It is well established that increasing training intensity can improve athletic performance (1); however, prolonged high-intensity training results in the buildup of hydrogen ions (H+), leading to metabolic acidosis and, ultimately, fatigue. Research has shown that replacing longer bouts of exercise with short-duration, high-intensity intervals often leads to significant improvements in aerobic parameters while delaying muscle fatigue (51). The purpose of high-intensity interval training (HIIT) is to repeatedly stress the body, physiologically, resulting in chronic adaptations and improving metabolic and energy efficiency. When repeated high-intensity efforts are separated by short rest periods, subsequent exercise bouts begin at a much lower muscle pH level, indicating a greater accumulation of H+ (52). The large accumulation of H+ may serve as an important stimulus to improve intramuscular buffering capacity (βm) (10). Improvements in βm after HIIT has been shown to delay the onset of muscle fatigue as well as increase peak anaerobic power and the total amount of work done (32,36,37).
The critical power (CP) test, which measures both CP and anaerobic working capacity (AWC), has been shown to be both reliable in measuring aerobic and anaerobic parameters as well as sensitive to changes with high-intensity exercise training (18,31,42). The CP test involves a series of exhaustive work bouts at various supramaximal intensities, completed on a cycle ergometer, in which the total amount of work (work limit [Wlim]) and the time to exhaustion (time limit [Tlim]) are determined. The relationship between Wlim and Tlim has been reported to be highly linear (r > .98) (40,41) and can be expressed as a linear regression equation (Wlim = a + b[Tlim]). CP corresponds to the highest sustainable power output that can be maintained for an extended period of time, whereas AWC reflects an individual's total metabolic work capacity independent of oxygen use. Although very few studies have been able to demonstrate significant improvements in CP after HIIT, it has been shown to improve endurance capacity as measured by maximal oxygen consumption rate (O2PEAK) and time to exhaustion (12,24,51). Interestingly, HIIT may significantly improve AWC by increasing βm, thereby delaying the onset of fatigue (31).
The use of creatine (Cr) as an ergogenic aid in anaerobic performance is well established and has been demonstrated to increase muscle strength, power output, and muscle mass (3,15,33). The synthesis/resynthesis of phosphocreatine (PCr) by way of Cr supplementation can provide a rapid source of energy, which can be used in high-intensity, short-duration exercises. Cr supplementation has largely been studied in an effort to increase energy production in the adenosine triphosphate (ATP)-PCr energy system (53). Cr supplementation has been shown to improve AWC in men and women, which is primarily limited by the amount of energy available from stored ATP and PCr stores (15). Furthermore, despite the positive effects of Cr supplementation on AWC, it appears to have little effect on CP (30,47). Although there is limited evidence on the effects of Cr on endurance performance, we hypothesize combining Cr with HIIT may improve CP, an inherent characteristic of the aerobic energy system, as well as AWC. To date, no one has examined the effects of Cr supplementation and HIIT on CP and AWC.
Experimental Approach to the Problem
The anaerobic benefits of Cr are well documented (i.e., enhanced performance in resistance training and repeated sprint performance), whereas its use in aerobic performance has not been thoroughly investigated. In addition, very few studies have examined the effects of low-dose supplementation on aerobic and anaerobic measures. Therefore, the aim of the study was to determine whether the combination of low-dose Cr supplementation and HIIT could leaded to improvements in AWC as well as CP. The experimental design was double blind and involved 42 recreationally active college-aged men. During the course of the study, participants were asked to maintain their current exercise and dietary patterns and to abstain from any additional nutritional supplements. The time course of the experiment is represented in Figure 2. After pretesting, participants were randomly assigned to 1 of 3 treatment groups (Cr, n = 16; placebo [PL], n = 16; or control [CON], n = 10) and supplemented twice daily on training days for 30 days (10 day familiarization, 20 days baseline). Measurements of CP and AWC were chosen to best characterize changes in aerobic and anaerobic performance after Cr supplementation and HIIT.
Forty-two recreationally active (1-5 hr/wk aerobic exercise, resistance training, or recreational sports) college-aged men (mean ± SD; age: 23.62 ± 4.78 yr; height: 177.48 ± 7.05 cm; weight: 82.23 ± 12.70 kg; O2PEAK: 43.72 ± 9.96 mL/kg/min) volunteered to participate in this study. All procedures were approved by the University of Oklahoma Institutional Review Board for Human Subjects, and written informed consent was obtained from each participant before any testing. Supplement history was also recorded to ensure none of the participants had taken any supplement within 3 months before their initial testing date.
O2PEAK Bike Test
Participants performed a continuous graded exercise test on an electronically braked cycle ergometer (Lode, Corival 400, Groningen, The Netherlands) to determine the peak power output (PPO) in watts (W) at O2PEAK (O2PPO). Participants began pedaling at a cadence of 60 to 80 revolutions per minute (RPM) at a workload of 20 W. The workload increased 1 W every 3 seconds (a total of 20 W every min) until the participant was unable to maintain 60 to 80 RPM or until volitional fatigue. PPO was recorded as the value in W that occurred at O2PEAK. After a familiarizationO2PEAK bike test, participants returned 2 weeks later to perform a baseline O2PEAK bike test. Participants' values from the baseline test were used for the following 4 weeks of training.
Determination of Critical Power and Anaerobic Working Capacity
After a 48-hour rest period from the O2PEAK test, each participant returned to complete a CP test. The procedure for the CP test was based on the protocol previously described in detail by Housh et al. (27); participants completed 3 cycling bouts to exhaustion. Power outputs for the exercise bouts were established from the O2PEAK test. The first bout was performed at 110% of the participants' O2PPO; the second and third bouts were performed at power outputs prescribed specifically for each individual to elicit fatigue in 60- to 600 seconds. All 3 bouts were completed on the same day with a 15-minute rest period between each bout.
After a 5-minute warm-up at 50 W, participants started the test pedaling against zero resistance. Within 2 to 3 seconds of reaching a pedaling rate of 70 RPM, the appropriate power output was applied. Each participant was encouraged to maintain the required pedaling rate throughout the entire exercise bout. The exercise bout was immediately terminated when the participant could no longer maintain 65 RPM as determined by the monitor on the ergometer. Time limit (Tlim) was recorded to the nearest 0.1 second for each exercise bout, and work limit Wlim was calculated by multiplying power (P) in watts and Tlim (Wlim= P * Tlim). CP was represented by the slope of the line, whereas AWC corresponded with the y-intercept of the Wlim-Tlim relationship (40). Test-retest correlations of 0.92 and 0.97 for the slope and y-intercept, respectively, have been previously established (32). Test-retest reliability data for CP and AWC from the authors' laboratory for college-aged men (n = 10) measured 4 weeks apart resulted in an intraclass correlation of 0.96 and 0.61, respectively, with an SEM of 9.43W for CP and 2.85 kJ for AWC.
High-Intensity Interval Training
Participants were required to visit the laboratory 5 days per week for 6 weeks to perform HIIT. Participants trained at progressively increasing workloads, determined as a percentage of the participant's O2PPO, 3 days per week. One recovery day occurred between each of the difficult training sessions at an intensity of 80% O2PPO. Difficult training days increased in intensity, with the first session beginning at 90% O2PPO and progressing up to 120% O2PPO (Figure 1). Each training session started with a 5-minute warm up at 50 W, followed by a protocol of 5 or 6 2-minute exercise bouts at a predetermined percentage of their O2PPO, with 1 minute of complete rest in between exercise bouts. After a 2-week familiarization period, which consisted of 10 days of training and supplementation, participants retested O2PPO and CP before completing 4 additional weeks of training at intensities based on their new baseline O2PPO values. The dates and times of training were recorded for each participant to ensure training compliance. During the course of the study, participants were instructed to maintain their current physical activity level outside of the training completed in the laboratory.
After familiarization testing, participants were randomly assigned, in a double-blind fashion, to either a Cr or aPLgroup. Supplementation continued for 30 days (10 days of supplementation during the familiarization period followed by baseline testing and an additional 20 days of supplementing before post-testing) (Figure 2) at a dose of 10 g per day taken in 2 doses: 1 dose 30 minutes before training and 1 dose immediately after training. Participants only supplemented on training days (5 days/wk). Participants in the Cr group consumed 5 g of Cr mixed with 15 g dextrose in 4 to 8 ounces of water before and after their training, whereas participants in the PL group consumed 20 g of dextrose in 4 to 8 ounces of water before and after training. Both drinks were identical in appearance and taste.
Separate two-way mixed factorial analysis of variance (ANOVA) were run to determine changes in body mass (BM), CP, and AWC after treatment. Follow-up analyses included dependent t-tests and one-way ANOVAs. For effect size, the partial eta squared (η2) statistic was calculated, and, according to Green et al. (20), η2 of 0.01, 0.06, and 0.14 represents small, medium, and large effect sizes, respectively. Before all statistical analysis, the alpha level was set to p ≤ 0.05 to determine statistical significance.
Mean values for BM, CP, and AWC at baseline and after treatment for all 3 groups are presented in Table 1. There were no significant (p > 0.05) changes in BM from baseline to post-testing for any of the treatment groups. The results of the ANOVA for CP indicated there was a significant time × treatment interaction (p = 0.007, η2 = 0.32). Follow-up dependent t-tests reported a significant increase in CP for the Cr group (p = 0.013), no change for the PL group (p = 0.077), and a significant decrease for CP in the CON group (p = 0.034). The results of the ANOVA for AWC indicated no significant time × treatment interaction (p = 0.222, η2 = 0.01) and no significant main effects for time (p = 0.741) or treatment (p = 0.535).
Previous research has shown that HIIT is an effective training method for improving βm, delaying the onset of fatigue, and ultimately improving anaerobic power (32,36,37). Furthermore, Cr supplementation has been demonstrated to increase total muscle Cr stores, resulting in improved high-intensity exercise performance (2,14,21,22). The present study is the first to use a low-dose Cr supplementation protocol, in conjunction with HIIT, to assess changes in CP and AWC. The primary finding of this investigation was that 4 weeks of Cr supplementation along with HIIT significantly increased CP, whereas no significant increase was observed in the training-only (PL) or CON group. Furthermore, there was no change in AWC in any of the groups.
Interval training, described as periods of high-intensity work alternated with periods of low-intensity rest/work, is regarded as an overall effective training method for improving metabolic and energy efficiency (4,35). More specifically, during bouts of high-intensity exercise, the large accumulation of H+ and lactate impairs muscular force production and ultimately leads to fatigue. HIIT has been shown to improve βm, which, in turn, prolongs the onset of fatigue, resulting in improved performance (6,16). A few studies, including the current one, have examined the effects of HIIT on CP. Studies by Gaesser and Wilson (18) and Poole et al. (43) demonstrated increases in CP of 15% and 10%, respectively, after a HIIT intervention. The current investigation, however, revealed no significant increase in CP in the training-only group. A possible explanation for this would be the difference in training volume. Both Gaesser and Wilson and Poole et al. used a protocol of 10 2-minute work bouts, compared with the 5 bouts used in the current study; the higher training volume may have led to greater increases in aerobic performance. In support of our findings, Jenkins and Quigley (32), using a protocol similar to the current one, examined the effects of HIIT on CP and reported no significant change after 8 weeks of training. They concluded that the strong involvement of anaerobic glycolysis in response to the exercise demand resulted in an increase in the potential for anaerobic metabolism rather than aerobic. It was suggested, however, that an increase in intramuscular βm may have occurred because of the intense nature of the protocol. Training-induced elevations in βm, observed in previous studies (44,45) may help to attenuate the rate of intramuscular acidosis, thereby delaying the onset of fatigue.
PCr, a major component of biological buffering, has been reported to be significantly increased with Cr supplementation. During intensive exercise, PCr stores are rapidly depleted, leading to impairments in ATP production and the onset of muscle fatigue (9,28,48). Several studies have demonstrated that increasing total Cr stores can result in greater pre-exercise PCr availability, improved βm capacity, and an acceleration of PCr resynthesis during recovery (5,14,22). Therefore, combining high-intensity training with Cr supplementation should, theoretically, lead to substantial improvements in βm capacity and possibly aerobic performance because of a delay in the onset of fatigue. In support of this hypothesis, the current investigation demonstrated a significant increase in CP (6.72% ± 2.54%) for those participants supplementing with Cr. To our knowledge, this is the first study to show that HIIT, with concomitant Cr supplementation, leads to significant improvements in CP. Although the current study did not directly measure intramuscular levels of PCr, it has been repeatedly shown that supplementing with 3 to 20 g/day of Cr can significantly increase intramuscular levels of PCr (8,23,29,50). Furthermore, Harris et al. (23) showed that Cr uptake was enhanced through exercise. The majority of studies using Cr supplementation examine short-term (e.g. 5-7 days) Cr loading (20-30 g/day) on anaerobic performance. However, the present study demonstrated that low-dose Cr supplementation (10 g/day for 20 days) may significantly enhance CP, an aerobic measure of performance. It is hypothesized that the increase in PCr stores that occurs with Cr supplementation may prolong muscle fatigue by delaying the accumulation of adenosine diphosphate and inorganic phosphate (Pi), which are activators of anaerobic glycolysis (11). Reducing the reliance on anaerobic glycolysis may, in turn, improve exercise efficiency and aerobic performance, as demonstrated in this study.
Maximal anaerobic work is limited by the amount of energy available from stored ATP and PCr (13). Therefore, AWC represents the maximal workload potential associated with muscle reserves of ATP and PCr (7,40,41). As previously mentioned, during high-intensity exercise, metabolites including H+ and Pi accumulate in the muscle, leading to fatigue and a decrease in performance (34). HIIT has been shown to enhance the muscle's ability to remove such metabolites and facilitate the resynthesis of energy stores (17,26,46). Such adaptations should enhance anaerobic power during bouts of repeated exercise (19). Furthermore, Cr supplementation has been shown to improve performance through elevated stores of PCr, which are involved with the resynthesis of ATP, as well as the buffering of H+ during periods of maximal exercise (38). The current investigation, which used Cr supplementation in conjunction with HIIT, revealed no significant change in AWC in either training group. Training at exercise intensities that induce fatigue within 60 seconds may be necessary to elicit significant changes in AWC (32). The current study, which used a 2 minute:1 minute work-to-rest ratio, may have allowed a high proportion of the energy to be supplied aerobically, resulting in no significant change in AWC. In support, Medbo and Tabata (39) found that the energy contribution during a 2-minute maximal effort work bout was 65% aerobic and 35% anaerobic. Furthermore, several studies examining the effects of Cr supplementation on AWC found significant increases using a loading protocol (30,47,49). The purpose of using a 5- or 6-day loading phase is to maximize Cr concentrations in the muscle in a short period of time (29). The low-dose protocol used in the present investigation may not have increased PCr content enough to improve AWC. In support, Hoffman et al. (25) reported no significant improvements in anaerobic exercise performance after consuming 6 g of Cr for 6 days. Therefore, it may be necessary to use a 4- to 7-day loading phase to maximize Cr levels before a low-dose maintenance phase to see improvements in anaerobic performance and AWC.
The primary findings of this study support the use of Cr supplementation in conjunction with HIIT as a method for improving aerobic parameters, specifically CP. These results may be particularly useful for recreationally active athletes looking to improve aerobic performance but have been reluctant to supplement with Cr because of the perception that it leads to weight gain. In addition, these findings may provide a foundation for future studies examining the effects of low-dose Cr and HIIT on performance in endurance-trained or elite athletes.
The authors declare that they have no competing interests. The results of the present study do not constitute endorsement of the product by the authors or the NSCA. The authors thank FSI Nutrition Fortress Systems, LLC for supplying the supplements for this study.
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