PCr breakdown in the regulation of mitochondrial respiration (7,29,317,29,317,29,31). However, because PCr kinetics are slower after Cr loading, any energy-buffer effect of Cr loading would be possible only with a progressive, time-dependent greater PCr split amplitude throughout the trial. While there is no study investigating the PCr amplitude decrease during an all-out time trial, studies with calf or knee extensors at constant-load, heavy-intensity exercise have suggested an increased (14) or non-altered (28) PCr amplitude after Cr loading. Therefore, while a slower V˙O2 response and an increased Pan at the onset of an all-out exercise after Cr loading found in the present study would suggest that the increased PCr content into the skeletal muscle may buffer a decrease in [ATP]/[ADP] ratio (the main parameter governing V˙O2 response), further research is required to investigate this hypothesis.
Time to complete and total mechanical work performed during the task were not improved after Cr loading. A nonimproved overall performance is in accordance with other studies evaluating the effect of Cr supplementation on performance during continuous exercise lasting more than 30 s (5). In a meta-analysis (5), Branch verified that the effect of Cr loading is more evident in continuous exercise lasting up to 30 s. When exercise takes longer, the Cr loading effects tend to be less pronounced, probably due to the progressive increase in the contribution of other energy systems (i.e., lactic and aerobic energy systems). Unfortunately, most previous studies have evaluated the effects of Cr supplementation on performance during constant-load tasks (20,22,3620,22,3620,22,36); therefore, the extrapolation of these results to time trials is limited. In the present study, Cr loading was unable to promote significant improvements in overall performance during a self-paced, 1-km cycling time trial.
It is important to recognize that we did not measure intramuscular PCr or Cr; therefore, we are unable to provide definitive evidence of a connection between PCr, Cr, and the V˙O2 response during an all-out time trial. However, the participants showed a high compliance with the supplementation procedures and all Cr-supplemented participants increased their body mass and fat-free mass after the loading period. Several studies have observed that an increase in Cr and PCr content after a Cr-loading period is associated with an increased body mass and fat-free mass (11,1411,14). Furthermore, many studies have reported an increase in muscle PCr and Cr content following the Cr loading protocol used in the present study (18,19,2618,19,2618,19,26). In addition, alterations of the V˙O2 response in the Cr, but not the placebo group, suggest that muscle PCr and Cr were likely increased in the Cr group in the present study. Another limitation may be that, like others (33,3733,37), we used only a single test to characterize the V˙O2 response during the supramaximal test. It has been recommended that at least 4–6 tests should be performed to increase signal-to-noise ratio and the confidence of the kinetics parameters during moderate exercise. However, during supramaximal tests (i.e., above PPO), signal-to-noise ratio is naturally increased and a reduced number of tests is required (32). In addition, because we used a monoexponential model, we were able to use an amplitude-fixed term, which increased the confidence of the MRT parameter. The fact that no supplementation effect was found in the placebo group suggests a good reproducibility for the MRT values in the present study. Finally, a multivariate analysis could have provided a better understanding about which of the dependent variables (i.e., V˙O2 response, iEMG, and anaerobic contribution) interacted to influence metabolic control, and which of the variable contributed the most to the observed differences. However, because we have investigated several dependent variables, multivariate analysis would demand a much larger sample size (13), which is often difficult because investigations of this nature involve invasive procedures, multiple measurements, and many days of tests. Future studies should consider recruiting larger sample sizes and using a multivariate approach.
While previous studies provided important advances in our understating of respiratory regulation during constant-load exercise (26,2826,28), the experimental design of the present study enabled us to better understand the critical role of an increased metabolic capacitance on the V˙O2 response during an all-out, self-paced exercise. In this study, we observed that 5 d of dietary Cr supplementation resulted in a significant slowing of the V˙O2 response in the transition from rest to the completion of a self-paced, 1-km, cycling time trial performed using an all-out strategy. An increased Pan at the onset of the exercise and an increased total AOD accompanied this lower V˙O2 response. To the best of our knowledge, this is the first study to demonstrate that an intervention designed to alter total muscle [PCr] results in a corresponding effect on the V˙O2 response and anaerobic contribution, which may indicate a Cr loading-induced PCr buffer effect at the onset of a supramaximal, all-out exercise. As the iEMG signal was not altered, it seems improbable that Cr loading has any effect on the muscle recruitment pattern; this restrains the Cr-loading effects to intramuscular alterations. Our physiological results are also in line with other studies that implicate metabolic capacitance (i.e., PCr content) as a critical component affecting respiratory control at the onset of exercise (28,30,3828,30,3828,30,38).
In conclusion, Cr loading was associated with a slower V˙O2 response and a lower total O2 consumed during a self-paced 1-km time trial performed at a supramaximal intensity. Cr loading also induced an increased mechanical anaerobic contribution and an increased AOD, with no change in the number of motor units recruited and/or the firing frequency of these motor units (as indicated by iEMG) or plasma lactate. Cr loading did not alter pacing and did not improve the overall time trial performance.
Kleiner Marcio de Andrade Nemezio is grateful to Coordination for the Improvement of Higher Education Personnel (CAPES) for his masters scholarship. The authors thank all of the cyclists who took part in this study.
No financial support was received.
The authors declare no conflict of interest.
The results of this study do not constitute endorsement by the American College of Sports Medicine.
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