Phosphocreatine is a high-energy storage molecule found within skeletal muscle. During intense exercise, phosphocreatine provides immediate replenishment of ATP (Adenosine-5′-triphosphate), which transports chemical energy from within cells for metabolic processes.

Traditional endurance training induces physiological adaptations such as improved aerobic capacity, along with a reduction in glycogen utilization and lactate accumulation. HITT has been shown to be an efficient way to produce similar effects.

Multiple HIIT bouts are designed to deplete phosphocreatine stores in the working skeletal muscle, and so reducing power output.

It takes more than six minutes to fully recover phospho-creatine stores after exercise-induced depletion. Therefore, if recovery intervals during HIIT bouts are less than six minutes, phosphocreatine may not be fully replenished, leading to a reduced performance.

Supplementing with creatine has been demonstrated to effectively increase muscle phosphocreatine stores. Specifically, one study showed a 20% increase in muscle creatine with ingestion of 20 g of creatine per day for just 5 days.

The purpose of the study published in the Journal of the International Society of Sports Nutrition was to determine the effects of high-intensity interval training and creatine supplementation on cardiorespiratory fitness and endurance performance.

The subjects were 43 college-aged men who typically performed 1-5 hours of exercise per week. None had taken sports supplements, including any form of creatine, in the three months prior to the beginning of the study.

Participants were randomly assigned to a creatine or a placebo group and supplemented for 30 days at a dose of 10 g per training day, taken in two doses – one dose 30 minutes prior to and one dose immediately following training.

• Creatine group consumed 5 g of creatine citrate mixed with 15 g dextrose
• Placebo group consumed 20 g of dextrose

A control group, consumed no supplements nor completed the high-intensity interval training, and instead only completed the testing measurements.

Participants performed HIIT five days per week, for six weeks at progressively increasing workloads, determined as a percentage of the participant’s baseline VO_2PEAK max workload. Training increased in intensity each session beginning at 90% of their VO_2PEAK max workload and progressing up to 120% of their VO_2PEAK max workload.

Each training session began with a five-minute warm up, followed by a protocol of five sets of two-minute exercise bouts, with one minute of passive rest in between exercise bouts.

A maximal graded exercise test on a cycle ergometer was used to determine:

• maximal oxygen consumption (VO_2PEAK)
• maximum heart rate (HRmax).
• total work done (TWD)
• time to exhaustion (VO_2PEAKTTE)
• ventilatory threshold (VT), this is the point during graded exercise in which venitilation increases disproportionately to oxygen uptake.

Endurance performance is commonly assessed using a measure of aerobic capacity. While the HIIT program was effective in improving maximal oxygen consumption by 9%, creatine supplementation had no further influence on aerobic capacity. Ventilatory Threshold (VT) is another useful predictor of endurance performance as an indicator of the ability of the cardiovascular system to adequately supply oxygen to the working muscles. Performing exercise at intensities greater than VT often result in an inadequate supply of oxygen to the working muscles, quickly leading to fatigue.

If VT can be improved it could increase the time to exhaustion, reduce fatigue, and may enhance the efficiency of the body to supply oxygen to the working muscles.

However, when measuring VT in this study, significant improvements were only observed in the creatine group (16%), although the placebo group demonstrated a trend for improved VT (10%).

The researchers from University of Oklahoma found that HITT is an effective way to improve maximal endurance performance. The study also demonstrated an improvement to time to exhaustion with HIIT. However, whilst adding the use of creatine did improved ventilatory threshold, it did not increase total work done.

Previous studies have shown improvements in TWD after creatine supplementation which included a loading phase (20 g/d for 5-7 days). A loading phase was not used in the current study, so it may be possible that muscle phosphocreatine levels were not increased enough to aid in improving TWD.

The researchers did suggest that it is possibile that any benefits of low-dose creatine supplementation were masked by the effectiveness of HIIT alone.

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