Elevation in cerebral blood flow velocity with aerobic fitness throughout healthy human ageing

It is known that cerebral blood flow declines with age in sedentary adults, although previous studies have involved small sample sizes, making the exact estimate of decline imprecise and the effects of possible moderator variables unknown. Animal studies indicate that aerobic exercise can elevate cerebral blood flow; however, this possibility has not been examined in humans. We examined how regular aerobic exercise affects the age-related decline in blood flow velocity in the middle cerebral artery (MCAv) in healthy humans. Maximal oxygen consumption, body mass index (BMI), blood pressure and MCAv were measured in healthy sedentary (n = 153) and endurance-trained (n = 154) men aged between 18 and 79 years. The relationships between age, training status, BMI and MCAv were examined using analysis of covariance methods. Mean +/- s.e.m. estimates of regression coefficients and 95% confidence intervals (95% CI) were calculated. The age-related decline in MCAv was -0.76 +/- 0.04 cm s(-1) year(-1) (95% CI = -0.69 to -0.83, r(2) = 0.66, P < 0.0005) and was independent of training status (P = 0.65). Nevertheless, MCAv was consistently elevated by 9.1 +/- 3.3 cm s(-1) (CI = 2.7-15.6, P = 0.006) in endurance-trained men throughout the age range. This approximately 17% difference between trained and sedentary men amounted to an approximate 10 year reduction in MCAv 'age' and was robust to between-group differences in BMI and blood pressure. Regular aerobic-endurance exercise is associated with higher MCAv in men aged 18-79 years. The persistence of this finding in older endurance-trained men may therefore help explain why there is a lower risk of cerebrovascular disease in this population.

Mechanisms of orthostatic intolerance following very prolonged exercise

Nine men completed a 24-h exercise trial, with physiological testing sessions before (T1, ∼0630), during (T2, ∼1640; T3, ∼0045; T4, ∼0630), and 48-h afterwards (T5, ∼0650). Participants cycled and ran/trekked continuously between test sessions. A 24-h sedentary control trial was undertaken in crossover order. Within testing sessions, participants lay supine and then stood for 6 min, while heart rate variability (spectral analysis of ECG), middle cerebral artery perfusion velocity (MCAv), mean arterial pressure (MAP; Finometer), and end-tidal Pco2 (PetCO2) were measured, and venous blood was sampled for cardiac troponin I. During the exercise trial: 1) two, six, and four participants were orthostatically intolerant at T2, T3, and T4, respectively; 2) changes in heart rate variability were only observed at T2; 3) supine MAP (baseline = 81 ± 6 mmHg) was lower ( P < 0.05) by 14% at T3 and 8% at T4, whereas standing MAP (75 ± 7 mmHg) was lower by 16% at T2, 37% at T3, and 15% at T4; 4) PetCO2 was reduced ( P < 0.05) at all times while supine (−3–4 Torr) and standing (−4–5 Torr) during exercise trial; 5) standing MCAv was reduced ( P < 0.05) by 23% at T3 and 30% at T4 during the exercise trial; 6) changes in MCAv with standing always correlated ( P < 0.01) with changes in PetCO2 ( r = 0.78–0.93), but only with changes in MAP at T1, T2, and T3 ( P < 0.05; r = 0.62–0.84); and 7) only two individuals showed minor elevations in cardiac troponin I. Recovery was complete within 48 h. During prolonged exercise, postural-induced hypotension and hypocapnia exacerbate cerebral hypoperfusion and facilitate syncope.

Endogenous and exogenous female sex hormones and renal electrolyte handling: effects of an acute sodium load on plasma volume at rest

This study was conducted to investigate effects of an acute sodium load on resting plasma volume (PV) and renal mechanisms across the menstrual cycle of endurance-trained women with natural (NAT) or oral contraceptive pill (OCP) controlled cycles. Twelve women were assigned to one of two groups, according to their usage status: 1) OCP [ n = 6, 29 yr (SD 6), 59.4 kg (SD 3.2)], or 2) NAT [ n = 6, 24 yr (SD 5), 61.3 kg (SD 3.6)]. The sodium load was administered as a concentrated sodium chloride/citrate beverage (164 mmol Na+/l, 253 mosmol/kgH2O, 10 ml/kg body mass) during the last high-hormone week of the OCP cycle (OCPhigh) or late luteal phase of the NAT cycle (NAThigh) and during the low-hormone sugar pill week of OCP (OCPlow) or early follicular phase of the NAT cycle (NATlow). The beverage (∼628 ml) was ingested in seven portions across 60 min. Over the next 4 h, PV expanded more in the low-hormone phase for both groups (time-averaged change): OCPlow 6.1% (SD 1.1) and NATlow 5.4% (SD 1.2) vs. OCPhigh 3.9% (SD 0.9) and NAThigh 3.5% (SD 0.8) ( P = 0.02). The arginine vasopressin increased less in the low-hormone phase [1.63 (SD 0.2) and 1.30 pg/ml (SD 0.2) vs. 1.82 (SD 0.3) and 1.57 pg/ml (SD 0.5), P = 0.0001], as did plasma aldosterone concentration (∼64% lower, P = 0.0001). Thus PV increased more and renal hormone sensitivity was decreased in the low-hormone menstrual phase following sodium/fluid ingestion, irrespective of OCP usage.

The effects of ageing and passive heating on cardiorespiratory and cerebrovascular responses to orthostatic stress in humans

We tested the hypothesis that older adults, relative to younger adults, would be more prone to critical reductions in cerebral blood flow and oxygenation upon standing during passive heat stress. Six older (70+/-4 years, mean+/-s.d.) and six younger males (29+/-4 years) were heated (oesophageal temperature raised 0.5 degrees C) in a water-perfused suit. Blood flow velocity in the middle cerebral artery (MCAv), cerebral oxygenation, mean arterial pressure (MAP) and end-tidal partial pressure of carbon dioxide (PET,CO2) were measured continuously before and during 3 min standing in each thermal state. At supine normothermic baseline, MCAv was 47% lower in older participants (P<0.001), whilst MAP and cerebral oxygenation were similar between groups (P>0.05). Heating lowered the supine MAP more in younger adults, and elevated heart rate only in this group. Upon initial standing in normothermia, older participants had a greater drop in MCAv (P<0.05 versus young), a lesser drop in MAP (approximately 24 and approximately 42% in older and younger participants, respectively), but slower recovery of MAP (27.3+/-6.8 versus 18.6+/-4.7 s, mean+/-s.d., P=0.004); heating did not exacerbate any postural responses in either age group. During the last minute of standing, MCAv and PET,CO2 were lower in older participants, though age differences were not evident in cerebral oxygenation (normothermic or heated). Thus, independent of heat stress, in addition to lower resting MCAv, there are further age-related reductions in MCAv and slower corrections of MAP following standing. However, these asymptomatic changes seem to represent a physiologically acceptable insult which can be well tolerated in otherwise healthy older participants even during heat stress.

Increased fat oxidation and regulation of metabolic genes with ultraendurance exercise

AIM

Regular endurance exercise stimulates muscle metabolic capacity, but effects of very prolonged endurance exercise are largely unknown. This study examined muscle substrate availability and utilization during prolonged endurance exercise, and associated metabolic genes.

METHODS

Data were obtained from 11 competitors of a 4- to 5-day, almost continuous ultraendurance race (seven males, four females; age: 36 +/- 11 years; cycling Vo(2peak): males 57.4 +/- 5.9, females 48.1 +/- 4.0 mL kg(-1) min(-1)). Before and after the race muscle biopsies were obtained from vastus lateralis, respiratory gases were sampled during cycling at 25 and 50% peak aerobic power output, venous samples were obtained, and fat mass was estimated by bioimpedance under standardized conditions.

RESULTS

After the race fat mass was decreased by 1.6 +/- 0.4 kg (11%; P < 0.01). Respiratory exchange ratio at the 25 and 50% workloads decreased (P < 0.01) from 0.83 +/- 0.06 and 0.93 +/- 0.03 before, to 0.71 +/- 0.01 and 0.85 +/- 0.02, respectively, after the race. Plasma fatty acids were 3.5 times higher (from 298 +/- 74 to 1407 +/- 118 micromol L(-1); P < 0.01). Muscle glycogen content fell 50% (from 554 +/- 28 to 270 +/- 25 nmol kg(-1) d.w.; n = 7, P < 0.01), whereas the decline in muscle triacylglycerol (from 32 +/- 5 to 22 +/- 3 mmol kg(-1) d.w.; P = 0.14) was not statistically significant. After the race, muscle mRNA content of lipoprotein lipase and glycogen synthase increased (P < 0.05) 3.9- and 1.7-fold, respectively, while forkhead homolog in rhabdomyosarcoma, pyruvate dehydrogenase kinase 4 and vascular endothelial growth factor mRNA tended (P < 0.10) to be higher, whereas muscle peroxisome proliferator-activated receptor gamma co-activator-1beta mRNA tended to be lower (P = 0.06).

CONCLUSION

Very prolonged exercise markedly increases plasma fatty acid availability and fat utilization during exercise. Exercise-induced regulation of genes encoding proteins involved in fatty acid recruitment and oxidation may contribute to these changes.

Preexercise sodium loading aids fluid balance and endurance for women exercising in the heat

This study was conducted during the high-hormone phase of both natural and oral contraceptive pill (OCP)-mediated menstrual cycles to determine whether preexercise ingestion of a concentrated sodium beverage would increase plasma volume (PV), reduce physiological strain, and aid endurance of moderately trained women cycling in warm conditions. Thirteen trained cyclists [peak O2 uptake 52 ml·kg−1·min−1 (SD 2), age 26 yr (SD 6), weight 60.8 kg (SD 5)] who were oral contraceptive users ( n = 6) or not ( n = 7) completed this double-blind, crossover experiment. Cyclists ingested a concentrated-sodium (High Na+: 164 mmol Na+/l) or low-sodium (Low Na+: 10 mmol Na+/l) beverage (10 ml/kg) before cycling to exhaustion at 70% Peak O2 uptake in warm conditions (32°C, 50% relative humidity, air velocity 4.5 m/s). Beverage (∼628 ml) was ingested in seven portions across 60 min beginning 105 min before exercise, with no additional fluid given until the end of the trial. Trials were separated by one to two menstrual cycles. High Na+ increased PV (calculated from hematocrit and hemoglobin concentration) before exercise, whereas Low Na+ did not [−4.4 (SD 1.1) vs. −1.9% (SD 1.3); 95% confidence interval: for the difference 5.20, 6.92; P < 0.0001], and it involved greater time to exhaustion [98.8 (SD 25.6) vs. 78.7 (SD 24.6) min; 95% confidence interval: 13.3, 26.8; P < 0.0001]. Core temperature rose more quickly with Low Na+ [1.6°C/h (SD 0.2)] than High Na+ [1.2°C/h (SD 0.2); P = 0.04]. Plasma [AVP], [Na+] concentration, and osmolality, and urine volume, [Na+], and osmolality decreased with sodium loading ( P < 0.05) independent of pill usage. Thus preexercise ingestion of a concentrated sodium beverage increased PV, reduced thermoregulatory strain, and increased exercise capacity for women in the high-hormone phase of natural and oral contraceptive pill-mediated menstrual cycles, in warm conditions.

Effect of post-exercise sauna bathing on the endurance performance of competitive male runners

The physiological adaptations to sauna bathing could enhance endurance performance. We have therefore performed a cross-over study in which six male distance runners completed 3 wk of post-training sauna bathing and 3 wk of control training, with a 3 wk washout. During the sauna period, subjects sat in a humid sauna at 89.9+/-2.0 degrees C (mean+/-standard deviation) immediately post-exercise for 31+/-5 min on 12.7+/-2.1 occasions. The performance test was a approximately 15 min treadmill run to exhaustion at the runner's current best speed over 5 km. The test was performed on the 1st and 2nd day following completion of the sauna and control periods, and the times were averaged. Plasma, red-cell and total blood volume were measured via Evans blue dye dilution immediately prior to the first run to exhaustion for each period. Relative to control, sauna bathing increased run time to exhaustion by 32% (90% confidence limits 21-43%), which is equivalent to an enhancement of approximately 1.9% (1.3-2.4%) in an endurance time trial. Plasma and red-cell volumes increased by 7.1% (5.6-8.7%) and 3.5% (-0.8% to 8.1%) respectively, after sauna relative to control. Change in performance had high correlations with change in plasma volume (0.96, 0.76-0.99) and total blood volume (0.94, 0.66-0.99), but the correlation with change in red cell volume was unclear (0.48, -0.40 to 0.90). We conclude that 3 wk of post-exercise sauna bathing produced a worthwhile enhancement of endurance running performance, probably by increasing blood volume.

Alterations in autonomic function and cerebral hemodynamics to orthostatic challenge following a mountain marathon

We examined potential mechanisms (autonomic function, hypotension, and cerebral hypoperfusion) responsible for orthostatic intolerance following prolonged exercise. Autonomic function and cerebral hemodynamics were monitored in seven athletes pre-, post- (<4 h), and 48 h following a mountain marathon [42.2 km; cumulative gain ∼1,000 m; ∼15°C; completion time, 261 ± 27 (SD) min]. In each condition, middle cerebral artery blood velocity (MCAv), blood pressure (BP), heart rate (HR), and cardiac output (Modelflow) were measured continuously before and during a 6-min stand. Measurements of HR and BP variability and time-domain analysis were used as an index of sympathovagal balance and baroreflex sensitivity (BRS). Cerebral autoregulation was assessed using transfer-function gain and phase shift in BP and MCAv. Hypotension was evident following the marathon during supine rest and on standing despite increased sympathetic and reduced parasympathetic control, and elevations in HR and cardiac output. On standing, following the marathon, there was less elevation in normalized low-frequency HR variability ( P < 0.05), indicating attenuated sympathetic activation. MCAv was maintained while supine but reduced during orthostasis postmarathon [−10.4 ± 9.8% pre- vs. −15.4 ± 9.9% postmarathon (%change from supine); P < 0.05]; such reductions were related to an attenuation in BRS ( r = 0.81; P < 0.05). Cerebral autoregulation was unchanged following the marathon. These findings indicate that following prolonged exercise, hypotension and postural reductions in autonomic function or baroreflex control, or both, rather than a compromise in cerebral autoregulation, may place the brain at risk of hypoperfusion. Such changes may be critical factors in collapse following prolonged exercise.

The effect of multidirectional mechanical vibration on peripheral circulation of humans

The physiological response of humans to vibration has intrigued researchers for some time, and recently in relation to its potential as a non-pharmacological means to improve peripheral blood flow. A new vibration device [Arapal Technologies Ltd (ATL), Christchurch, New Zealand] for pain relief that purportedly delivers multidirectional vibration waveforms, has been developed. The aim of the study was to quantify the effect of 30 min of mechanical vibration (60 Hz) using two ATL massage devices concurrently upon local peripheral blood flow in healthy humans. On the basis of past work it was expected that acute exposure of the body to the vibratory stimulus would increase local peripheral blood flow. In a randomized cross-over design, mean blood flow (MBF) to the calf was measured using venous occlusion plethysmography before, during 3 min and after 30 min exposure to the vibratory devices or placebo (non-vibratory) devices. Statistical analysis revealed no consistent differences between conditions and considerable individual variability. The MBF increase tended to be higher in the vibration condition than the placebo condition (P=0.16, 95% likely range=-14.4% to 82.2%), the mean increase from resting blood flow at the post-test was 26+/-49% in the vibration condition and 12+/-39% in the placebo condition. It took approximately 22 min of exposure to the vibratory stimulus to elicit peak blood flow (18 min with the placebo). Improvements in local blood flow may be beneficial in the therapeutic alleviation of pain or other symptoms resulting from acute or chronic musculoskeletal injuries.

Hazardous drinking in New Zealand sportspeople: level of sporting participation and drinking motives

AIMS

To examine the relationship between athlete drinking motives and hazardous drinking across differing levels of sporting participation (club vs elite-provincial vs elite-international).

METHODS

Data from 1214 New Zealand sportspeople was collected. We assessed hazardous drinking with the WHO's AUDIT questionnaire and sportspeople's psychosocial reasons for drinking with the ADS. Level of sporting participation (club/social, provincial/state, or international/olympic level) was also assessed.

RESULTS

Hazardous drinking behaviours differed across levels of sporting participation, with elite-provincial sportspeople showing the highest level of hazardous drinking, club/social sportspeople the next highest and elite-international sportspeople the lowest. Sportspeople who placed a greater emphasis on drinking as a reward for participating in their sports tended to display more hazardous drinking behaviours, but other ADS motives differed over level of sporting participation. Elite-provincial sportspeople and elite-international sportspeople placed more emphasis on drinking as a way to cope with the stresses of participating in their sports. A relationship between team/group motives and AUDIT scores was fully mediated by positive reinforcement motives, and partially mediated by stress-related coping motives.

CONCLUSIONS

These findings have implications for alcohol education programs targeted at sportspeople and sport administration, and may help improve the efficacy and focus of intervention programs.

Exercise can be pyrogenic in humans

Exercise increases mean body temperature (T̄body) and cytokine concentrations in plasma. Cytokines facilitate PG production via cyclooxygenase (COX) enzymes, and PGE2 can mediate fever. Therefore, we used a COX-2 inhibitor to test the hypothesis that PG-mediated pyrogenicity may contribute to the raised T̄body in exercising humans. In a double-blind, cross-over design, 10 males [age: 23 yr (SD 5), V̇o2 max: 53 ml·kg−1·min−1 (SD 5)] consumed rofecoxib (50 mg/day; NSAID) or placebo (PLAC) for 6 days, 2 wk apart. Exercising thermoregulation was measured on day 6 during 45-min running (∼75% V̇o2 max) followed by 45-min cycling and 60-min seated recovery (28°C, 50% relative humidity). Plasma cytokine (TNF-α, IL-10) concentrations were measured at rest and 30-min recovery. T̄body was similar at rest in PLAC (35.59°C) and NSAID (35.53°C) and increased similarly during running, but became 0.33°C (SD 0.26) lower in NSAID during cycling (37.39°C vs. 37.07°C; P = 0.03), and remained lower throughout recovery. Sweating was initiated at T̄body of ∼35.6°C in both conditions but ceased at higher T̄body in PLAC than NSAID during recovery [36.66°C (SD 0.36) vs. 36.39°C (SD 0.27); P = 0.03]. Cardiac frequency averaged 6·min−1 higher in PLAC ( P < 0.01), whereas exercising metabolic rate was similar (505 vs. 507 W·m−2; P = 0.56). A modest increase in both cytokines across exercise was similar between conditions. COX-2 specific NSAID lowered exercising heat and cardiovascular strain and the sweating (offset) threshold, independently of heat production, indicating that PGE-mediated inflammatory processes may contribute to exercising heat strain during endurance exercise in humans.

Sodium Loading Aids Fluid Balance and Reduces Physiological Strain of Trained Men Exercising in the Heat

PURPOSE

This study was conducted to determine whether preexercise ingestion of a highly concentrated sodium beverage would increase plasma volume (PV) and reduce the physiological strain of moderately trained males running in the heat.

METHODS

Eight endurance-trained (.VO2max: 58 mL.kg(-1).min(-1) (SD 5); 36 yr (SD 11)) runners completed this double-blind, crossover experiment. Runners ingested a high-sodium (High Na+: 164 mmol Na+.L(-1)) or low-sodium (Low Na+: 10 mmol Na+.L(-1)) beverage (10 mL.kg(-1)) before running to exhaustion at 70% .VO2max in warm conditions (32 degrees C, 50% RH, V(a) approximately equal to 1.5 m.s(-1)). Beverages (approximately 757 mL) were ingested in seven portions across 60 min beginning 105 min before exercise. Trials were separated by 1-3 wk. Heart rate and core and skin temperatures were measured throughout exercise. Urine and venous blood were sampled before and after drinking and exercise.

RESULTS

High Na+ increased PV before exercise (4.5% (SD 3.7)), calculated from Hct and [Hb]), whereas Low Na+ did not (0.0% (SD 0.5); P = 0.04), and involved greater time to exercise termination in the six who stopped because of an ethical end point (core temperature 39.5 degrees C: 57.9 min (SD 6) vs 46.4 min (SD 4); P = 0.04) and those who were exhausted (96.1 min (SD 22) vs 75.3 min (SD 21); P = 0.03; High Na+ vs Low Na+, respectively). At equivalent times before exercise termination, High Na+ also resulted in lower core temperature (38.9 vs 39.3 degrees C; P = 0.00) and perceived exertion (P = 0.01) and a tendency for lower heart rate (164 vs 174 bpm; P = 0.08).

CONCLUSIONS

Preexercise ingestion of a high-sodium beverage increased plasma volume before exercise and involved less thermoregulatory and perceived strain during exercise and increased exercise capacity in warm conditions.

Ventilatory changes in heat-stressed humans with spinal-cord injury

STUDY DESIGN

Single trial using matched subjects under tightly-controlled experimental conditions.

OBJECTIVE

Humans with spinal-cord injury have a reduced ability to dissipate heat. The current project examined the possibility that, in such people, an elevated ventilatory response (panting) may act as a supplementary avenue for heat loss.

SETTING

Australia, New South Wales.

METHODS

Breathing frequency was measured during a resting heat exposure (<or=2 h) in 10 subjects with spinal-cord injury (C4-L5), and in 10 mass- and age-matched, able-bodied subjects.

RESULTS

Subjects with spinal-cord injury displayed a ventilatory sensitivity, relative to mean body temperature change (2.4 breaths/min/ degrees C +/-0.9), more than twice that of able-bodied subjects (1.1 breaths/min/ degrees C +/-0.6; P=0.042). Furthermore, the higher the level of spinal-cord injury, the greater was the ventilatory response (r2=0.51, P=0.048).

CONCLUSION

While these ventilatory changes were apparently thermally mediated, they did not represent a true panting response, nor did the increased breathing frequency confer a physiologically significant thermoregulatory benefit that may help compensate for the loss of sympathetic flow to eccrine sweat glands and cutaneous blood vessels in people with spinal-cord injury.

The distribution of cutaneous sudomotor and alliesthesial thermosensitivity in mildly heat-stressed humans: an open-loop approach

The distribution of cutaneous thermosensitivity has not been determined in humans for the control of autonomic or behavioural thermoregulation under open-loop conditions. We therefore examined local cutaneous warm and cool sensitivities for sweating and whole-body thermal discomfort (as a measure of alliesthesia). Thirteen males rested supine during warming (+4 degrees C), and mild (-4 degrees C) and moderate (-11 degrees C) cooling of ten skin sites (274 cm2), whilst the core and remaining skin temperatures were clamped above the sweat threshold using a water-perfusion suit and climate chamber. Local thermosensitivities were calculated from changes in sweat rates (pooled from sweat capsules on all limbs) and thermal discomfort, relative to the changes in local skin temperature. Thermosensitivities were examined across local sites and body segments (e.g. torso, limbs). The face displayed stronger cold (-11 degrees C) sensitivity than the forearm, thigh, leg and foot (P = 0.01), and was 2-5 times more thermosensitive than any other segment for both sudomotor and discomfort responses (P = 0.01). The face also showed greater warmth sensitivity than the limbs for sudomotor control and discomfort (P = 0.01). The limb extremities ranked as the least thermosensitive segment for both responses during warming, and for discomfort responses during moderate cooling (-11 degrees C). Approximately 70% of the local variance in sudomotor sensitivity was common to the alliesthesial sensitivity. We believe these open-loop methods have provided the first clear evidence for a greater facial thermosensitivity for sweating and whole-body thermal discomfort.

Effects of Indomethacin and Celecoxib on Renal Function in Athletes

INTRODUCTION

Strenuous exercise induces a marked reduction in renal hemodynamics. Prostaglandins (PG) play an important role in maintaining renal integrity in the face of hemodynamic changes. Inhibition of cyclooxygenase (COX) and thus PG formation can further compromise renal perfusion. The role of selective COX-2 inhibition on renal hemodynamics during exercise has not been investigated.

METHODS

Twelve healthy males (22-47 yr) took part in a randomized placebo controlled study investigating the effects of nonselective COX inhibition (indomethacin) and COX-2 selective inhibition (celecoxib) on renal hemodynamics during exercise. Renal blood flow (RBF), glomerular filtration rate (GFR), and free water clearance were measured using standard clearance techniques. Each experimental session was performed at least a week apart. The medications were taken for 36 h before study with the last dose at 0700 h on the day of study. Following baseline studies, each participant exercised for 30 min at 80% of their maximal aerobic power. Renal function was monitored for 2 h post-recovery.

RESULTS

RBF and GFR fell by 40% after exercise with no significant difference between placebo, indomethacin, or celecoxib. Indomethacin (-2.43 +/- 0.95 mL x min(-1), P < 0.007) and celecoxib (-3.88 +/- 0.94 mL x min(-1), P < 0.0001) significantly reduced free water clearance compared with placebo during recovery.

CONCLUSION

This study has confirmed that selective and nonselective COX inhibition can induce significant inhibition of free water clearance, indicating that these acute changes are regulated predominantly via COX-2. Acute cerebral edema with hyponatremia has been reported after major endurance sporting events. Identifiable risk factors include excessive hydration and use of NSAID. Impaired free water clearance during exercise potentiated by COX inhibition provides a pathophysiological explanation for these observations.

Effect of glycerol-induced hyperhydration on thermoregulation and metabolism during exercise in heat

This study examined the effect of glycerol ingestion on fluid homeostasis, thermoregulation, and metabolism during rest and exercise. Six endurance-trained men ingested either 1 g glycerol in 20 ml H2O x kg(-1) body weight (bw) (GLY) or 20 ml H2O x kg(-1) bw (CON) in a randomized double-blind fashion, 120 min prior to undertaking 90 min of steady state cycle exercise (SS) at 98% of lactate threshold in dry heat (35 degrees C, 30% RH), with ingestion of CHO-electrolyte beverage (6% CHO) at 15-min intervals. A 15-min cycle, where performance was quantified in kJ, followed (PC). Pre-exercise urine volume was lower in GLY than CON (1119 +/- 97 vs. 1503 +/- 146 ml x 120 min(-1); p < .05). Heart rate was lower (p < .05) throughout SS in GLY, while forearm blood flow was higher (17.1 +/- 1.5 vs. 13.7 +/- 3.0 ml x 100 g tissue x min(-1); p < .05) and rectal temperature lower (38.7 +/- 0.1 vs. 39.1 +/- 0.1 degrees C; p < .05) in GLY late in SS. Despite these changes, skin and muscle temperatures and circulating catecholamines were not different between trials. Accordingly, no differences were observed in muscle glycogenolysis, lactate accumulation, adenine nucleotide, and phosphocreatine degradation or inosine 5'-monophosphate accumulation when comparing GLY with CON. Of note, the work performed during PC was 5% greater in GLY (252 +/- 10 vs. 240 +/- 9 kJ; p < .05). These results demonstrate that glycerol, when ingested with a bolus of water 2 hours prior to exercise, results in fluid retention, which is capable of reducing cardiovascular strain and enhancing thermoregulation. Furthermore, this practice increases exercise performance in the heat by mechanisms other than alterations in muscle metabolism.

The influence of whole-body vs. torso pre-cooling on physiological strain and performance of high-intensity exercise in the heat

Little research has been reported examining the effects of pre-cooling on high-intensity exercise performance, particularly when combined with strategies to keep the working muscle warm. This study used nine active males to determine the effects of pre-cooling the torso and thighs (LC), pre-cooling the torso (ice-vest in 3 degrees C air) while keeping the thighs warm (LW), or no cooling (CON: 31 degrees C air), on physiological strain and high-intensity (45-s) exercise performance (33 degrees C, 60% rh). Furthermore, we sought to determine whether performance after pre-cooling was influenced by a short exercise warm-up. The 45-s test was performed at different (P<0.05) mean core temperature [(rectal+oesophageal)/2] [CON: 37.3+/-0.3 (S.D.), LW: 37.1+/-0.3, LC: 36.8+/-0.4 degrees C] and mean skin temperature (CON: 34.6+/-0.6, LW: 29.0+/-1.0, LC: 27.2+/-1.2 degrees C) between all conditions. Forearm blood flow prior to exercise was also lower in LC (3.1+/-2.0 ml 100 ml tissue(-1) x min(-1)) than CON (8.2+/-2.5, P=0.01) but not LW (4.3+/-2.6, P=0.46). After an exercise warm-up, muscle temperature (Tm) was not significantly different between conditions (CON: 37.3+/-1.5, LW: 37.3+/-1.2, LC: 36.6+/-0.7 degrees C, P=0.16) but when warm-up was excluded, T(m) was lower in LC (34.5+/-1.9 degrees C, P=0.02) than in CON (37.3+/-1.0) and LW (37.1+/-0.9). Even when a warm-up was performed, torso+thigh pre-cooling decreased both peak (-3.4+/-3.8%, P=0.04) and mean power output (-4.1+/-3.8%, P=0.01) relative to the control, but this effect was markedly larger when warm-up was excluded (peak power -7.7+/-2.5%, P=0.01; mean power -7.6+/-1.2%, P=0.01). Torso-only pre-cooling did not reduce peak or mean power, either with or without warm-up. These data indicate that pre-cooling does not improve 45-s high-intensity exercise performance, and can impair performance if the working muscles are cooled. A short exercise warm-up largely removes any detrimental effects of a cold muscle on performance by increasing Tm.

Effect of pre-cooling, with and without thigh cooling, on strain and endurance exercise performance in the heat

Body cooling before exercise (i.e. pre-cooling) reduces physiological strain in humans during endurance exercise in temperate and warm environments, usually improving performance. This study examined the effectiveness of pre-cooling humans by ice-vest and cold (3 degrees C) air, with (LC) and without (LW) leg cooling, in reducing heat strain and improving endurance performance in the heat (35 degrees C, 60% RH). Nine habitually-active males completed three trials, involving pre-cooling (LC and LW) or no pre-cooling (CON: 34 degrees C air) before 35-min cycle exercise: 20 min at approximately 65% VO2peak then a 15-min work-performance trial. At exercise onset, mean core (Tc, from oesophagus and rectum) and skin temperatures, forearm blood flow (FBF), heart rate (HR), and ratings of exertion, body temperature and thermal discomfort were lower in LW and LC than CON (P<0.05). They remained lower at 20 min [e.g. Tc: CON 38.4+/-0.2 (+/-S.E.), LW 37.9+/-0.1, and LC 37.8+/-0.1 degrees C; HR: 177+/-3, 163+/-3 and 167+/-3 b.p.m.), except that FBF was equivalent (P=0.10) between CON (15.5+/-1.6) and LW (13.6+/-1.0 ml.100 ml tissue(-1) x min(-1)). Subsequent power output was higher in LW (2.95+/-0.24) and LC (2.91+/-0.25) than in CON (2.52+/-0.28 W kg(-1), P=0.00, N=8), yet final Tc remained lower. Pre-cooling by ice-vest and cold air effectively reduced physiological and psychophysical strain and improved endurance performance in the heat, irrespective of whether thighs were warmed or cooled.

Human sudomotor responses to heating and cooling upper-body skin surfaces: cutaneous thermal sensitivity

The influence of local skin temperature (Tskl) on the control of local and whole-body sweating was evaluated in eight healthy males. A water-perfusion garment (37 degrees C) and a climatic chamber (36.45 +/- 0.78 degrees C; [+/- SD]; relative humidity 60.3 +/- 1.6%) were used to raise and clamp skin and core temperatures. Warm and cool stimuli were applied to four upper-body skin regions (face, arm, forearm, hand) using perfusion patches (249.0 +/- 0.2 cm2). Heating elevated, while cooling suppressed sweat rate (msw) locally, and at other skin surfaces. However, the tendency for Tskl manipulations to induce localized sweat responses was no more powerful than it was at stimulating sweating in non-treated regions (P > 0.05). Accordingly, neither thermal stimulus produced significantly greater local sudomotor influences than were elicited contralaterally (P > 0.05). No statistical support was found for the notion of inter-regional differences in upper-body cutaneous thermal sensitivity for sudomotor control, and, regardless of the stimulation site, whole-body sudomotor responses to localized thermal treatments were equivalent (P > 0.05).

Effects of artificially-induced anaemia on sudomotor and cutaneous blood flow responses to heat stress

The influence of artificially induced anaemia on thermal strain was evaluated in trained males. Heat stress trials (38.6 degrees C, water vapour pressure 2.74 kPa) performed at the same absolute work rates [20 min of seated rest, 20 min of cycling at 30% peak aerobic power (VO2pcak), and 20 min cycling at 45% VO2peak] were completed before (HST1) and 3-5 days after 3 units of whole blood were withdrawn (HST2). Mild anaemia did not elevate thermal strain between trials, with auditory canal temperatures terminating at 38.5 degrees C [(0.16), HST1] and 38.6 degrees C [(0.13), HST2; P > 0.05]. Given that blood withdrawal reduced aerobic power by 16%, this observation deviates from the close association often observed between core temperature and relative exercise intensity. During HST2, the absolute and integrated forearm sweat rate (mSW) exceeded control levels during exercise (P < 0.05), while a suppression of forehead mSW occurred (P < 0.05). These observations are consistent with a possible peripheral redistribution of sweat secretion. It was concluded that this level of artificially induced anaemia did not impact upon heat strain during a 60-min heat stress test.

Sweat distribution before and after repeated heat exposure

We investigated the impact of short-term, moderate humidity heat acclimation upon sweat distribution. Eight males completed six daily heat exposures [cycling: ambient temperature 39.5 (0.2) degrees C, relative humidity 59.2 (0.8)%], during which auditory canal temperature (T(ac)) was maintained 1.4 degrees C above pre-exposure levels for 70 min by manipulating the work rate. On days 1 and 6, T(ac) and local sweat rates (m(sw): eight sites) were monitored. The pre-exposure, resting T(ac) and the T(ac) sweat threshold decreased from day 1 to day 6 [36.83 (0.05) degrees C vs 36.62 (0.05) degrees C, and 36.90 (0.05) degrees C vs 36.75 (0.05) degrees C, respectively; both P < 0.05]. However, the sweat-onset time, sweat sensitivity (delta m(sw)/deltaT(ac)) and established m(sw) were unaltered (P > 0.05). There was also no evidence of a post-acclimation redistribution in established m(sw) between the eight skin regions, though both the sweat sensitivity and established m(sw) for the forehead and hand were significantly greater than at the remaining sites (P < 0.05). It is concluded that the 5-day heat acclimation regimen provided only a minimal stimulus for sudomotor adaptation.

The topography of eccrine sweating in humans during exercise

The purpose of this study was to investigate the distribution of steady-state sweating rates (msw), during stressful exercise and heat exposures. Six men completed 42-min trials: 2-min rest and 40-min cycling at 40% peak power in 36.6 degrees C (relative humidity 46.0%). The msw was monitored using ventilated capsules at the forehead, and at three additional sites. Repeat trials allowed monitoring from eleven skin surfaces. Auditory canal temperature (Tac) and 11 skin temperatures were measured. After normalising msw to the forehead response within subjects, differences in Tac and onset time thresholds, and transient and steady-state msw were examined. The pooled, lower torso msw onset [mean 45.5 (SEM 42.0) s] preceded that of the head [mean 126.5 (SEM 34.8) s, P < 0.05], but was not significantly different from the legs [mean 66.6 (SEM 25.7) s], upper torso [mean 80.2 (SEM 36.8) s] or arms [mean 108.6 (SEM 31.2) s]. Transient msw did not differ among regions (P = 0.16). Mean, steady-state forehead msw [3.20 (SEM 0.51) mg.cm-2.min-1] was not significantly greater than the scapula, forearm, hand, stomach and lower back msw (in descending order), but was greater than the chest [1.6 (SEM 0.2)], upperarm [1.6 (SEM 0.2)], calf [1.5 (SEM 0.3)] and thigh msw [1.0 (SEM 0.2), P < 0.05 for all comparisons]. The results did not support the caudal-to-rostral sweat onset evident during supine, resting heat stress. Equivalent Tac sweat thresholds existed between sites, while steady-state state msw topography varied among subjects and was not dominated by central regions.

Physiological assessment of the RNZAF constant wear immersion suit: laboratory and field trials

Laboratory and field immersion trials were undertaken to determine the thermal protection afforded by a constant wear immersion suit (CWIS) in operation with the Royal New Zealand Air Force (RNZAF). Six males wore each of two ensembles during head-out laboratory immersions in 5.0 +/- 0.1 degree C (mean +/- SD) water for a maximum of 3 h. Ensembles 1 and 2 consisted of the CWIS in addition to minimal and maximal likely undergarment insulations, respectively. Open sea field trials (water temperature = 13.8 +/- 0.7 degree C; Beaufort wind state = 0-4; Sea state = 0-2) were conducted for a maximum of 2 h, with subjects wearing ensemble two and remaining strike aircrew apparel (ensemble three). Analysis of rectal temperature (Tre) changes permitted calculation of time to hypothermia (t35) and the survival estimate of 34 degrees C (t34). Mean (+/- SEM) t35 was 78 +/- 11 (n = 6), 187 +/- 20 (n = 5, p < 0.05) and 98 +/- 5 min (n = 3) for ensembles one, two and three, respectively. Mean t34 was 96 +/- 15, 259 +/- 31 (p < 0.05), and 119 +/- 5 min, respectively. Immersed insulations of ensembles one and two were 0.035 +/- 0.002 and 0.150 +/- 0.015 degree C.m2.W-1, respectively. Thus, the difference between minimal and maximal operational insulation was a 4.3-fold increase in insulation, which facilitated a 2.7-fold increase in mean t34. The thermal protection afforded by the CWIS during field trials was not sufficient to ensure survival for the 12-h expected rescue time.(ABSTRACT TRUNCATED AT 250 WORDS)

Thermal tolerance following artificially induced polycythaemia

Polycythaemia has been shown to improve physical performance, possibly due to increased arterial oxygen transport. Enhanced thermoregulatory function may also accompany this manipulation, since a greater proportion of the cardiac output becomes available for heat dissipation. We further examined this possibility in five trained men, who participated in three-phase heat stress trials (20 min rest, 20 min cycling at 30% peak power (Wpeak) and 20 min at 45% Wpeak at 38.3 (SEM 0.7) degrees C [relative humidity 41.4 (SEM 2.9)%]. Trials were performed during normocythaemia (control) and polycythaemia, obtained by reinfusion of autologous red blood cells and resulting in significant elevation of arterial oxygen transport. During the polycythaemic trials, the subjects demonstrated diminished thermal strain, as evidenced by a significant reduction in cardiac frequency (fc: 12 beats.min-1 lower throughout the test; P < 0.05), and reduced auditory canal temperatures (Tac) during the latter 20-min phase (P < 0.05). Forearm sweat onset was more rapid (363.0 compared to 1083.0 s; P < 0.05), and forearm sweat rate (msw) sensitivity was elevated from 1.80 to 2.91.mg.cm-2.min-1.degrees C-1 (P < 0.05). Forehead msw was depressed during the final 20 min, while forearm msw was greater during all test phases, averaging 0.94 and 1.20 mg.cm-2.min-1, respectively, over the 60 min. Skin blood flows for the upper back, upper arm and forearm were reduced (P < 0.05). Polycythaemia enhanced thermoregulation, through an elevation in forearm sweat sensitivity and msw, but not via increased cutaneous blood flow. These modifications occurred simultaneously with decreases in fc and Tac, resulting in greater thermal tolerance.

Epidemiology of hypothermia: fatalities and hospitalisations in New Zealand

BACKGROUND

Hypothermia occurs within domestic and non-residential settings. Most epidemiological data originate from the northern hemisphere, with little data being generally available concerning cases from New Zealand and Australia.

AIMS

The National Health Statistics Centre (New Zealand) records hospital discharges and deaths. This study isolated hypothermia cases, to quantify its incidence and identify risk groups.

METHODS

The morbidity and mortality files for the years 1979-86 (cases = 3,808,717) and 1977-86 (cases = 259,325; respectively) were searched by three investigators.

RESULTS

Hypothermia hospitalisations were identified (6.9 per 100,000 per year). There were 176 deaths from hypothermia, representing 0.07% of the 259,325 deaths from all causes for the same period (0.537 per 100,000 people per year); of these fatalities, 72.2% were classified as domestic, and 27.8% as non-residential; of the domestic fatalities, 86.6% were 65 + years and 35.5% of these were male. Within the non-residential category, 75.5% were aged 13-65, of which 94.6% were male. The hospitalisation incidence was 12.7 times the fatality incidence, with the majority of hospitalisations being of domestic origin (88.4% of total), and occurring mostly within the lower and upper age extremes. Neonatal domestic hypothermia accounted for 72.6% of all domestic hospitalisations, and the elderly constituted 72.0% of the remaining cases. The proportion of New Zealand fatalities caused by hypothermia was 0.067%; lower than reported in the United Kingdom.

CONCLUSIONS

The two main non-neonatal groups contributing to cases of hypothermia were males aged 13-65 years, and the elderly. In the aged, the proportion of hypothermia-related deaths was no different from that associated with other disorders, however, the case-fatality ratio was three times greater, highlighting the need for improving prevention and management strategies.

Functional torque-velocity and power-velocity characteristics of elite athletes

Technical limitations of some isokinetic dynamometers have called into question the validity of some data on human muscle mechanics. The Biodex dynamometer has been shown to minimize the impact artefact while permitting automatic gravity correction. This dynamometer was used to study quadriceps muscle torque and power generation in elite power (n = 6) and elite endurance (n = 7) athletes over 12 randomly assigned isokinetic velocities from 30 degrees.s-1 to 300 degrees.s-1. The angle at peak torque varied as a negative, linear function of angular velocity, with the average angle across test velocities being 59.5 degrees (SD 10.2 degrees). Power athletes developed greater peak torque at each angular velocity (P less than 0.05) and experienced a 39.7% decrement in torque over the velocity range tested. Endurance athletes encountered a 38.8% decline in peak torque. Torques measured at 60 degrees of knee flexion followed a similar trend in both groups; however the greatest torques were recorded at 60 degrees.s-1 rather than at 30 degrees.s-1. Leg extensor muscle power increased monotonically with angular velocity in both power (r2 = 0.728) and endurance athletes (r2 = 0.839); however these curves diverged significantly so that the power athletes produced progressively more power with each velocity increment. These inter group differences probably reflected a combination of natural selection and training adaptation.