Subject characteristics (age, height, body mass, percent fat, VO2peak, and training mileage) are presented in Table 1. Body mass was 2.0 kg higher after supplementation than before supplementation (P < 0.05). There were no differences between creatine and placebo groups for all other descriptive variables.
Sprint time
The final sprint times prior to supplementation were 64.4 ± 13.5 and 69.0 ± 24.8 seconds in the creatine and placebo groups, respectively (Figure 2). There was a main effect (P < 0.05) for sprint time pre to post supplementation, in that creatine and placebo groups both increased final sprint times following supplementation by approximately 25 seconds.
Power output
The power output for the final sprint prior to supplementation was 23,459 ± 6,430 and 19,509 ± 2,969 joules in the creatine and placebo groups, respectively. There was a main effect (P < 0.05) for power output pre to post supplementation, in that creatine and placebo groups both increased final power output after supplementation by approximately 33%. The power output for the final sprint after supplementation was 30,811 ± 10,198 and 26,599 ± 3,772 joules in the creatine and placebo groups, respectively.
Respiratory exchange ratio (RER) and oxygen consumption (VO2)
Mean RER values during the two-hour cycling bout were similar in both groups prior to supplementation and decreased from approximately 0.91 to 0.82 from 7 to 119 minutes of the cycling bout. RER during the ride was not affected by the type of supplementation, in that both creatine and placebo groups demonstrated a decline in RER over time (Figure 3a). There was an interaction in submaximal VO2 (Figure 3b) at minute 119 of the cycling bout due to the lower oxygen consumption after than before creatine ingestion and the higher oxygen consumption after than before placebo ingestion.
Blood glucose and lactate
There was a main effect for plasma glucose pre- to post-supplementation (P < 0.05; Figure 4a) resulting from higher plasma glucose concentrations after than before supplementation in both creatine and placebo groups. Blood lactate was higher in the creatine group than the placebo group during the 2-hour cycling bout both before and after supplementation (Figure 4b). There was a four- to six-fold increase in blood lactate from rest to the end of each set of sprints, although blood lactate was only two- to three-fold higher than resting at the end of each 15-minutes of cycling at 60% VO2peak. Blood lactate was not different after, compared to before, supplementation in either creatine or placebo groups.
Hemoglobin, hematocrit, and plasma volume
Hemoglobin and hematocrit were approximately 10% higher in the creatine group (48% and 17 mg/dl) than placebo group (43.5% and 15.5 mg/dl) both before and after supplementation: there was no effect of supplementation on either variable (Figures 5a and 5b). The changes in hemoglobin and hematocrit were reflective of changes in resting plasma volume from pre- to post-supplementation of +4.7 ± 4.7% and +0.5 ± 2.1% in the creatine and placebo groups, respectively (P = N.S.). Changes in plasma volume from pre- to post-supplementation were significantly greater in the creatine group (+14.0 ± 6.3%) than the placebo group (-10.4 ± 4.4%; P < 0.05) at 90 minutes of exercise.
Muscle creatine, total creatine, creatine phosphate, and adenosine triphosphate
Resting muscle total creatine concentrations (Figure 6a) were higher in the creatine than placebo groups both before and after supplementation, although muscle total creatine increased following supplementation in both groups. When calculating the increase in muscle creatine for each individual pre- to post-supplementation, the mean increase in muscle total creatine was 24 ± 11% in the creatine group and 15 ± 3% in the placebo group (p = N.S.).
Muscle creatine phosphate (CP; Figure 6b) at rest was not different between creatine and placebo groups prior to supplementation, although muscle CP was higher following supplementation in the creatine than placebo group (P < 0.05). When calculating the increase in muscle CP during supplementation on an individual basis, the increase in resting muscle CP was 38 ± 27% in the creatine group and 14 ± 11% in the placebo group. There was a significant drop in muscle CP by the end of the two-hour ride after supplementation in the placebo group (P < 0.05), although this drop was not as evident in the creatine group (Figure 6b). There was no correlation between the change in muscle creatine phosphate and the change in sprint performance from pre- to post-supplementation.
Resting muscle creatine concentration (Figure 6c) was increased by supplementation in the creatine group (P < 0.05). Muscle creatine concentration was increased (P < 0.05) to a similar extent during the two-hour cycling bout in creatine and placebo groups.
With respect to muscle ATP content (Figure 6d), there was a significant main effect for time, in that there was a drop in muscle ATP over the two-hour cycling bout prior to supplementation that was not seen following supplementation in either creatine or placebo groups. There was therefore no effect of supplementation on muscle ATP content in resting or exercising muscle.
Muscle lactate and glycogen
Muscle lactate (Figure 7a) concentration increased for both creatine and placebo groups from rest to the end of the two-hour cycling bout before supplementation; however, after supplementation both groups exhibited less of an increase in muscle lactate during the two-hour cycling bout. Muscle glycogen content (Figure 7b) was reduced (P < 0.05) by approximately 600 mmol/kg dry mass both before and after supplementation in creatine and placebo groups. After supplementation, muscle glycogen content at the end of the two-hour ride was higher in the creatine than placebo group (P < 0.05) due to the higher resting muscle glycogen content after supplementation in the creatine than placebo group.
Muscle fiber composition
Fiber type percentage in the creatine group was 46.8 ± 3.6, 42.7 ± 2.4, and 10.5 ± 2.5% for type I, type IIa, and type IIb fibers, respectively. Fiber type percentage in the placebo group was not different from that of the creatine group, with fiber type percentages of 42.5 ± 2.3, 48.7 ± 3.8, and 8.5 ± 3.0% for type I, type IIa, and type IIb fibers, respectively. Type I fiber percentage was correlated with muscle total creatine (r = 0.62, P < 0.05) and muscle creatine phosphate (r = 0.65, P < 0.05). Fiber type percentage was not significantly correlated with sprint performance time, nor with the change in muscle creatine concentration from pre- to post-supplementation.
Side effects
Regarding side effects (data not shown), two of the 12 subjects reported experiencing muscle cramps at rest following supplementation. There were no reports of muscle cramping prior to supplementation. Both of the subjects who reported muscle cramping following supplementation were in the creatine group. There were no other reports of side effects (chest pain, fatigue, upper-respiratory and auditory problems, autoimmune reactions, gastrointestinal difficulties, syncope, joint discomfort, appetite, headache, memory, stress and mood changes) that were unique to the creatine supplementation.