In a classic series demonstrating the shift of energy usage in skeletal muscle, Hirvonen et al. examined the changes in the intramuscular concentrations of muscle ATP, PCr, and blood lactic acid concentration in sprinters running distances of 40 to 400 meters lasting approximately 4.5 to 50 seconds . Blood lactic acid is a reflection of muscle glycogen usage or glycolysis. Even on casual observation it is interesting to note that despite the increase in running distance, ATP stores initially decline but appear to reach a minimal or critical level after which no further decrease is observed. In contrast, PCr continually and rapidly decreases while the appearance of blood lactic acid or muscle glycogen usage increases. At this point, the reader should embrace two key points. One is to recognize the immediate contribution of all energy systems simultaneously and cooperatively to facilitate energy needs. These energy contributions do not occur sequentially (i.e., one after the other), but instead are time and intensity dependent as to which system dominates. This continued energy production has been conceptualized as a metabolic flux or energy currency that transforms stored energy into muscle contraction.

The maximum work attainable from any energy source can be characterized as both ATP capacity (amount of ATP produced per mole of available substrate) and ATP power (the rate of ATP produced per substrate storage depot). Despite the low intramuscular stores of ATP and PCr within skeletal muscle, their energy production capabilities are exceptional . A review by Sahlin provides an excellent consensus of research findings elaborating on the available energy capacity (mol ATP), maximal ATP power produced from each source (per mmol ATP/kg of dry muscle), as well as the exercise intensity supported and duration of activity allowed per endogenous energy source. It should become readily apparent that events of shorter duration and higher intensity necessitate physical training and nutritional support aimed toward the enhancement of ATP, PCr, and muscle glycogen as opposed to alternative energy sources such as liver glycogen and adipose stores. These sources serve as less adequate sources of ATP power.

Research has shown an improvement in endogenous energy stores of glycogen, PCr, and ATP following 5 months of heavy resistance training although this correlation has not been universally shown. Although changes in resting PCr concentrations might enhance performance during anaerobic activities (e.g., sprinting, weightlifting, etc.), increases in skeletal muscle glycogen may not confer a similar advantage in these types of activities. Strong evidence shows that the dietary manipulation of glycogen stores does not improve various anaerobic performance indices. Also, oral ATP administration does not appear to be prudent owing to the presence of phosphatase enzymes in the blood and gut. These enzymes readily cleave the phosphate portions of ATP. Thus, it appears that oral ATP does not present itself as a suitable ergogenic aid. The same point may be argued for PCr as well. Thus, it is plausible that creatine ingestion may be the best way of augmenting athletic performance vis-a-vis changes in the phosphagen energy system.

One study that examined PCr supplementation showed a performance benefit, albeit to a smaller degree than creatine supplementation. However, any effect of orally ingested PCr would be expected to be mediated by creatine alone because gut phosphatase enzymes would readily cleave off the phosphate portion of the molecule, liberating free creatine in a smaller quantity than when taking the monohydrate form. To date, no human studies have evaluated PCr’s oral absorption and intramuscular uptake. Additionally, blood serum also possesses high phosphatase activity, leading to rapid breakdown of intravenously administered PCr to creatine and phosphate. A study by Peeters et al. determined that PCr-supplemented subjects exhibited a performance response that was approximately 50% less than than that of a creatine monohydrate group. Given that the creatine portion of the monohydrate form makes up about 92% of the molecule and only 50% of the PCr molecule, these results are not surprising. Similarly, although glycolysis is initiated at muscle contraction, increasing glycogen stores may be more advantageous to longer, high-intensity work efforts (>400 m) because of its lower ATP power. These same objectives do not appear to apply to the use of creatine supplementation because both early and more recent studies involving creatine show that it is readily found in food, is absorbed intact, appears rapidly in the blood, and increases intramuscular stores of total creatine and PCr.