Browsing by Subject "Medical Physiology"

Now showing 1 - 6 of 6
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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 effect of muscle glycogen status on control of substrate metabolism during exercise
    (1998) Weltan, Sandra Mary; Noakes, Timothy D; Dennis, Steve
    Glycogen depletion has frequently been shown to result in a decrease in respiratory exchange ratio (RER). However, the metabolic response to glycogen depletion has generally been studied in overnight fasted subjects or in subjects who were already fatigued, or hypoglycaemic, or both, raising the question of whether the differences seen were due to general "carbohydrate deficiency" or due specifically to muscle or liver glycogen depletion. If euglycaemia and especially hyperglycaemia is maintained, the " carbohydrate deficiency" is overcome. In addition, because insulin stimulates muscle glucose uptake and not liver glucose uptake during euglycaemia (except at very high concentrations), insulin infusion would differentiate between liver and muscle glycogen depletion, since if the decrease in RER previously observed is abolished with insulin infusion while euglycaemia is maintained, this would indicate that the decrease is specifically due to muscle glycogen depletion. Thus, the aim of this study was to investigate the metabolic effect of glycogen content while an adequate amount or an excess of carbohydrate was provided in the form of an intravenous glucose infusion and when plasma insulin concentrations are raised.
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    Factors limiting the exercise tolerance of patients with end-stage renal failure undergoing maintenance haemodialysis
    (1994) Diesel, Wayne Jonathan; Noakes, Timothy D
    Exercise tolerance, measured as peak oxygen consumption (VO₂ peak), is very low in patients with end-stage renal failure undergoing maintenance haemodialysis. Due to their associated anaemia and low peak heart rates during maximal exercise it has been argued that the reduced blood oxygen carrying capacity and central cardiovascular limitations are primarily responsible for the poor exercise tolerance of these patients. However, others suggest that peripheral (skeletal muscle) limitations including impaired substrate utilization, muscle weakness caused by peripheral neuropathy and myopathy, malnutrition and general physical deconditioning are responsible for the poor exercise tolerance. The present thesis was therefore designed to study whether central cardiovascular function or anaemia or muscle weakness causes patients with end-stage renal failure to terminate exercise at workrates well below those achieved by healthy controls.
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    Limits to exogenous glucose oxidation by skeletal muscle during prolonged, moderate-intensity exercise in man
    (1993) Hawley, John Alan; Noakes, Timothy D; Dennis, Steve
    Several factors may determine the rate. at which exogenous carbohydrate (CHO) is utilised by the human working muscles during prolonged (> 90 min moderate-intensity (63% of peak sustained power output [PPO]) exercise. These include i) the rate of gastric emptying of an ingested fluid, ii) the rate of digestion, absorption and subsequent transport of glucose into the systemic circulation, and iii) the rate of glucose uptake and oxidation by the working muscles. To test the hypothesis that the rate of gastric emptying is the primary factor limiting the rate of CHO delivery to the working muscles during exercise, uniformly labelled ¹⁴carbon (U-¹⁴C) tracer techniques were used in association with conventional gas exchange measurements and post-exercise gastric aspiration to compare the rates of gastric emptying, intestinal CHO delivery and ingested CHO oxidation from 15 g/100 ml solutions of glucose, maltose, a 22 chain-length glucose polymer, and an isocaloric 'soluble' starch preparation. Two groups of six highly-trained male cyclists or triathletes each ingested two of the test drinks which were given as a 400 ml loading bolus immediately before and then as eight 100 ml feedings at 10 min intervals during 90 min of continuous cycling at a work rate of 63% of PPO (~70% of maximal oxygen consumption [VO₂ₘₐₓ]).
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    Physiological and metabolic responses to constant and variable load cycling performance
    (1999) Palmer, Gary Stanley; Hawley, John
    The experiments described in this thesis comprise a series of related, yet independent investigations examining the physiological and metabolic responses of well-trained amateur cyclists under conditions designed to mimic actual competitive situations, during individual and mass start races. In Section A the physiological responses to constant load and steady state exercise are determined. In Section B, the metabolic factors associated with constant and variable load cycling performance are examined.
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    Selected exercise and skeletal muscle characteristics of African distance runners
    (1996) Weston, Adele Robyn; Myburgh, Kathryn Helen
    African runners dominate distance running both in South Africa and internationally. Therefore, the aim of this thesis was to compare selected exercise and skeletal muscle characteristics in well-trained African and Caucasian 10 km runners to determine if evidence exists of differences between these groups with respect to these physiological and biochemical characteristics. Furthermore, the relationship between exercise and skeletal muscle characteristics was investigated. Sedentary individuals from each population group were also studied to determine if differences existed in untrained skeletal muscle between groups. Maximal oxygen consumption and peak treadmill speed were measured using an incremental treadmill protocol whilst submaximal exercise characteristics were measured during a specifically designed protocol consisting of four sequential submaximal workloads relative to the peak treadmill speed of the individual. The final workload was maintained until fatigue with resistance to fatigue defined as total test time. Running economy was measured at a treadmill speed of 16.1 km/hr. Race pace characteristics were measured directly at race pace. Characteristics measured during exercise tests were oxygen uptake, minute ventilation, respiratory exchange ratio and heart rate whilst plasma lactate concentration was determined immediately after exercise. Skeletal muscle characteristics were determined by needle biopsy of the vastus lateralis muscle. Skeletal muscle enzymes citrate synthase, phosphofructokinase, 3-hydroxyacyl CoA dehydrogenase, hexokinase and carnitine palmityl transferase were assayed spectrophotometrically. Skeletal muscle buffering capacity was measured using by titration and fibre type proportions were analysed histochemically. Comparisons between groups were made with the Student's t-test for unpaired data whilst the relationships between variables were analysed using the Pearson's correlation coefficient. The first major finding was that when exercising at the same relative percentage of individual maximal treadmill velocity, African distance runners were able to exercise for longer than the Caucasians (1376±227 vs 1137±126 sec, p<0.01) with lower plasma lactate accumulation (4.8±3.2 vs 7.7±2.8 mmol/l,p<0.05). Time to fatigue was significantly related to a lower plasma lactate concentration (r=-0.63) and a lower respiratory exchange ratio (r=-0.53). The second major finding indicated that African runners were able to race 10 km at a higher percentage of their maximal oxygen uptake (93.5 vs 86.0%, p<0.005), whilst eliciting only a comparable plasma lactate concentration and respiratory exchange ratio. The third main finding was that the African runners were more economical than the Caucasian runners (p<0.05). The fourth main finding is that the African runners had a 50% greater activity of citrate synthase (p<0.005) and 3-hydroxyacyl CoA dehydrogenase (p<0.01) in the vastus lateralis than the Caucasians and this could not be explained by fibre type proportions, because the proportion of type I fibres was lower in the African runners (p<0.05). Citrate synthase activity, was related to the runners' ability to resist fatigue at high intensity relative to their individual peak treadmill velocity (r=0.70, p<0.05). A higher CS activity was related to a lower plasma lactate concentration and a lower RER. The sixth main finding of this thesis was that skeletal muscle buffering capacity of the Caucasian runners was higher than that of the African runners (p<0.05). A methodological study of buffering capacity in rats showed the buffering capacity was largely dependent upon fibre type and protein concentration, however these parameters could not explain the difference observed between the African and Caucasian runners. Furthermore, despite the differences in skeletal muscle characteristics observed between African and Caucasian runners in the current thesis, there was no evidence of these differences being inherently present in sedentary African and Caucasian individuals. In conclusion, the current series of studies do provide evidence of differences in selected exercise and skeletal muscle characteristics between African and Caucasian distance runners, with the African runners possessing exercise and skeletal muscle profiles that are considered to be more advantageous for endurance performance.