Initial fitness, maturity status, and total training explain small and inconsistent proportions of the variance in physical development of adolescent footballers across one season.

To investigate how initial fitness, maturity status, and training time explain changes in physical performance across one season. Eighty-eight adolescent male footballers, representing four age categories (Under 15 [n = 12], Under 14 [n = 21], Under 13 [n = 25], Under 12 [n = 30]), were tested using physical performance tests (20 m sprint, change of direction, squat jump and yo-yo intermittent recovery test level 1 [YYIRTL1]) and maturity offset at the season start (Test 1) and end (Test 2). Multiple regression determined the proportion of variance in test score changes, explained by three predictor variables: initial fitness (i.e., Test 1), maturity offset change, and training time. With combined categories, predictor variables explained 0.051 to 0.297 of the variance in physical performance score changes. Analysing age categories separately, predictor variables explained 0.047 to 0.407 (20 m sprint), 0.202 to 0.626 (change of direction), 0.336 to 0.502 (squat jump), and 0.196 to 0.777 (YYIRTL1) of variance in test score changes. Of the limited differences in relative predictor contribution, Test 1 was the strongest predictor of test score change. Initial fitness, maturity status change, and training time explain small and inconsistent proportions of variance in adolescent footballers' physical development across one season.

Contrasting effects of heat stress on neuromuscular performance.

NEW FINDINGS: What is the central question of this review? Is exposure to a hot environment detrimental to neuromuscular performance? What is the main finding and what is its importance? Elevating body temperature improves peak power during short-duration, high-intensity exercise but trades off with an accelerated rate of decay. Higher muscle temperatures and cross-bridge cycling rate resemble a shift in contractile characteristic to a faster phenotype. Prolonged moderate-intensity exercise capacity is impaired in a hot environment. Fatigue appears to combine a reduced drive from the CNS and increased cardiovascular strain to maintain skeletal muscle perfusion and thermoregulation. ABSTRACT: The effect of thermal stress on human work capacity and neuromuscular function has been of interest to physiologists since the 19th century. The aim of the present review is to examine the impact of exposure to heat stress on neuromuscular performance. Exposure to heat stress during exercise is known to increase strain on the cardiovascular system owing to the competing demands of skeletal muscle perfusion and homeostatic thermoregulation. The effects of exposure to heat stress on the neuromuscular system are more complex, because in some circumstances an elevation in muscle temperature leads to an improvement in function, whereas in other circumstances an increase in temperature leads to a decrement in function that is a consequence of the mode, metabolic demand and duration of the exercise. The ability to sustain isometric tension is impaired with an elevated muscle temperature and so too is locomotor capacity over prolonged periods of time. In contrast, peak power production is enhanced by increasing muscle temperature but is achieved at the expense of maintaining power output, owing to a higher rate of decay in power production. The different effects on neuromuscular function at an elevated muscle temperature are explained, in part, by a higher rate of energy turnover. In addition, the effect of an elevated core temperature also appears to impair neuromuscular performance either owing to a reduced voluntary drive in motor unit recruitment or to a failure in muscle afferent feedback, or a combination of the two.

PHOSPHO1 is a skeletal regulator of insulin resistance and obesity.

BACKGROUND: The classical functions of the skeleton encompass locomotion, protection and mineral homeostasis. However, cell-specific gene deletions in the mouse and human genetic studies have identified the skeleton as a key endocrine regulator of metabolism. The bone-specific phosphatase, Phosphatase, Orphan 1 (PHOSPHO1), which is indispensable for bone mineralisation, has been recently implicated in the regulation of energy metabolism in humans, but its role in systemic metabolism remains unclear. Here, we probe the mechanism underlying metabolic regulation by analysing Phospho1 mutant mice. RESULTS: Phospho1-/- mice exhibited improved basal glucose homeostasis and resisted high-fat-diet-induced weight gain and diabetes. The metabolic protection in Phospho1-/- mice was manifested in the absence of altered levels of osteocalcin. Osteoblasts isolated from Phospho1-/- mice were enriched for genes associated with energy metabolism and diabetes; Phospho1 both directly and indirectly interacted with genes associated with glucose transport and insulin receptor signalling. Canonical thermogenesis via brown adipose tissue did not underlie the metabolic protection observed in adult Phospho1-/- mice. However, the decreased serum choline levels in Phospho1-/- mice were normalised by feeding a 2% choline rich diet resulting in a normalisation in insulin sensitivity and fat mass. CONCLUSION: We show that mice lacking the bone mineralisation enzyme PHOSPHO1 exhibit improved basal glucose homeostasis and resist high-fat-diet-induced weight gain and diabetes. This study identifies PHOSPHO1 as a potential bone-derived therapeutic target for the treatment of obesity and diabetes.

The Effect of Maturation on Performance During Repeated Sprints With Self-Selected Versus Standardized Recovery Intervals in Youth Footballers.

PURPOSE: The purpose of this experiment was to assess performance during repeated sprints utilizing self-selected recovery intervals in youth football (soccer) players at different stages of maturation. METHODS: Quota sampling method was used to recruit 14 prepeak height velocity (PHV) and 14 post-PHV participants for the study (N = 28; age = 13 [0.9] y, stature = 162.5 [10.8] cm, mass = 50.2 [12.7] kg). Players performed repeated sprints comprising 10 × 30 m efforts under 2 experimental conditions: using 30-second and self-selected recovery intervals. Magnitude of effects for within- and between-group differences were reported using effect size (ES) statistics ± 90% confidence intervals and percentage differences. RESULTS: The decline in sprint performance was likely lower in the pre-PHV compared with the post-PHV group during the standardized recovery trial (between-group difference = 37%; ES = 0.41 ± 0.51), and likely lower in the post-PHV group during the self-selected recovery trial (between-group difference = 50%; ES = 0.45 ± 0.54). Mean recovery duration was likely shorter in the pre-PHV compared with the post-PHV group during the self-selected recovery trial (between-group difference = 26.1%; ES = 0.47 ± 0.45). CONCLUSION: This is the first study to show that during repeated sprints with self-selected recovery, pre-PHV children have an impaired ability to accurately interpret physical capabilities in the context of the task compared with post-PHV adolescents.

Physiological, Perceptual and Performance Responses Associated With Self-Selected Versus Standardized Recovery Periods During a Repeated Sprint Protocol in Elite Youth Football Players: A Preliminary Study.

PURPOSE: To examine the physiological and perceptual responses of youth footballers to a repeated sprint protocol employing standardized and self-selected recovery. METHODS: Eleven male participants (13.7 ± 1.1 years) performed a repeated sprint assessment comprising 10 × 30 m efforts. Employing a randomized cross-over design, repeated sprints were performed using 30 s and self-selected recovery periods. Heart rate was monitored continuously with ratings of perceived exertion (RPE) and lower body muscle power measured 2 min after the final sprint. The concentration of blood lactate was measured at 2, 5 and 7 min post sprinting. Magnitude of effects were reported using effect size (ES) statistics ± 90% confidence interval and percentage differences. Differences between trials were examined using paired student t tests (p < .05). RESULTS: Self-selected recovery resulted in most likely shorter recovery times (57.7%; ES 1.55 ± 0.5; p < .01), a most likely increase in percentage decrement (65%; ES 0.36 ± 0.21; p = .12), very likely lower heart rate recovery (-58.9%; ES -1.10 ± 0.72; p = .05), and likely higher blood lactate concentration (p = .08-0.02). Differences in lower body power and RPE were unclear (p > .05). CONCLUSION: Self-selected recovery periods compromise repeated sprint performance.

The effect of sodium acetate ingestion on the metabolic response to prolonged moderate-intensity exercise in humans.

At rest, administration of the short-chain fatty acid acetate suppresses fat oxidation without affecting carbohydrate utilization. The combined effect of increased acetate availability and exercise on substrate utilization is, however, unclear. With local ethics approval, we studied the effect of ingesting either sodium acetate (NaAc) or sodium bicarbonate (NaHCO3) at a dose of 4 mmol·kg-1 body mass 90 min before completing 120 min of exercise at 50% VO2peak. Six healthy young men completed the trials after an overnight fast and ingested the sodium salts in randomized order. As expected NaAc ingestion decreased resting fat oxidation (mean ± SD; 0.09 ± 0.02 vs. 0.07 ± 0.02 g·min-1 pre- and post-ingestion respectively, p < .05) with no effect upon carbohydrate utilization. In contrast, NaHCO3 ingestion had no effect on substrate utilization at rest. In response to exercise, fat and CHO oxidation increased in both trials, but fat oxidation was lower (0.16 ± 0.10 vs. 0.29 ± 0.11 g·min-1, p < .05) and carbohydrate oxidation higher (1.67 ± 0.35 vs. 1.44 ± 0.22 g·min-1, p < .05) in the NaAc trial compared with the NaHCO3 trial during the first 15 min of exercise. Over the final 75 min of exercise an increase in fat oxidation and decrease in carbohydrate oxidation was observed only in the NaAc trial. These results demonstrate that increasing plasma acetate concentration suppresses fat oxidation both at rest and at the onset of moderate-intensity exercise.

Remote sensing of blood oxygenation using red-eye pupil reflection.

OBJECTIVE: To develop a technique for remote sensing of systemic blood oxygenation using red-eye pupil reflection. APPROACH: The ratio of the intensities of light from the bright pupil reflections at oxygen sensitive and isosbestic wavelengths is shown to be sensitive to the oxygenation of blood in the eye. A conventional retinal camera, fitted with an image-replicating imaging spectrometer, was used at standoff range to record snapshot spectral images of the face and eyes at eight different wavelengths. In our pilot study we measured optical-density ratios (ODRs) of pupil reflections at wavelengths of 780 nm and 800 nm, simultaneous with pulse oximetry, for ten healthy human subjects under conditions of normoxia and mild hypoxia (15% oxygen). The low absorption at these infrared wavelengths localises the sensing to the choroid. We propose that this can be used for as a proxy for systemic oximetry. MAIN RESULTS: A significant reduction (P < 0.001) in ODR of the pupil images was observed during hypoxia and returned to baseline on resumption of normoxia. We demonstrate that measurement of the choroidal ODR can be used to detect changes in blood oxygenation that correlate positively with pulse oximetry and with a noise-equivalent oximetry precision of 0.5%. SIGNIFICANCE: We describe a new method to remotely and non-invasively sense the oxygen saturation of choroidal blood. The methodology provides a proxy for remote sensing of cerebral and systemic blood oxygenation. We demonstrate the technique at short range but it has potential for systemic oximetry at large standoff ranges.

The interaction between nutrition and exercise for promoting health and performance.

The theme of The Nutrition Society Spring Conference 2017 was on the interaction between nutrition and exercise for promoting healthy ageing, maintaining cognitive function and improving the metabolic health of the population. The importance of this theme is highlighted by the public health issues surrounding obesity, diabetes and the age-related loss of skeletal muscle mass (sarcopenia). The opening symposium provided a historical perspective of both invasive and non-invasive methodologies for measuring exercise energetics and energy balance. Data derived from these techniques underpin current understanding regarding the metabolic response to nutrition and exercise. Further symposia examined the importance of skeletal muscle for healthy ageing in older men and postmenopausal women. From a nutritional perspective, the potential for animal- v. plant-based protein sources to offset the age-related decline in muscle mass was discussed. The day concluded by discussing the link(s) between nutrition, exercise and brain function. Day 2 commenced with examples of applied equine research illustrating the link between nutrition/exercise and insulin resistance to those of a human model. The final symposium examined the combined role of nutrition and exercise in reducing risk of type 2 diabetes and dyslipidaemia. The overall conclusion from the meeting was that the interaction between diet and physical activity confers greater benefits to human health and performance than either component alone.

Neuromuscular and hormonal responses to a single session of whole body vibration exercise in healthy young men.

Whole body vibration (WBV) has been proposed as an alternative exercise stimulus to produce adaptive responses similar to resistance exercise. Few studies have analysed acute hormonal responses to WBV. Purpose To evaluate neuromuscular and hormonal responses to an acute bout of isometric half-squat exercise with and without superimposition of WBV. Methods Seven healthy males (22.3+/-2.7 years) performed 10 sets of half squat isometric exercise for 1 min with 1-min rest between sets. Two separate trials were conducted either with WBV [30 Hz; 3.5 g (1 g=9.81 m.s2)] or without vibration (Control). Salivary concentration of testosterone and cortisol was collected and maximal isometric unilateral knee extensions (MVC) were completed before, immediately after, 1, 2 and 24 h after treatment. Results Significant decreases in MVC were observed immediately after (229.4+/-53.2 Nm), 1 h (231.6+/-59.9 Nm), and 2 h (233.0+/-59.1 Nm) after WBV compared with baseline (252.7+/-56.4 Nm; P<0.05). No significant change in MVC was recorded in Control. Rate of torque development in the first 200 ms (RTD200 ms), and salivary testosterone and cortisol concentrations were unaffected in both conditions. However, there was a trend for change over time in cortisol (P=0.052), with an increase after WBV and decrease after Control. Conclusion A 10 min session of intermittent WBV was shown to produce an acute reduction in MVC in healthy individuals, which recovered after 24 h. No significant changes were identified in salivary concentration of testosterone and cortisol suggesting that WBV with low acceleration does not represent a stressful stimulus for the neuroendocrine system.

Haemostasis, inflammation and renal function following exercise in patients with intermittent claudication on statin and aspirin therapy.

BACKGROUND: Previous studies have suggested that exercise in patients with intermittent claudication (IC) may induce a systemic thrombo-inflammatory response. The effect of secondary prevention therapy on this response is unknown. This study aimed to investigate the effects of treadmill exercise on markers of coagulation activation, inflammation and renal function in patients with IC, receiving aspirin and statin therapy compared to healthy controls. METHODS: Samples were taken before, immediately and 1 hour after exercising on a treadmill in 20 patients with IC and 20 healthy volunteers. Interleukin-6 (IL-6), thrombin-anti-thrombin complex (TAT) and fibrin D-dimer were measured by ELISA. High sensitivity CRP (HsCRP) and urinary albumin were measured via a nephelometric technique, urinary protein via a turbidometric assay and N-acetyl-beta-D-glucosaminidase (NAG) via a colorimetric assay. RESULTS: Elevated baseline levels of Hs-CRP, IL-6, white cell counts, D-dimer and urinary NAG occurred in patients with IC compared to volunteers (p > 0.05). Following exercise there was no increase in Hs CRP or IL-6. D-dimer levels significantly increased following exercise in the patients and volunteers. TAT levels increased immediately after exercise in the patient group only and were significantly increased at 1 hour in both patients and volunteers. A transient rise in the protein creatinine ratio occurred in both groups (p < 0.007), and in albumin creatinine ratio in the patient group. There was no change in urinary NAG. CONCLUSION: Elevated markers of inflammation occurred in patients with IC on statin and aspirin therapy but these did not increase following exercise. However, acute exercise resulted in a prothrombotic state evident in both groups, although this was more prolonged in patient with IC. The clinical significance of these findings in patients who are known to be at an increased risk of cardiac and other thrombotic event are unclear.

Metabolic and endocrine response to exercise: sympathoadrenal integration with skeletal muscle.

Skeletal muscle has the capacity to increase energy turnover by ∼1000 times its resting rate when contracting at the maximum force/power output. Since ATP is not stored in any appreciable quantity, the muscle requires a coordinated metabolic response to maintain an adequate supply of ATP to sustain contractile activity. The integration of intracellular metabolic pathways is dependent upon the cross-bridge cycling rate of myosin and actin, substrate availability and the accumulation of metabolic byproducts, all of which can influence the maintenance of contractile activity or result in the onset of fatigue. In addition, the mobilisation of extracellular substrates is dependent upon the integration of both the autonomic nervous system and endocrine systems to coordinate an increase in both carbohydrate and fat availability. The current review examines the evidence for skeletal muscle to generate power over short and long durations and discusses the metabolic response to sustain these processes. The review also considers the endocrine response from the perspective of the sympathoadrenal system to integrate extracellular substrate availability with the increased energy demands made by contracting skeletal muscle. Finally, the review briefly discusses the evidence that muscle acts in an endocrine manner during exercise and what role this might play in mobilising extracellular substrates to augment the effects of the sympathoadrenal system.

Effects of prior heavy exercise on VO(2) kinetics during heavy exercise are related to changes in muscle activity.

We hypothesized that the elevated primary O(2) uptake (VO(2)) amplitude during the second of two bouts of heavy cycle exercise would be accompanied by an increase in the integrated electromyogram (iEMG) measured from three leg muscles (gluteus maximus, vastus lateralis, and vastus medialis). Eight healthy men performed two 6-min bouts of heavy leg cycling (at 70% of the difference between the lactate threshold and peak VO(2)) separated by 12 min of recovery. The iEMG was measured throughout each exercise bout. The amplitude of the primary VO(2) response was increased after prior heavy leg exercise (from mean +/- SE 2.11 +/- 0.12 to 2.44 +/- 0.10 l/min, P < 0.05) with no change in the time constant of the primary response (from 21.7 +/- 2.3 to 25.2 +/- 3.3 s), and the amplitude of the VO(2) slow component was reduced (from 0.79 +/- 0.08 to 0.40 +/- 0.08 l/min, P < 0.05). The elevated primary VO(2) amplitude after leg cycling was accompanied by a 19% increase in the averaged iEMG of the three muscles in the first 2 min of exercise (491 +/- 108 vs. 604 +/- 151% increase above baseline values, P < 0.05), whereas mean power frequency was unchanged (80.1 +/- 0.9 vs. 80.6 +/- 1.0 Hz). The results of the present study indicate that the increased primary VO(2) amplitude observed during the second of two bouts of heavy exercise is related to a greater recruitment of motor units at the onset of exercise.

Does the human heart fatigue subsequent to prolonged exercise?

A reduction in left ventricular systolic and diastolic function subsequent to prolonged exercise in healthy humans, often called exercise-induced cardiac fatigue (EICF), has recently been reported in the literature. However, our current understanding of the exact nature and magnitude of EICF is limited. To date, there is no consensus as to the clinical relevance of such findings and whether such alterations in function are likely to impact upon performance. Much of the existing literature has employed field-based competitions. Whilst ecologically valid, this approach has made it difficult to control many factors such as the duration and intensity of effort, fitness and training status of subjects and environmental conditions. The impact of such variables on EICF has not been fully evaluated and is worthy of further research. To date, most EICF studies have been descriptive, with limited success in elucidating mechanisms. To this end, the assessment of humoral markers of cardiac myocyte or membrane disruption has produced contradictory findings partially due to controversy over the validity of specific assays. It is, therefore, important that future research utilises reliable and valid biochemical techniques to address these aetiological factors as well as develop work on other potential contributors to EICF such as elevated free fatty acid concentrations, free radicals and beta-adrenoceptor down-regulation. In summary, whilst some descriptive evidence of EICF is available, there are large gaps in our knowledge of what specific factors related to exercise might facilitate functional changes. These topics present interesting but complex challenges to future research in this field.

Effects of massage on limb and skin blood flow after quadriceps exercise.

PURPOSE: At present, there is little scientific evidence that postexercise manual massage has any effect on the factors associated with the recovery process. The purpose of this study was to compare the effects of massage against a resting control condition upon femoral artery blood flow (FABF), skin blood flow (SKBF), skin (SKT), and muscle (MT) temperature after dynamic quadriceps exercise. METHODS: Thirteen male volunteers participated in 3 x 2-min bouts of concentric quadriceps exercise followed by 2 x 6-min bouts of deep effleurage and pétrissage massage or a control (rest) period of similar duration in a counterbalanced fashion. Measures of FABF, SKBF, SKT, MT, blood lactate concentration (BLa), heart rate (HR), and blood pressure (BP) were taken at baseline, immediately after exercise, as well as at the midpoint and end of the massage/rest periods. Data were analyzed by two-way ANOVA. RESULTS: Significant main effects were found for all variables over time due to effects of exercise. Massage to the quadriceps did not significantly elevate FABF (end-massage 760 +/- 256 vs end-control 733 +/- 161 mL x min(-1)), MT, BL, HR, and BP over control values (P < 0.05). SKBF (end-massage 150 +/- 49 vs end control 6 +/- 4 au) SKT (end-massage 32.2 +/- 0.9 vs end-control 31.1 +/- 1.3degreesC) were elevated after the application of massage compared with the control trial (P < 0.05). CONCLUSION: From these data it is proposed that without an increase in arterial blood flow, any increase in SKBF is potentially diverting flow away from recovering muscle. Such a response would question the efficacy of massage as an aid to recovery in postexercise settings.

The cardiospecificity of the third-generation cTnT assay after exercise-induced muscle damage.

PURPOSE: The purpose of the present study was to examine the cardiospecificity of cTnI and the new third-generation cTnT assay, in the presence of exercise-induced muscle damage in highly trained individuals, and to examine the impact of a maximal-ramping treadmill test on cardiac function. METHODS: Eight highly trained male triathletes (mean +/- SD; age: 29 +/- 9 yr; height: 1.79 +/- 0.10 m; body mass: 77 +/- 10 kg; .VO(2max): 67.4 +/- 6.3 mL.kg(-1).min(-1)) completed two bouts of exercise. On the first occasion, subjects completed a maximal-ramping treadmill test. On a separate occasion, the subjects completed 30 min of downhill running (15% gradient) at a speed equivalent to 70% of maximal running velocity attained during the maximal-ramping treadmill test. All subjects were assessed using ECG, echocardiography, and blood analysis. Measurements were taken at rest, immediately after, and 48 h postexercise for each bout of exercise. Echocardiographic analysis was used to determine left ventricular systolic and diastolic function. Blood samples were analyzed for markers of myocyte damage. RESULTS: Echocardiographic results indicated normal left ventricular function before and after both exercise bouts. Total CK and CKMB were significantly elevated 48 h after the downhill run. cTnT and cTnI were not elevated at any stage of the study. CONCLUSIONS: Neither the maximal-ramping treadmill test nor the 30-min downhill run produced cardiac dysfunction or myocardial damage in young, healthy trained subjects. The elevated total CK and CKMB within the downhill study are noncardiac in origin as demonstrated by the lack of cTnT and cTnI. The cTnI and new third-generation cTnT assays may be used to detect cardiac damage in the presence of elevated total CK and CKMB associated with exercise-induced skeletal muscle damage.

Sodium acetate decreases phosphorylation of hormone sensitive lipase in isoproterenol-stimulated 3T3-L1 mature adipocytes.

Lipolysis, the process of hydrolysis of stored triacylglycerol into glycerol and non-esterified fatty acids (NEFA), is reported to be reduced by short chain fatty acids (SCFA) but the mechanism of this inhibition is poorly understood. The aim of this study was to measure the phosphorylation at serine residue 563 of hormone sensitive lipase with and without exposure to sodium acetate. Using the 3T3-L1 cell line, we identified that stimulating the cells with isoproterenol increased phosphorylated hormone sensitive lipase (pHSL) expression by 60% compared with the basal state. In the presence of the SCFA acetate in stimulated cells, pHSL decreased by 15% compared with stimulated cells alone. These results were mirrored by the NEFA release from stimulated cells that had significantly decreased in the presence of sodium acetate after 60 min (from 0.53 µmol mg(-1) protein to 0.41 µmol mg(-1) protein, respectively, P = 0.004); and 180 min (1.73 µmol mg(-1) protein to 1.13 µmol mg(-1) protein, P = 0.020); however, treatment had no effect on glycerol release (P = 0.109). In conclusion, exposure to 4 mM acetate reduced the level of phosphorylation of HSL(SER563) in mature 3T3-L1 adipocytes and led to a significant reduction in NEFA release, although glycerol release was not affected.

Assessment of acute mild hypoxia on retinal oxygen saturation using snapshot retinal oximetry.

PURPOSE: To study the effect of acute mild hypoxia on retinal oxygen saturation. METHODS: Spectral retinal images were acquired under normoxic and hypoxic conditions for 10 healthy human volunteers (six male, four female, aged 25 ± 5 years [mean ± SD]) using a modified fundus camera fitted with an image-replicating imaging spectrometer (IRIS). Acute, mild hypoxia was induced by changing the oxygen saturation of inhaled air from 21% to 15% using a hypoxia generator with subjects breathing the hypoxic gas mixture for 10 minutes. Peripheral arterial oxygen saturation of the subjects was monitored using fingertip-pulse oximetry. Images were processed to calculate oxygen saturation, arteriovenous difference in oxygen saturation, and vessel diameter. Data are presented as mean ± SD and were analyzed using paired sample t-test with significance accepted at P < 0.05. RESULTS: The retinal arterial and venous oxygen saturation was 98.5% ± 1.6% and 70.7% ± 2.7% during normoxia. A reduction in the fraction of inspired oxygen resulted in a decline (P < 0.001) in both retinal-arterial and venous oxygen saturation to 90.3% ± 2.0% and 62.4% ± 2.2%, respectively. The arteriovenous oxygen saturation difference in normoxia (27.8% ± 2.9%) and hypoxia (27.9% ± 2.1%) did not change. Retinal arteriolar and venular diameters increased (P < 0.001) by 4% and 3%, respectively, under hypoxia. CONCLUSIONS: The acute inhalation of a hypoxic gas mixture resulted in a decline in both retinal-arterial and venous saturation, while arteriovenous oxygen difference was maintained with an accompanying significant increase in retinal vessel diameter. This may suggest an autoregulatory response to acute mild hypoxia.

Sodium acetate induces a metabolic alkalosis but not the increase in fatty acid oxidation observed following bicarbonate ingestion in humans.

We conducted this study to quantify the oxidation of exogenous acetate and to determine the effect of increased acetate availability upon fat and carbohydrate utilization in humans at rest. Eight healthy volunteers (6 males and 2 females) completed 2 separate trials, 7 d apart in a single-blind, randomized, crossover design. On each occasion, respiratory gas and arterialized venous blood samples were taken before and during 180 min following consumption of a drink containing either sodium acetate (NaAc) or NaHCO3 at a dose of 2 mmol/kg body mass. Labeled [1,2 -13C] NaAc was added to the NaAc drink to quantify acetate oxidation. Both sodium salts induced a mild metabolic alkalosis and increased energy expenditure (P < 0.05) to a similar magnitude. NaHCO3 ingestion increased fat utilization from 587 +/- 83 kJ/180 min to 693 +/- 101 kJ/180 min (P = 0.01) with no change in carbohydrate utilization. Following ingestion of NaAc, the amount of fat and carbohydrate utilized did not differ from the preingestion values. However, oxidation of the exogenous acetate almost entirely (90%) replaced the additional fat that had been oxidized during the bicarbonate trial. We determined that 80.1 +/- 2.3% of an exogenous source of acetate is oxidized in humans at rest. Whereas NaHCO3 ingestion increased fat oxidation, a similar response did not occur following NaAc ingestion despite the fact both sodium salts induced a similar increase in energy expenditure and shift in acid-base balance.

Effect of muscle temperature on rate of oxygen uptake during exercise in humans at different contraction frequencies.

The effect of elevated human muscle temperature on energy turnover was investigated during cycling exercise (at 85 % of (VO(2)max)) at a contraction frequency of 60 revs min(-1). Muscle temperature was passively elevated prior to exercise by immersion of the legs in a hot water bath (42 degrees C). During exercise at this low pedalling rate, total energy turnover was higher (P<0.05) when muscle temperature was elevated compared with normal temperature (70.4+/-3.7 versus 66.9+/-2.4 kJ min(-1), respectively). Estimated net mechanical efficiency was found to be lower when muscle temperature was elevated. A second experiment was conducted in which the effect of elevated human muscle temperature on energy turnover was investigated during cycling exercise (at 85 % of (VO(2)max)) at a contraction frequency of 120 revs min(-1). Under the conditions of a high pedalling frequency, an elevated muscle temperature resulted in a lower energy turnover (P<0.05) compared with the normal muscle temperature (64.9+/-3.7 versus 69.0+/-4.7 kJ min(-1), respectively). The estimated net mechanical efficiency was therefore higher when muscle temperature was elevated. We propose that, in these experiments, prior heating results in an inappropriately fast rate of cross-bridge cycling when exercising at 60 revs min(-1), leading to an increased energy turnover and decreased efficiency. However, at the faster pedalling rate, the effect of heating the muscle shifts the efficiency/velocity relationship to the right so that cross-bridge detachment is more appropriately matched to the contraction velocity and, hence, energy turnover is reduced.