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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.

Since the discovery of creatine in 1832, it has fascinated scientists with its central role in skeletal muscle metabolism. In humans, over 95% of the total creatine (Crtot) content is located in skeletal muscle, of which approximately a third is in its free (Crf) form. The remainder is present in a phosphorylated (Crphos) form. Crf and Crphos levels in skeletal muscle are subject to individual variations and are influenced by factors such as muscle fibre type, age and disease, but not apparently by training or gender. Daily turnover of creatine to creatinine for a 70kg male has been estimated to be around 2g. Part of this turnover can be replaced through exogenous sources of creatine in foods, especially meat and fish. The remainder is derived via endogenous synthesis from the precursors arginine, glycine and methionine. A century ago, studies with creatine feeding concluded that some of the ingested creatine was retained in the body. Subsequent studies have shown that both Crf and Crphos levels in skeletal muscle can be increased, and performance of high intensity intermittent exercise enhanced, following a period of creatine supplementation. However, neither endurance exercise performance nor maximal oxygen uptake appears to be enhanced. No adverse effects have been identified with short term creatine feeding. Creatine supplementation has been used in the treatment of diseases where creatine synthesis is inhibited.

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.

The effect of high-intensity intermittent training on the adenine nucleotide content of skeletal muscle was studied. Eleven male subjects (group A) performed high-intensity intermittent training on a cycle ergometer three times per week for 6 wk, followed by 1 wk of the same kind of training with two sessions per day. Nine males (group B) exclusively performed 1 wk of training with two sessions per day. In group A, skeletal muscle total adenine nucleotide (TAN) levels decreased from 25.1 +/- 0.7 (SE) to 22.0 +/- 0.6 mmol/kg dry wt over the 6-wk period (P < 0.01). The subsequent intensive week did not further alter TAN levels. In group B, the intensive week of training reduced TAN levels from 25.1 +/- 0.5 to 19.4 +/- 0.6 mmol/kg dry wt (P < 0.001). The decrease was sustained 72 h after training (P < 0.001). During the intensive week, there was no change in plasma creatine kinase activity in either group A or group B. The plasma activity was, however, higher in group B than in group A on days 4 and 7 of the intensive week (P < 0.05). The results from this study indicate that high-intensity intermittent exercise causes a decrease in resting levels of skeletal muscle adenine nucleotide without a concomitant indication of muscle damage. A training-induced adaptation appears to occur with training by which a further loss of adenine nucleotides is prevented despite an increased training dose.

Six male football players competed in a 90 min game (4-a-side) on two occasions following an exercise and diet (either high- approximately 65% or low- approximately 30% carbohydrate intake) regimen designed to manipulate muscle glycogen concentrations. Movement and technical parameters of performance and selected physiological responses were measured. Pre-game muscle glycogen concentrations following the high carbohydrate diet (mean +/- SD) (395.6 +/- 78.3 mmol x kg(-1) dw) were significantly higher than following the low carbohydrate diet (287.1 +/- 85.4 mmol x kg(-1) dw). The results of the movement analysis showed that the players performed significantly more (approximately 33%) high intensity exercise in the game played following the high carbohydrate diet. No significant differences were found, between the two dietary conditions, in any of the measured technical variables. Plasma FFA and glycerol concentrations in the game played following the low carbohydrate diet were significantly higher after 45 min (905 +/- 103 and 293 +/- 23 micromol x l(-1)) and post exercise (1388 +/- 122 and 366 +/- 36 micromol x l(-1)) compared to the game played following the high carbohydrate diet (532 +/- 137 and 202 +/- 55 micromol x l(-1) and 888 +/- 192 and 266 +/- 27 micromol x l(-1), respectively). Post-exercise blood glucose levels were significantly lower in the game played following the low carbohydrate diet (5.8 +/- 0.3 vs 7.2 +/- 0.3 mmol x l(-1)). No significant differences were found in the mean blood lactate values (3.5 +/- 0.6 and 3.9 +/- 0.5 mmol x l(-1)) or mean heart rates (162 vs. 163.5 beats x min(-1)) between the high and low carbohydrate conditions, respectively. The main finding from this study was that the carbohydrate content of the diet influenced the amount of high intensity exercise performed during a small-sided football game. This suggests that to optimise performances, in not only football but possibly also other multiple sprint sports of similar duration, a high carbohydrate diet should be administered in preparation for intense training and competition.

Harridge, S. D. R., R. Bottinelli, M. Canepari, M. Pellegrino, C. Reggiani, M. Esbjörnsson, P. D. Balsom, and B. Saltin. Sprint training, in vitro and in vivo muscle function, and myosin heavy chain expression. J. Appl. Physiol. 84(2): 442–449, 1998.—Sprint training represents the condition in which increases in muscle shortening speed, as well as in strength, might play a significant role in improving power generation. This study therefore aimed to determine the effects of sprint training on 1) the coupling between myosin heavy chain (MHC) isoform expression and function in single fibers, 2) the distribution of MHC isoforms across a whole muscle, and 3) in vivo muscle function. Seven young male subjects completed 6 wk of training (3-s sprints) on a cycle ergometer. Training was without effect on maximum shortening velocity in single fibers or in the relative distribution of MHC isoforms in either the soleus or the vastus lateralis muscles. Electrically evoked and voluntary isometric torque generation increased ( P < 0.05) after training in both the plantar flexors (+8% at 50 Hz and +16% maximal voluntary contraction) and knee extensors (+8% at 50 Hz and +7% maximal voluntary contraction). With the shortening potential of the muscles apparently unchanged, the increased strength of the major lower limb muscles is likely to have contributed to the 7% increase ( P < 0.05) in peak pedal frequency during cycling.

Seven male subjects performed 15 x 40m sprints, on three occasions, with rest periods of either 120 s (R120), 60 s (R60) or 30 s (R30) between each sprint. Sprint times were recorded with four photo cells placed at 0, 15, 30 and 40 m. The performance data indicated that whereas running speed over the last 10 m of each sprint decreased in all three protocols (after 11 sprints in R120, 7 sprints in R60 and 3 sprints in R30), performance during the initial acceleration period from 0-15 m was only affected with the shortest rest periods increasing from (mean +/- SEM) 2.58 +/- .03 (sprint 1) to 2.78 +/- .04 s (spring 15) (p < .05). Post-exercise blood lactate concentration was not significantly different in R120 (12.1 +/- 1.3 mmol.l-1) and R60 (13.9 +/- 1.2 mmol.l-1), but a higher concentration was found in R30 (17.2 +/- .7 mmol.l-1) (p < .05). After 6 sprints there was no significant difference in blood lactate concentration with the different recovery durations, however, there were significant differences in sprint times at this point, suggesting that blood lactate is a poor predictor of performance during this type of exercise. Although the work bouts could be classified primarily as anaerobic exercise, oxygen uptake measured during rest periods increased to 52, 57 and 66% of maximum oxygen uptake in R120, R60 and R30, respectively. Evidence of adenine nucleotide degradation was provided by plasma hypoxanthine and uric acid concentrations elevated post-exercise in all three protocols. Post-exercise uric acid concentration was not significantly affected by recovery duration.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological responses to repeated bouts of short duration maximal-intensity exercise were evaluated. Seven male subjects performed three exercise protocols, on separate days, with either 15 (S15), 30 (S30) or 40 (S40) m sprints repeated every 30 s. Plasma hypoxanthine (HX) and uric acid (UA), and blood lactate concentrations were evaluated pre- and postexercise. Oxygen uptake was measured immediately after the last sprint in each protocol. Sprint times were recorded to analyse changes in performance over the trials. Mean plasma concentrations of HX and UA increased during S30 and S40 (P less than 0.05), HX increasing from 2.9 (SEM 1.0) and 4.1 (SEM 0.9), to 25.4 (SEM 7.8) and 42.7 (SEM 7.5) mumol.l-1, and UA from 372.8 (SEM 19) and 382.8 (SEM 26), to 458.7 (SEM 40) and 534.6 (SEM 37) mumol.l-1, respectively. Postexercise blood lactate concentrations were higher than pretest values in all three protocols (P less than 0.05), increasing to 6.8 (SEM 1.5), 13.9 (SEM 1.7) and 16.8 (SEM 1.1) mmol.l-1 in S15, S30 and S40, respectively. There was no significant difference between oxygen uptake immediately after S30 [3.2 (SEM 0.1) l.min-1] and S40 [3.3 (SEM 0.4) l.min-1], but a lower value [2.6 (SEM 0.1) l.min-1] was found after S15 (P less than 0.05). The time of the last sprint [2.63 (SEM 0.04) s] in S15 was not significantly different from that of the first [2.62 (SEM 0.02) s]. However, in S30 and S40 sprint times increased from 4.46 (SEM 0.04) and 5.61 (SEM 0.07) s (first) to 4.66 (SEM 0.05) and 6.19 (SEM 0.09) s (last), respectively (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)