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Two intermittent high-intensity exercise protocols were performed before and after the administration of either creatine or a placebo, and performance characteristics and selected physiological responses were studied. Each exercise protocol consisted of 10 6-s bouts of high-intensity cycling at 2 exercise intensities (130 rev/min [EX130]: ∼820 W and 140 rev/min [EX140)]: ∼ 880 W) so that in EX130 the same amount of exercise was performed before and after the administration period, whereas an exercise intensity in EX140 was chosen to induce fatigue over the 10 exercise bouts. Sixteen healthy male subjects were randomly assigned to the 2 experimental groups. A double-blind design was used in this study. There were no significant changes in the placebo group for any of the measured parameters. Performance towards the end of each exercise bout in EX140 was enhanced following creatine supplementation, as shown by a smaller decline in work output from baseline along the 10 trials. Although more work was performed in EX140, after vs before the administration period, blood lactate accumulation decreased (mean and SEM), from 10.8 (0.5) to 9.1 (0.8) mmol·l−1 and plasma accumulation of hypoxanthine decreased from 21.1 (0.4) to 16.7 (0.8) μmol·l−1, but there was no change in oxygen uptake measured during 3 exercise and recovery periods [3.18 (0–1) vs 3.14 (0.1) l·min−1]. In EX130 blood lactate accumulation decreased, from 7.0 (0.5) to 5.1 (0.5) mmol·l−1, and oxygen uptake was also lower, decreasing from 2.84 (0.1) to 2.78 (0.1) l·min−1. A significant increase in body mass (11 kg: range 0.3 to 2.5 kg) was found in the creatine group. The mechanism responsible for the improved performance with creatine supplementation are postulated to be both a higher initial creatine phosphate content availability and an increased rate of creatine phosphate resynthesis during recovery periods. The lower blood lactate and hypoxanthine accumulation can also be explained by these mechanisms.

This study investigated the effects of muscle glycogen availability on performance and selected physiological and metabolic responses during high-intensity intermittent exercise. Seven male subjects completed a regimen of exercise and dietary intake (48 h) to either lower and keep low (LOW-CHO) or lower and then increase (HIGH-CHO) muscle glycogen stores, on two separate occasions at least a week apart. On each occasion the subjects completed a short-term (10 min) and prolonged (30 min) intermittent exercise (IEX) protocol, 24 h apart, which consisted of 6-s bouts of high-intensity exercise performed at 30-s intervals on a cycle ergometer. Glycogen concentration (mean +/- SEM) in m. vastus lateralis before both IEx(short) and IEx(long) was significantly lower following LOW-CHO [180 (14), 181 (17) mmol kg (dw)(-1)] compared with HIGH-CHO [397 (35), 540 (25) mmol kg (dw)(-1)]. In both IEx(short) and IEx(long), significantly less work was performed following LOW-CHO compared with HIGH-CHO. In IEx(long), the number of exercise bouts that could be completed at a pre-determined target exercise intensity increased by 265% from 111 (14) following LOW-CHO to 294 (29) following HIGH-CHO (P0.05). At the point of fatigue in IEx(long), glycogen concentration was significantly lower with the LOW-CHO compared with HIGH-CHO [58 (25) vs. 181 (46) mmol kg (dw)(-1), respectively]. The plasma concentrations of adrenaline and nor-adrenaline (in IEx(short) and IEx(long)), and FFAand glycerol (in IEx(long)), increased several-fold above resting values with both experimental conditions. Oxygen uptake during the exercise periods in IEx(long), approached 70% of Vo2max. These results suggest that muscle glycogen availability can affect performance during both short-term and more prolonged high-intensity intermittent exercise and that with repeated exercise periods as short as 6 s, there can be a relatively high aerobic contribution.

Seven male subjects performed repeated bouts of high-intensity exercise, on a cycle ergometer, before and after 6 d of creatine supplementation (20 g Cr H2O day-1). The exercise protocol consisted of five 6-s exercise periods performed at a fixed exercise intensity, interspersed with 30-s recovery periods (Part I), followed (40 s later) by one 10 s exercise period (Part II) where the ability to maintain power output was evaluated. Muscle biopsies were taken from m. vastus lateralis at rest, and immediately after (i) the fifth 6 s exercise period in Part I and (ii) the 10 s exercise period in Part II. In addition, a series of counter movement (CMJ) and squat (SJ) jumps were performed before and after the administration period. As a result of the creatine supplementation, total muscle creatine [creatine (Cr) + phosphocreatine (PCr)] concentration at rest increased from (mean +/- SEM) 128.7 (4.3) to 151.5 (5.5) mmol kg-1 dry wt (P0.05). This was accompanied by a 1.1 (0.5) kg increase in body mass (P0.05). After the fifth exercise bout in Part I of the exercise protocol, PCr concentration was higher [69.7 (2.3) vs. 45.6 (7.5) mmol kg-1 dry wt, P0.05], and muscle lactate was lower [26.2 (5.5) vs. 44.3 (9.9) mmol kg-1 dry wt, P0.05] after vs. before supplementation. In Part II, after creatinine supplementation, subjects were better able to maintain power output during the 10-s exercise period (P0.05). There was no change in jump performance as a result of the creatine supplementation (P0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

This study examined the influence of reduced oxygen availability on the ability to perform repeated bouts of high intensity exercise on a cycle ergometer. Seven male physical education students performed 10 exercise bouts (of 6 s each), interspersed with 30-s recovery periods, under hypoxic and normoxic conditions. The hypoxic condition was carried out in a low pressure chamber at 526 mmHg. Subjects were instructed to try to maintain a target pedalling speed of 140 rev min-1 during each exercise period. The mean power output of the first exercise bout was approximately 950 W. In both experimental conditions, all subjects were able to maintain the target speed for the first 3 s of each of the 10 exercise bouts. During the last 3-s interval of each exercise period the target speed was not maintained in both conditions over the 10 sprints. However, the reduction was greater in the hypoxic condition (P0.05). Post-exercise blood lactate accumulation was higher with hypoxia [10.3 (0.7) vs. 8.5 (0.8) mmol l-1, P0.05]. Oxygen uptake, measured during the exercise and recovery periods of sprints 6-9, was lower in the hypoxic condition [3.03 (0.2) vs. 3.19 (0.2) 1 min-1, P0.05]. These results indicate that a reduction in oxygen availability during high intensity intermittent exercise results in a higher accumulation of blood lactate and a lower oxygen uptake. The ability to maintain a high power output is impaired.

The effect of intermittent high-intensity training on the activity of enzymes involved in purine metabolism and on the concentration of plasma purines following acute short-term intense exercise was investigated. Eleven subjects performed sprint training three times per week for 6 weeks. Muscle biopsies for determination of enzyme activities were obtained prior to and 24 h after the training period. After training, the activity of adenosine 5′-phosphate (AMP) deaminase was lower (P < 0.001) whereas the activities of hypoxanthine phosphoribosyl transferase (HPRT) and phosphofructokinase were significantly higher compared with pre-training levels. The higher activity of HPRT with training suggests an improved potential for rephosphorylation of intracellular hypoxanthine to inosine monophosphate (IMP) in the trained muscle. Before and after the training period the subjects performed four independent 2-min tests at intensities from a mean of 106 to 135 % of Vomax. Venous blood was drawn prior to and after each test. The accumulation of plasma hypoxanthine following the four tests was lower following training compared with prior to training (P < 0.05). The accumulation of uric acid was significantly lower (46% of pre-training value) after the test performed at 135% of Fo2mM (P < 0.05). Based on the observed alterations in muscle enzyme activities and plasma purine accumulation, it is suggested that high intensity intermittent training leads to a lower release of purines from muscle to plasma following intense exercise and, thus, a reduced loss of muscle nucleotides.