Effects of caffeine and high ambient temperature on haemodynamic and body temperature responses to dynamic exercise.

Caffeine can enhance mean arterial blood pressure (MAP) and attenuate forearm blood flow (FBF) and forearm vascular conductance (FVC) during exercise in thermal neutral conditions without altering body temperature. During exercise at higher ambient temperatures, where a greater transfer of heat from the body core to skin would be expected, caffeine-induced attenuation of FBF (i.e. cutaneous blood flow) could attenuate heat dissipation and increase body temperature (T(re)). We hypothesized that during exercise at an ambient temperature of 38 degrees C, caffeine increases MAP, and attenuates FBF and FVC such that T(re) is increased. Eleven caffeine-naive, active men, were studied at rest and during exercise after ingestion of a placebo or 6 mg kg(-1) of caffeine. MAP, heart rate (HR), FBF, FVC, T(re) skin temperature (T(sk)) and venous lactate concentrations (lactate) were assessed sequentially during rest at room temperature, after 45 min of exposure to an ambient temperature of 38 degrees C, and during 35 min of submaximal cycling. Heat exposure caused increases in MAP, FBF, FVC and T(sk) that were not altered by caffeine. HR, T(re), and lactate were unaffected. During exercise, only MAP (95 +/- 2 vs. 102 +/- 2 mmHg), HR (155 +/- 10 vs. 165 +/- 10 beats min(-1)), and lactate (2.0 +/- 0.4 vs. 2.3 +/- 0.4 mmol l(-1)) were increased by caffeine. These data indicate that increases in cutaneous blood flow during exercise in the heat are not reduced by caffeine. This may be because of activation of thermal reflexes that cause cutaneous vasodilation capable of offsetting caffeine-induced reductions in blood flow. Caffeine-induced increases in lactate, MAP and HR during exercise suggest that this drug and high ambient temperatures increase production of muscle metabolites that cause reflex cardiovascular responses.     

Osmotic responses and tolerance limits to changes in external osmolalities, and oolemma permeability characteristics, of human in vitro matured MII oocytes

BACKGROUND: Oocyte cryopreservation remains a realistic objective, provided that more systematic approaches are applied, such as thorough analysis of the oocyte oolemma permeability to water and diverse cryoprotectants. METHODS: We prospectively investigated volume changes over time at different temperatures (30 degrees C, 22 degrees C and 8 degrees C) of human metaphase II (MII) oocytes (obtained in stimulated ICSI cycles and matured in vitro from the germinal vesicle stage) when exposed to changes in external osmolality. We also investigated human in vitro matured (IVM) oocytes membrane permeability characteristics at 22 degrees C to 1,2-propanediol (PG) and dimethylsulphoxide (DMSO) and at 30 degrees C, 22 degrees C and 8 degrees C to ethylene glycol (EG), and calculated corresponding oocyte oolemma permeability coefficients (Lp and Pcpa). Furthermore, we investigated the osmotic tolerance limits of IVM oocytes exposed to changes in external osmolality as assessed by their developmental competence during the course of 72 h after ICSI. RESULTS: The results of our studies describe human oocyte membrane permeability coefficients for EG at 30 degrees C (2.85+/-0.15x10(-3) cm/min), 22 degrees C (1.17+/-0.60x10(-3) cm/min) and 8 degrees C (0.37+/-0.15x10(-3) cm/min). Furthermore, at 22 degrees C the EG oolemma permeability coefficient was lower than that of PG and DMSO (1.17+/-0.60x10(-3) cm/min versus 2.15+/-0.70x10(-3) and 1.56+/-0.38x10(-3) cm/min, respectively). Our results also indicate, that human IVM MII oocytes tolerated exposure to solutions in the range of 39-2264 mOsmol/kg H2O as assessed by the oocytes' developmental competence after exposure. CONCLUSIONS: The results of the present study may contribute to a better understanding of the biology and cryobiology of human oocytes, and to the design of better and more robust cryopreservation (freezing or vitrification) protocols.     

Enhanced heat loss responses induced by short-term endurance training in exercising women

We investigated the effects of short-term endurance training and detraining on sweating and cutaneous vasodilatation during exercise in young women, taking into account changes in maximal oxygen uptake (VO2max) and the phase of the menstrual cycle. Eleven untrained women participated in endurance training; cycle exercise at approximately 60% VO2max for 60 min day(-1), 4-5 days week(-1) (30 degrees C, 45% relative humidity) for three complete menstrual cycles. The standard exercise test consisted of exercise at 50% VO2max for 30 min (25 degrees C, 45% relative humidity), and was conducted before training (Pre), during training sessions (T1, T2 and T3) and after cessation of training (D1 and D2). Values of VO2max increased significantly from 32.7 +/- 1.2 to 37.8 +/- 1.2 ml min(-1) kg(-1) at the end of the training. Local sweat rate in the chest and thigh, but not in the back and forearm, were significantly greater during T1 and T2 only in women who started training from the midfollicular phase. Cutaneous blood flow did not change with training. The threshold oesophageal temperatures for heat loss responses were significantly decreased during T1 versus Pre (averaged values for each body site: sweating, 37.49 +/- 0.08 versus 37.22 +/- 0.12 degrees C; and cutaneous vasodilatation, 37.40 +/- 0.07 versus 37.17 +/- 0.10 degrees C) and maintained through T3; the sensitivities of heat loss responses were not altered. These changes returned to the Pre level by D1. Our data indicate that physical training improves heat loss responses by decreasing the threshold temperatures and that these effects occur within a month of training and disappear within a month after cessation of training. The degree of increase in sweating with training differs among body sites and might be affected by the phase of the menstrual cycle.     

Cardiac output decline in prolonged dynamic exercise is affected by the exercise mode

The aim of this study was to compare the cardiovascular responses to prolonged submaximal cycling and running. Eleven males [maximal oxygen uptake (VO(2max)): 3.58+/-0.15 l min(-1) for running and 3.84+/-0.16 l min(-1) for cycling; mean+/-SE] either cycled (C) or ran (R) for 90 min at 60% of mode-specific VO(2max), on two randomly assigned occasions. Cardiac output declined after 85 min of exercise in C (-1.9+/-0.5 l min(-1), P<0.01) but not in R, as a result of a more pronounced decrease in stroke volume in the former exercise mode (-22.7+/-3.8 ml beat(-1) vs -14.3+/-1.9 ml beat(-1), P<0.01) since heart rate did not differ between trials. Stroke volume responses were despite a higher level of dehydration (-3.3+/-0.2% in R vs -2.8+/-0.2% in C, P<0.05) and hyperthermia in R (39.6+/-0.1 vs 38.8+/-0.1 degrees C in C at 90 min, P<0.01). Finally, mean skin blood flow was lower in R than C (72+/-8 vs 89+/-10%; P<0.05). In conclusion, stroke volume and cardiac output decline was more pronounced in cycling than in running despite lower dehydration and rectal temperature in the former exercise mode.     

Effect of skin temperature on the cholinergic sensitivity of the human eccrine sweat gland

Although sweat gland activity is directly controlled by the central nervous system, which detects changes in core body temperature, sweat glands can also be influenced by local cutaneous thermal conditions. OBJECTIVE: The present study sought to determine the effect of local skin temperature on pilocarpine-induced sweating within a range of typical skin temperatures. METHODS: Thirteen subjects (30 +/- 6 years; 172 +/- 11 cm; 72.8 +/- 11.0 kg) had forearm sweat rates measured at rest following pilocarpine iontophoresis at each of three skin temperatures in randomized order: warm (T(warm) = 37.1 +/- 0.9 degrees C), control (T(con) = 32.3 +/- 1.4 degrees C), and cool (T(cool) = 26.6 +/- 1.3 degrees C). T(skin) was raised and lowered with an electric heating pad and gel ice pack, respectively. Forearm T(skin) was measured with a skin temperature probe. Pilocarpine iontophoresis was used on an approximately 7 cm(2) area of the anterior forearm to stimulate localized sweating. Following stimulation, sweat was collected from the area for 15 min with a Macroduct Sweat Collection System. RESULTS: There was a higher sweat rate at T(warm) (p = 0.001) and T(con) (p = 0.006) compared to that at T(cool). However, there was no difference between the sweat rate at T(warm) and that at T(con) (p = 0.127). CONCLUSION: These results indicated that skin temperatures below approximately 32 degrees C affect local sweat production primarily by altering glandular sensitivity to the neurotransmitter, whereas skin temperatures above approximately 32 degrees C predominantly affect neurotransmitter release. Furthermore, sweat glands display maximal or near maximal cholinergic sensitivity at resting skin temperature in a thermoneutral environment.     

The effects of face cooling during hyperthermic exercise in man: evidence for an integrated thermal, neuroendocrine and behavioural response

The present study investigated whether face cooling reduced both the perceived exertion (RPE) and prolactin (PRL) release during hyperthermic exercise. Ten, non-heat-acclimated males (23 +/- 2 years; maximal oxygen consumption, 56 +/- 7 ml kg(-1) min(-1) [mean +/- s.d.]) exercised for 40 min on a cycle ergometer at 65% of their peak aerobic power, at an ambient temperature of 33 degrees C (27% relative humidity) with (FC) and without face cooling as a control (CON). With FC, forehead temperature was maintained approximately 6 degrees C lower than CON, while other skin sites were similar or slightly warmer in the FC condition. Rectal temperature increased by approximately 1.5 degrees C with the same time course in both conditions. A relative bradycardia was observed during FC, with heart rate approximately 5 beats min(-1) lower than CON (P < 0.05). Mean plasma lactate was lower during FC (FC, 5.0 +/- 0.3 mmol l(-1); CON, 5.9 +/- 0.3 mmol l(-1); P < 0.05) but no differences were observed for plasma glucose, which remained constant during exercise. Levels of PRL were maintained at 175 +/- 17 mIU l(-1) during exercise for FC, while values for CON increased to a peak of 373 +/- 22 mIU l(-1) so that towards the end of the exercise, for the same rectal temperature, PRL was significantly lower in the FC condition (P < 0.05). Global and breathing RPE were reduced but only towards the end of the 40 min of exercise during FC, whilst subjective thermal comfort was significantly lower during FC (P < 0.05). We confirm the substantial effect that FC has on the secretion of PRL during hyperthermic exercise but show that it makes a relatively small contribution to the perception of effort when compared to the effect of a cool total skin area as occurs with exercise in a thermoneutral environment.     

Evaluation of multielement catheter-cooled interstitial ultrasound applicators for high-temperature thermal therapy

Catheter-cooled (CC) interstitial ultrasound applicators were evaluated for their use in high-temperature coagulative thermal therapy of tissue. Studies in ex vivo beef muscle were conducted to determine the influences of applied electrical power levels (5-20 W per element), catheter flow rate (20-60 ml min(-1)), circulating water temperature (7-40 degrees C), and frequency (7-9 MHz) on temperature distribution and thermal lesion geometry. The feasibility of using multiple interstitial applicators to thermally coagulate a predetermined volume of tissue was also investigated. Results of these studies revealed that the directional shape of the thermal lesions is maintained with increasing time and power. Radial depths of the thermal lesions ranged from 10.7 +/- 0.7 mm after heating for 4 min with an applied power level of 5 W, to 16.2 +/- 1.4 mm with 20 W. The axial length of the thermal lesions is controlled tightly by the number of active transducers. A catheter flow rate of 20 to 40 ml min(-1) (52.2 +/- 5.5 kPa at 40 ml min(-1)) with 22 degrees C water was determined to provide sufficient cooling of the transducers for power levels used in this study. In vivo temperatures measured in the center of a 3-cm-diam peripheral implant of four applicators in pig thigh muscle reached 89.3 degrees C after 4 min of heating, with boundaries of coagulation clearly defined by applicator position and directivity. Conformability of heating in a clinically relevant model was demonstrated by inserting two directional CC applicators with a 2 cm separation within an in vivo canine prostate, and generating a thermal lesion measuring 3.8 cm x 2.2 cm in cross section while directing energy away from, and protecting the rectum. Maximum measured temperatures at midgland exceeded 90 degrees C within 20 min of heating. The results of this study demonstrate the utility of single or multiple CC applicators for conformal thermal coagulation and high temperature thermal therapy, with potential for clinical applications in sites such as prostate, liver, breast, or uterus.     

Oxygen consumption during pouch development of the macropod marsupial Setonix brachyurus

1. Measurements of O(2) consumption at 9 or 10 temperatures in the 20-40 degrees C ambient temperature range were made on joeys with ages selected to cover the 180-day period of pouch occupancy.2. The rate of O(2) consumption of joeys younger than 100 days increased directly with ambient temperature.3. After 100 days of age the O(2) consumption rate at low temperatures rose and at about 140 days of age a constant rate was maintained over the full ambient temperature range.4. Heat transfer from joey to mother commenced after 100 days of age.5. At 150-180 days of age the rate of O(2) consumption at 20 degrees C was approximately 12 times greater than at ages less than 100 days. A thermal neutral zone was established in the range 32-36 degrees C by joeys older than 150 days.6. At the usual pouch temperature of 36.5 degrees C, O(2) consumption per unit wet body weight rose from 12 ml./kg.min at birth to 17 ml./kg.min at the end of pouch life. On a unit dry body weight basis it fell from 120 to 56 ml./kg.min. This decline parallels the decrease in growth rate.     

The prolactin responses to active and passive heating in man

The aim of this study was to compare the prolactin and blood pressure responses at identical core temperatures during active and passive heat stresses, using prolactin as an indirect marker of central fatigue. Twelve male subjects cycled to exhaustion at 60% maximal oxygen uptake (VO2peak) in a room maintained at 33 degrees C (active). In a second trial they were passively heated (passive) in a water bath (41.56 +/- 1.65 degrees C) until core temperature was equal to the core temperature observed at exhaustion during the active trial. Blood samples were taken from an indwelling venous cannula for the determination of serum prolactin during active heating and at corresponding core temperatures during passive heating. Core temperature was not significantly different between the two methods of heating and averaged 38.81 +/- 0.53 and 38.82 +/- 0.70 degrees C (data expressed as means +/- s.d.) at exhaustion during active heating and at the end of passive heating, respectively (P > 0.05). Mean arterial blood pressure was significantly lower throughout passive heating (active, 73 +/- 9 mmHg; passive, 62 +/- 12 mmHg; P < 0.01). Despite the significantly reduced blood pressure responses during passive heating, during both forms of heating the prolactin response was the same (active, 14.9 +/- 12.6 ng ml(-1); passive, 13.3 +/- 9.6 ng ml(-1); n.s.). These results suggest that thermoregulatory, i.e. core temperature, and not cardiovascular afferents provide the key stimulus for the release of prolactin, an indirect marker of central fatigue, during exercise in the heat.     

Capillary and cell wall permeability to potassium in isolated dog hearts.

From venous tracer-dilution curves recorded after 36 pulse injections of 42KCl and 131I-labeled albumin into the coronary artery inflow of 15 isolated canine heart preparations, we calculated maximal fractional extractions (Emax) and capillary permeability-surface area products (PScap) for 42K+ over a range of plasma flows (FP) from 0.3 to 1.7 ml min-1 g-u. At low FP (less than 1.0), Emax was 0.60 +/- 0.0l (mean +/- SD) and PScap was 0.72 +/- 0.20 ml min-1 g-1; at high FP (greater than 1.0), Emax decreased to 0.49 +/- 0.05 and PScap increased to 1.06 +/- 0.18. Continuous recording (gamma detector) of residual myocardial 42K+ in seven hearts showed that the mean fractional escape rate of tracer between 30 and 60 min after injection was 0.011-0.023 min-1; higher rates were observed at high FP, when the residue of 42K+ decreased to less than 10% of the injected dose by 60 min. Using PScap measured at high FP and considering the virtual intracellular volume of distribution for K+ to be 20 ml/g, we calculated the permeability-surface area product for sarcolemma (PScw) as 0.54-0.73 ml min-1 g-1, or about 50% of PScap. Considering sarcolemmal surface area (Scw) as 4,200 cm2/g and capillary surface area (Scap) as 500 cm2/g, cell permeability is low, with Pcw:Pcap being less than 0.08.     

Influence of adiposity on cooling efficiency in hyperthermic individuals

This study evaluated the effect of body adiposity on core cooling rates, as measured by decreases in rectal (T (re)), esophageal (T (es)) and aural canal (T (ac)) temperatures, of individuals rendered hyperthermic by dynamic exercise in the heat. Seventeen male participants were divided into two groups; low body fat (LF, 12.9 +/- 1.9%) and high body fat (HF, 22.3 +/- 4.3%). Participants exercised at 65% of their maximal oxygen uptake at an ambient air temperature of 40 degrees C until T (re) increased to 40 degrees C or until volitional fatigue. Following exercise, participants were immersed up to the clavicles in an 8 degrees C circulated water bath until T (re) returned to 37.5 degrees C. No significant differences were found between the LF and HF in the time to reach a T (re) of 39.5 degrees C (P = 0.205), 38.5 degrees C (P = 0.343) and 37.5 degrees C (P = 0.923) during the immersion. Overall cooling rate for T (re) was also similar between groups (0.23 +/- 0.09 degrees C/min (LF) vs. 0.20 +/- 0.09 degrees C/min (HF), P = 0.647) as well as those for T (es) (P = 0.502) and T (ac) (P = 0.940). Furthermore, mean rate of non-evaporative heat loss (702 +/- 217 W/m(2) (LF) vs. 612 +/- 141 W/m(2) (HF), P = 0.239) was not different between groups. These results suggest that a difference of approximately 10% of body adiposity does not affect core cooling rates in active individuals under 25% body fat rendered hyperthermic by exercise.     

The effects of heat stress on neuromuscular activity during endurance exercise

This study analysed the effect of hot (35 degrees C) and cold (15 degrees C) environments on electromyographic (EMG) signal characteristics, skin and rectal temperatures and heart rate during progressive endurance exercise. Eight healthy subjects performed three successive 15-min rides at 30%, 50% and 70% of their peak sustained power output and then cycled at increasing (15 W/min) work rates to exhaustion in both 35 degrees C and 15 degrees C environments. Skin and rectal temperatures, heart rate and EMG data were measured during the trials. The skin temperatures were higher and the subjects felt more uncomfortable in the hot conditions (Bedford scale) ( P<0.01). Rectal temperature was slightly, but not significantly, higher under hot conditions. Heart rate was significantly higher in the hot group (between condition P<0.05). Peak power output (267.4+/-67.7 W vs. 250.1+/-61.5 W) and time-to-exhaustion (55.7+/-16.7 min vs. 54.5+/-17.1 min) (COLD vs. HOT) were not different between conditions. There were no differences in integrated EMG (IEMG) or mean power frequency spectrum between conditions. Rating of perceived exertion increased similarly in both conditions over time. Although the hot conditions increased heart rate and skin temperature, there were no differences in muscle recruitment or maximal performance, which suggests that the thermal stress of 35 degrees C, in combination with exercise, did not impair maximal performance in this study.     

Effect of Temperature on Muscle Metabolism During Submaximal Exercise in Humans

To study the effect of temperature on muscle metabolism during submaximal exercise, six endurance-trained men had one thigh warmed and the other cooled for 40 min prior to exercise using water-perfused cuffs. One cuff was perfused with water at 50-55 degrees C (HL) with the other being perfused with water at 0 degree C (CL). With the cuffs still in position, subjects performed cycling exercise for 20 min at a work load corresponding to 70% VO2,peak (where VO2,peak is peak pulmonary oxygen uptake) in comfortable ambient conditions (20-22 degrees C). Muscle biopsies were obtained prior to and following exercise and forearm venous blood was collected prior to and throughout the exercise period. Muscle temperature (Tmus) was not different prior to treatment, but treatment resulted in a large difference in pre-exercise Tmus (difference = 6.9 +/- 0.9 degrees C; P < 0.01). Although this difference was reduced following exercise; it was nonetheless significant (difference = 0.4 +/- 0.1 degree C; P < 0.05). Intramuscular [ATP] was not affected by either exercise or muscle temperature. [Phosphocreatine] decreased (P < 0.01) and [creatine] increased (P < 0.01) with exercise but were not different when comparing HL with CL. Muscle lactate concentration was not different prior to treatment nor following exercise when comparing HL with CL. Muscle glycogen concentration was not different when comparing the trials before treatment, but the post-exercise value was lower (P < 0.05) in HL compared with CL. Thus, net muscle glycogen use was greater during exercise with heating (208 +/- 23 vs. 118 +/- 22 mmol kg-1 for HL and CL, respectively; P < 0.05). These data demonstrate that muscle glycogen use is augmented by increases in intramuscular temperature despite no differences in high energy phosphagen metabolism being observed when comparing treatments. This suggests that the increase in carbohydrate utilization occurred as a direct effect of an elevated muscle temperature and was not secondary to allosteric activation of enzymes mediated by a reduced ATP content.     

Elevation of arterial blood pressure in the squirrel monkey at 10 degrees C.

Systemic arterial blood pressure, heart rate, and total body oxygen consumption were measured in seven unanesthetized squirrel monkeys exposed to ambient temperatures of 28 degrees C and 10 degrees C. At 28 degrees C, subjects sat quietly, the average mean arterial blood pressure was 116 +/- 16 (mean +/- SD, n - 7) mmHg, heart rate was 274 +/- 31 beats/min, and oxygen consumption was 14 +/- 1.4 ml/kg-min. At 10 degrees C, the animals shivered vigorously, the average mean arterial blood pressure was 139 +/- 16 mmHg, heart rate was 328 +/- 18 beats/min, and oxygen consumption was 31.6 +/- 3.9 ml/kg-min. Thus, the oxygen consumption more than doubled, the blood pressure rose by approximately 21%, and the heart rate by approximately 20%. Elevations in heart rate as well as systemic mean arterial blood pressure during exposure to low ambient temperature were probably mediated by sympathetic-adrenal discharges as well as by activity of skeletal muscles.     

Increasing exercise duration does not affect the postexercise elevation in esophageal temperature.

It has previously been observed that (a) following 15 min of intense exercise, esophageal temperature (Tes) remains elevated at a plateau value equal to that at which active vasodilation had occurred during exercise (i.e., esophageal temperature threshold for cutaneous vasodilation [ThVD]); and (b) exercise/recovery cycles of identical intensity and duration, when sequential, result in progressively higher Tes at the beginning and end of exercise. In the latter case, parallel increases in both the exercise ThVD and postexercise plateau of Tes were noted. This study was conducted to determine if the elevated postexercise Tes is related to increases in whole-body heat content. On separate occasions, 9 subjects completed 3 bouts of treadmill exercise at 70% VO2 max, 29 degrees C ambient temperature. Each exercise bout lasted either 15, 30, or 45 min and was followed by 60 min of inactive recovery. Esophageal temperatures were similar at the start of each exercise bout, but the rise in Tes during exercise nearly doubled from 1.0 degree C after 15 min of exercise to 1.9 degrees C after 45 min of exercise. There were no intercondition differences among the exercise ThVD (approximately 0.36 degree C above baseline) or postexercise plateau values for Tes (approximately 0.40 degree C above baseline). Thus the relationship between the ThVD during exercise and the postexercise Tes did not appear to be dependent on changes in whole-body heat content as produced by endogenous heating during exercise of different duration.     

Sweating responses to a sustained static exercise is dependent on thermal load in humans

The purpose of this project was to test the hypothesis that internal temperature modulates the sweating response to sustained handgrip exercise. Ten healthy male subjects immersed their legs in 43 degrees C water for 30-40 min at an ambient temperatures of 30 degrees C and a relative humidity of 50%. Sweating responses to 50% maximal voluntary contraction isometric handgrip exercise (IH) were measured following the onset of sweating (i.e. following slight increases in internal temperature), and after more pronounced increases in internal temperature. Oesophageal temperature (Tes) was significantly lower during the first bout of exercise (37.54 +/- 0.07 degrees C) relative to the second bout (37.84 +/- 0.12 degrees C; P < 0.05). However, the increase in mean sweating rate (SR) from both the chest and forearm (non-glabrous skin) was significantly greater during the first IH bout relative to the second bout (P < 0.05). Increases in mean arterial blood pressure and palm SR (glabrous skin) did not differ significantly between exercise bouts, while heart rate and rating of perceived effort were significantly greater during the second bout of IH. As Tes and mean skin temperature did not change during either bout of exercise, the changes in SR from non-glabrous skin between the bouts of IH were likely because of non-thermal factors. These data suggest that sweating responses from non-glabrous skin during IH vary depending on the magnitude of thermal input as indicated by differing internal temperatures between bouts of IH. Moreover, these data suggest that the contribution of non-thermal factors in governing sweating from non-glabrous skin may be greatest when internal temperature is moderate (37.54 degrees C), but has less of an effect after greater elevations in internal temperature (i.e. 37.84 degrees C).     

Drink temperature influences fluid intake and endurance capacity in men during exercise in a hot, dry environment

The effect of different drink temperatures on the perception of exertion and exercise endurance has not been extensively investigated. Consequently, the purpose of the present study was to examine the effect of drink temperature on fluid intake and endurance during cycling in the heat. Eight healthy, non-acclimated males (26 +/- 7 years; maximum oxygen uptake, 54 +/- 5 ml kg(-1) min(-1); mean +/- S.D.) cycled to exhaustion at 34 degrees C and at 65% of their peak aerobic power, consuming a drink at either 19 degrees C (CON) or 4 degrees C (COLD). Six of the eight subjects cycled for longer during COLD, with exhaustion occurring at 62 +/- 4 min, compared to 55 +/- 4 min for CON (P < 0.05; mean +/- S.E.M.). Subjects consumed significantly more fluid during COLD compared to CON (1.3 +/- 0.3 l h(-1) compared to 1.0 +/- 0.2 l h(-1); P < 0.05). Heart rate tended to be lower by approximately 5 beats min(-1) during COLD, and rectal temperature during the second half of the exercise period was approximately 0.25 degrees C lower during the COLD trial; however, these trends were not significant (P = 0.08 and P = 0.07, respectively). No differences were observed between trials for ventilation, concentrations of prolactin, glucose and lactate or perceived exertion. It is concluded that a drink at 4 degrees C during exercise in the heat enhances fluid consumption and improves endurance by acting as a heat sink, attenuating the rise in body temperature and therefore reducing the effects of heat stress.     

Differences in cardiorespiratory responses during and after arm crank and cycle exercise

The differences in cardiorespiratory responses were examined during and after intermittent progressive maximal arm-crank and cycle exercise. Arm-crank exercise was performed in a standing position using no torso restraints to maximize the amount of active skeletal muscle mass. Recovery was followed for 16 min. In the tests a variety of ventilatory gas exchange variables, heart rate, the blood pressure, and the arm venous blood lactate concentration were measured in 21 untrained healthy men aged 24-45 years. At equal submaximal external workloads for arm cranking and cycling (50 and 100 W) the respiratory frequency, tidal volume, pulmonary ventilation, oxygen uptake, carbon dioxide output, the respiratory exchange ratio, heart rate, the arm venous blood lactate concentration, and the ventilatory equivalent for oxygen were higher (P less than 0.001) during arm cranking than cycling. The maximal workload for arm cranking was 44% lower than that for cycling (155 +/- 37 vs 277 +/- 39 W, P less than 0.001) associated with significantly (P less than 0.001) lower maximal tidal volume (-20%), oxygen uptake (-22%), carbon dioxide output (-28%), systolic blood pressure (-17%) and oxygen pulse (-22%) but a higher ventilatory equivalent for carbon dioxide (+22%) and arm venous blood lactate concentration (+37%). However, these responses after arm-crank and cycle exercises behaved almost similarly during recovery. The high cardiorespiratory stress induced by arm work should be taken into account when the work stress and work-rest regimens in actual manual tasks are assessed, and when arm work is used for clinical testing, and in physiotherapy particularly for patients with heart or pulmonary diseases.     

Validity of devices that assess body temperature during outdoor exercise in the heat

CONTEXT: Rectal temperature is recommended by the National Athletic Trainers' Association as the criterion standard for recognizing exertional heat stroke, but other body sites commonly are used to measure temperature. Few authors have assessed the validity of the thermometers that measure body temperature at these sites in athletic settings. OBJECTIVE: To assess the validity of commonly used temperature devices at various body sites during outdoor exercise in the heat. DESIGN: Observational field study. SETTING: Outdoor athletic facilities. PATIENTS OR OTHER PARTICIPANTS: Fifteen men and 10 women (age = 26.5 +/- 5.3 years, height = 174.3 +/- 11.1 cm, mass = 72.73 +/- 15.95 kg, body fat = 16.2 +/- 5.5%). INTERVENTION(S): We simultaneously tested inexpensive and expensive devices orally and in the axillary region, along with measures of aural, gastrointestinal, forehead, temporal, and rectal temperatures. Temporal temperature was measured according to the instruction manual and a modified method observed in medical tents at local road races. We also measured forehead temperatures directly on the athletic field (other measures occurred in a covered pavilion) where solar radiation was greater. Rectal temperature was the criterion standard used to assess the validity of all other devices. Subjects' temperatures were measured before exercise, every 60 minutes during 180 minutes of exercise, and every 20 minutes for 60 minutes of postexercise recovery. Temperature devices were considered invalid if the mean bias (average difference between rectal temperature and device temperature) was greater than +/-0.27 degrees C (+/-0.5 degrees F). MAIN OUTCOME MEASURE(S): Temperature from each device at each site and time point. RESULTS: Mean bias for the following temperatures was greater than the allowed limit of +/-0.27 degrees C (+/-0.5 degrees F): temperature obtained via expensive oral device (-1.20 degrees C [-2.17 degrees F]), inexpensive oral device (-1.67 degrees C [-3.00 degrees F]), expensive axillary device (-2.58 degrees C [-4.65 degrees F]), inexpensive axillary device (-2.07 degrees C [-3.73 degrees F]), aural method (-1.00 degrees C [-1.80 degrees F]), temporal method according to instruction manual (-1.46 degrees C [-2.64 degrees F]), modified temporal method (-1.36 degrees C [-2.44 degrees F]), and forehead temperature on the athletic field (0.60 degrees C [1.08 degrees F]). Mean bias for gastrointestinal temperature (-0.19 degrees C [-0.34 degrees F]) and forehead temperature in the pavillion (-0.14 degrees C [-0.25 degrees F]) was less than the allowed limit of +/-0.27 degrees C (+/-0.5 degrees F). Forehead temperature depended on the setting in which it was measured and showed greater variation than other temperatures. CONCLUSIONS: Compared with rectal temperature (the criterion standard), gastrointestinal temperature was the only measurement that accurately assessed core body temperature. Oral, axillary, aural, temporal, and field forehead temperatures were significantly different from rectal temperature and, therefore, are considered invalid for assessing hyperthermia in individuals exercising outdoors in the heat.     

The effect of sweetness on the efficacy of carbohydrate supplementation during exercise in the heat.

The aim of the present study was to investigate potential mechanisms responsible for the improvement in prolonged exercise capacity in hot environments with exogenous carbohydrate. Eight endurance-trained men (VO(2)max 60.5 +/- 2.4 ml.kg(-1).min(-1), mean +/- SE) cycled to exhaustion on three occasions at 60% VO(2)max at an ambient temperature of 35 degrees C. They ingested either a sweet 6.4% carbohydrate solution (SC), a nonsweet 6.4% carbohydrate solution (NSC), or water (W). Exercise capacity was significantly increased with SC and NSC compared to W, the improvements corresponding to 15.8% and 11.8%, respectively. No difference in exercise capacity was seen between SC and NSC solutions. Plasma glucose concentrations were higher during the SC and NSC trials compared to W, significantly so at 10 min and at fatigue. Rates of carbohydrate oxidation were higher in the SC and NSC trials, although the rates never declined below 2.1 +/- 0.2 g.min(-1) in the W trial. There was no difference in the rate of rise of rectal temperature between trials, but there was a trend for subjects to fatigue at higher temperatures during the two carbohydrate trials. In conclusion, exogenous carbohydrate, independent of sweetness, improves exercise capacity in the heat compared to water alone.