Browsing by Subject "Exertion - physiology"

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    The effect of carbohydrate-loading and carbohydrate ingestion on fuel substrate kinetics during prolonged cycling
    (1995) Bosch, Andrew Norman; Noakes, Timothy D; Dennis, Steve
    It has been well established that both carbohydrate-loading before and carbohydrate ingestion during exercise can enhance endurance performance by supplying carbohydrate for oxidation. However, the precise mechanism(s) underlying the proposed ergogenic effects of these procedures remain to be established. The studies in this thesis were therefore designed to examine the effects of carbohydrate-loading and carbohydrate ingestion on fuel substrate kinetics.
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    The effects of prior oral creatine supplementation on performance and metabolism after 7 days of sprint cycle training
    (1996) Bold, Antoinette; Myburgh, Kathryn Helen
    Oral creatine supplementation has been shown to increase skeletal muscle total creatine (TCr) content, and in some cases improve performance in high-intensity short duration exercise. A variety of factors related to an enhanced efficacy of adenine nucleotide metabolism have been demonstrated as partly responsible for this ergogenic effect. Also, there is evidence that high-intensity sprint training results in a decrease in muscle total adenine nucleotide (TAN) and/or ATP stores. This placebo controlled double-blind study examined whether an oral creatine supplementation regimen would 1) increase muscle TCr content, 2) attenuate any loss in TAN or ATP during intermittent sprint training, and 3) have an ergogenic effect on performance after sprint training. Thirteen male endurance trained cyclists ingested 20 g of creatine monohydrate supplement or placebo per day for 7 days, after which they ingested a maintenance dose of 2 g creatine or placebo per day for the remainder of the trial (15d). While on the maintenance dose, subjects performed intermittent sprint training (ST) on a cycle ergometer (10 x 10 s sprints with 140 s active recovery) for 6 consecutive days and a 7th day after one day of rest. Performance tests were performed before and after ST, and metabolic tests were performed on the 1st and 7th day of ST. TCr increased significantly with creatine supplementation (creatine group pre: 121 ± 4, post: 147 ± 9; vs. placebo group pre: 122 ± 4, post: 125 ± 4 mmol/kg dm; mean± SEM; p<0.05). The increase in TCr correlated with the percentage Type IIB fibres (r=0.95, p<0.005). By day 7 of ST, TCr content was no longer significantly higher than pre-supplementation levels despite the maintenance dose of creatine. ST resulted in a significant decrease in resting muscle TAN and ATP content in both groups (ATP content in creatine group pre: 24.1 ± 0.8, post: 17.2 ± 0.5; and placebo group pre: 26.5 ± 1.1, post: 18.0 ± 0.6 mmol/kg dm; p<0.001). During and in recovery from ST on day 7, both groups had lower plasma ammonia (p<0.05), hypoxanthine (p<0.001) and urate (p<0.001) accumulation than on day 1 of ST. There was no improvement in 1-hr cycle distance performance after ST, but peak sustained power output increased in the creatine group and not in the placebo group after ST (p<0.05). Peak and mean power during a 30 s Wingate test increased significantly (p<0.05) after ST but there was no additional ergogenic effect of creatine supplementation. In conclusion, this study shows that 1) the efficacy of muscle creatine uptake was dependent on the percentage of Type IIB fibres, 2) creatine supplementation and maintenance (2 g/d) did not attenuate ATP or TAN loss during 7 days of ST, 3) ST decreased the accumulation of plasma products of adenine nucleotide degradation and improved 30 s sprint performance, and 4) creatine supplementation and ST did not improve I-hr cycle distance performance.
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    The effects of water ingestion on high intensity cycling performance in a moderate ambient temperature
    (1994) Robinson, Tracy Anne; Hawley, John; Dennis, Steve
    Eight endurance~trained cyclists rode as far as possible in 1 h on a stationary cyclesimulator in a moderate environment (20°C, 60% relative humidity, 3 m/s wind speed) while randomly receiving either no fluid (NF) or attempting to replace their ~1.7 l sweat loss measured in a previous 1 h familiarisation performance ride at ~85% of peak oxygen uptake (VO₂ peak) with artificially sweetened, coloured water (F). During F the cyclists drank 1.49 ± 0.14 1 (values are mean± SEM), of which 0.27 ± 0.08 1 remained in the stomach at the end of exercise and 0.20 ± 0.05 1 was urinated after the trial. Thus, only 1.02 ± 0.12 l of the ingested fluid was available to replace sweat losses during the 1 h performance ride. That fluid decreased the average heart rate from 166 ± 3 to 157 ± 5 beats/min (P < 0.0001) and reduced the final serum [Na+] and osmolalities from 143 ± 0.6 to 139 ± 0.6 mEq/1 (P < 0.005) and from 294 ± 1.7 to 290 ± 1.9 mOsm/1 (P = 0.05), respectively. Fluid ingestion did not attenuate rises in plasma anti diuretic hormone and angiotensin concentrations, or decrease the ~-15% falls in estimated plasma volume in the F and NF trials. Nor did fluid ingestion significantly effect the ~1.7 l/h sweat rates, the rises in rectal temperature (~36.6° to 38.3°C) or the ratings of perceived exertion in the two trials. Ingestion of ~1.5 l of fluid produced an uncomfortable stomach fullness and reduced the distance covered in 1 h from 43.1 ± 0. 7 to 42.3 ± 0.6 km (P<0.05). Thus, trying to replace > 1.0 l/h sweat losses during high-intensity, short duration exercise in a moderate environment does not induce beneficial physiological effects, and may impair exercise performance.