Research Review By Christopher Howard©

Date Posted:

December 2009

Study Title:

Effects of Four Weeks of High-Intensity Interval Training and Creatine Supplementation on Critical Power and Anaerobic Working Capacity in College-Aged Men


Kendall KL, Smith AE, Graef JL et al.

Author's Affiliations:

Metabolic and Body Composition Laboratory; and Biophysics Laboratory, Department of Health and Exercise Science, University of Oklahoma, USA.

Publication Information:

Journal of Strength and Conditioning Research 2009; 23(6): 1663-1669.

Background Information:

Increasing training intensity can improve athletic performance, however prolonged high-intensity training results in the buildup of hydrogen ions, leading to acidosis and fatigue. High-Intensity Interval Training (HIIT) leads to chronic adaptations that improve metabolic and energy efficiency.

During a session of repeated bouts of HIIT, each interval is started at a lower pH, due to hydrogen ion accumulation in the muscle. This hydrogen ion accumulation is believed to be an important stimulus in improving intramuscular buffering capacity. Improvements in intramuscular buffering capacity following a HIIT program has been shown to delay the onset of muscle fatigue as well as increase peak anaerobic power and total amount of work done.

The critical power test measures both critical power (CP) and anaerobic working capacity (AWC). 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 and time to exhaustion are determined. 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. HIIT has been shown to improve endurance capacity as measured by maximal oxygen consumption rate (VO2PEAK) and time to exhaustion.

The use of creatine as an ergogenic aid is well established. Creatine has been demonstrated to increase muscle strength, power output, and muscle mass. Creatine supplementation has also been shown to improve AWC in men and women, although it appears to have little effect on CP. Although there is limited evidence on the effects of creatine on endurance performance, the authors hypothesized that combining creatine with HIIT may improve CP. To date, no one has examined the effects of creatine supplementation and HIIT on CP and AWC.

Pertinent Results:

  • There were no significant changes in body mass from baseline to post-testing for any of the groups.
  • The results of the ANOVA for CP indicated time x treatment effects. Follow up t-tests reported a significant increase in CP for the creatine group (p=0.013), no change for the placebo group (p=0.077), and a significant decrease in the control group (p=0.034).
  • The ANOVA for AWC indicated no significant time x treatment interaction (p=0.222), nor significant main effects for time (p=0.741) or treatment (p=0.535).

Clinical Application & Conclusions:

This study sets the stage for additional investigation into the effects of creatine supplementation on measures of aerobic performance, namely critical power. Based on the findings of this study, it is within reason that recreationally active men could begin creatine supplementation to improve aerobic performance in addition to a high-intensity interval training program.

Study Methods:

Experimental Approach:
Few studies have examined the effects of low-dose creatine supplementation on aerobic and anaerobic measures. Therefore, the aim of the study was to determine whether the combination of low-dose creatine supplementation and HIIT could lead to improvements in AWC as well as CP. This study was double blinded and placebo controlled.

During the course of the study, participants were asked to maintain their current exercise and dietary habits and to abstain from any consuming additional nutritional supplements. After pre-testing, participants were randomly assigned to 1 of 3 treatment groups (Creatine, n = 16; placebo, n = 16; or control, 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 creatine supplementation and HIIT.

  • Forty-two recreationally active college-aged men participated in this study.
  • Recreationally active was defined as 1-5 hours per week of aerobic, resistance training, or recreational activities.
  • Participants were 23 years old, 177 cm tall, 82 kg in weight, with a VO2peak of 43, on average.
VO2peak 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 VO2peak.
  • 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 20W 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 VO2PEAK.
  • After a familiarization bike test, participants returned 2 weeks later to perform a baseline VO2PEAK bike test. The 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 VO2PEAK test, each participant returned to complete a CP test. Participants completed 3 cycling bouts to exhaustion. Power outputs for the exercise bouts were established from the VO2PEAK test.
  • The first bout was performed at 110% of the participants’ VO2PPO; 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. The exercise bout was immediately terminated when the participant could no longer maintain 65 RPM as determined by the monitor on the ergometer. CP and AWC were determined from this test.
High-Intensity Interval Training (HIIT):
  • 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 VO2PPO, 3 days per week. One recovery day occurred between each of the difficult training sessions at an intensity of 80% VO2PPO.
  • Difficult training days increased in intensity, with the first session beginning at 90% VO2PPO and progressing up to 120% VO2PPO.
  • 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 VO2PPO, 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 re-tested VO2PPO and CP before completing 4 additional weeks of training at intensities based on their new baseline VO2PPO values.
Supplementation Protocol:
  • After familiarization testing, participants were randomly assigned, in a double-blind fashion, to either a creatine or a placebo group.
  • Supplementation continued for 30 days (10 days of supplementation during the familiarization period followed by baseline testing and an additional 2 days of supplementing before post-testing) 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 creatine group consumed 5 g of Cr mixed with 15 g dextrose in 4 to 8 ounces of water before and after their training.
  • Participants in the placebo 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.

Study Strengths / Weaknesses:

It has previously been shown that vegetarians have lower levels of intramuscular creatine than non-vegetarians. No mention was made as to the status of the participants in this regard. More importantly and more to the point, pre-testing creatine phosphate levels in these subjects were unknown, therefore it is unknown how supplementation with creatine affected these subjects.

Furthermore, the supplementation protocol used (10g/day on training days only), may not have sufficiently elevated creatine levels and thus no change in AWC was seen.

With all that being said, this was a very well designed study. Allowing participants to become familiar with the exercise protocol prior to baseline testing was a good idea and most likely added to the accuracy of this study.

Additional References:

  1. Becque, MD, Lochmann, JD, and Melrose, DR. Effects of oral creatine supplementation on muscular strength and body composition. Med Sci Sports Exerc 2000; 32: 654–658.
  2. Creer, AR, Ricard, MD, Conlee, RK, Hoyt, GL, and Parcell, AC. Neural, metabolic, and performance adaptations to four weeks of high intensity sprint-interval training in trained cyclists. Int J Sports Med 2004; 25: 92–98.
  3. Holloszy, JO. Muscle metabolism during exercise. Arch Phys Med Rehabil 1982; 63: 231–234.
  4. Laursen, PB and Jenkins, DG. The scientific basis for high-intensity interval training: optimising training programmes and maximizing performance in highly trained endurance athletes. Sports Med 2002; 32: 53–73.