Self-paced exercise performance in the heat with neck cooling,menthol application,and abdominal cooling

Objectives

To investigate whether the exercise performance benefits with neck cooling in the heat are attributable to neck-specific cooling, general body cooling, a cooler site-specific thermal perception or a combination of the above.

Isolated perfused rat kidney and liver combined

In the late phase of the fever occurring 120 or more min after i.v. injection of endotoxin (1 ug/kg) to female rabbits, marked shifts of thresholds for respiratory evaporative heat loss and for peripheral vasodilatation to higher body core temperatures were observed. In contrast, the threshold body core temperature for cold thermogenesis was shifted downwards. As a result, the interthreshold zone was widened. Within the body temperature range of 37.4 to 39.9°C neither heat production or heat loss mechanisms were operant and the body temperature was determined mainly by passive heat transfer between the body and the environment. Outside this zone, the sensitivities of the heat and cold deference activities to changes in body core temperature appeared to be unchanged.     

Short-term recovery from prolonged constant pace running in a warm environment: the effectiveness of a carbohydrate-electrolyte solution

Recovery from prolonged exercise involves both rehydration and replenishment of endogenous carbohydrate stores. This study examined the influence of drinking a carbohydrate-electrolyte solution on short-term recovery and subsequent exercise capacity in a warm environment. Thirteen healthy male volunteers completed two trials, at least 7 days apart. On each occasion subjects performed an initial treadmill run at 60% of maximal oxygen uptake (VO_2max), for 90 min or until volitional fatigue (T1), in a warm environment (35 °C, 40% relative humidity, RH). Volitional ingestion of water was permitted during each of the exercise trials. During a subsequent 4-h recovery period (REC) subjects consumed either a 6.9% carbohydrate-electrolyte solution (CES) or a sweetened placebo (P), in a volume equivalent to 140% of body mass loss. Following REC, subjects ran to exhaustion at the same %VO_2max in order to assess their endurance capacity (T2). Mean (SEM) run times during T1 did not differ between the CES [74.8 (4.6) min] and P [72.5 (5.2) min] trials. Body mass was reduced (P < 0.01) by 1.9 (0.2)% (CES) and 1.7 (0.2)% (P), and plasma volume (P < 0.01) by 6.0 (0.9)% (CES) and 5.4 (1.0)% (P) during the T1 trials. During REC 2006 (176) ml and 1830 (165) ml of fluid was ingested, providing 138 (12) g and 0 g of carbohydrate in the CES and P trials, respectively. Prior to T2, plasma volume and net fluid balance were similarly restored [CES +58 (26) g; P −4 (68) g] in both trials. During T2 the exercise duration was longer (P < 0.01) in the CES compared to the P trial [CES 60.9 (5.5) min; P 44.9 (3.0) min]. Thus, provided that an adequate hydration status is maintained, inclusion of carbohydrate within an oral rehydration solution will delay the onset of fatigue during a subsequent bout of prolonged submaximal running in a warm environment. Accepted: 25 February 2000     

Exercise thermoregulation after 6 h of chair rest, 6° head-down bed-rest,and water immersion deconditioning in men

The purpose was to investigate the mechanism for the excessive exercise hyperthermia following deconditioning (reduction of physical fitness). Rectal (T _re) and mean skin ( ) temperatures and thermoregulatory responses were measured in six men [mean (SD) age, 32 (6) years; mass, 78.26 (5.80) kg; surface area, 1.95 (0.11)m²; maximum oxygen uptake ( ), 48 (6) ml·min^–1·kg^–1; whilst supine in air at dry bulb temperature 23.2 (0.6)°C, relative humidity 31.1 (11.1)% and air speed 5.6 (0.1) m·min^–1] during 70 min of leg cycle exercise [51 (4)% ] in ambulatory control (AC), or following 6 h of chair rest (CR), 6° head-down bed rest (BR), and 20° (WI20) and 80° (WI80) foot-down water immersion [water temperature, 35.0 (0.1)°C]. Compared with the AC exercise T _re [mean (SD) 0.77 (0.13)°C], T _re after CR was 0.83 (0.08)°C (NS), after BR 0.92 (0.13)°C (^*P<0.05), after WI80 0.96 (0.13)°C^*, and after WI20 1.03 (0.09)°C^*. All responded similarly to exercise: they decreased (NS) by 0.5–0.7°C in minutes 4–8 and equilibrated at +0.1 to +0.5°C at 60–70. Skin heat conductance was not different among the five conditions (range = 147–159 kJ·m^–2·h^–1·°C^–1). Results from an intercorrelation matrix suggested that total body sweat rate was more closely related toT _re at 70 min (T _re70) than limb sweat rate or blood flow. Only 36% of the variability inT _re70 could be accounted for by total sweating, and less than 10% from total body dehydration. It would appear that multiple factors are involved which may include change in sensitivity of thermo- and osmoreceptors.     

Influence of angiotensin II blockade during exercise in the heat

The effect of the blockade of the renin angiotensin system (RAS) on thermorgulatory, cardiovascular and renal function during moderate exercise in a hot [mean (SEM) 34.4 (0.1)°C] environment was evaluated. Six men and three women cycled at 60% peak oxygen uptake for 45 min following acute administration of a placebo (PLAC) or enalapril (ENAL), an angiotensin converting enzyme inhibitor (ACE-I). Resting mean arterial pressure (MAP) was reduced by ENAL, but the pressor response to exercise was unaffected [MAP = 7.8 (1.4) mmHg for both trials (P > 0.05)]. Peak esophageal temperature [T _es = 38.7 (1.0)°C (PLAC) vs 38.4 (0.2)°C (ENAL)] and mean skin temperatures [ _sk = 36.5 (0.1)°C (PLAC) vs 36.6 (0.1)°C (ENAL)] were similar for both drug treatments during the exercise. Both aldosterone and plasma renin activity (PRA) increased five fold above resting values during exercise; however, only the PRA response [16.7 (3.2) ng angiotensin I (Ang I) · ml^–1 · h^–1 (ENAL) vs 7.4 (1.2) ng Ang I · ml^–1 · h^–1 (PLAC)] was significantly altered by ENAL treatment (P < 0.05). Urine flow, sodium excretion and glomerular filtration rates, determined from creatinine clearance, were similarly reduced following exercise for both ENAL and PLAC treatments. These results suggest acute administration (5 mg) of ACE-I does not impair thermoregulatory, cardiovascular or renal responses during moderate exercise in the heat.     

A model of human thermoregulation during water immersion

A mathematical model for thermoregulation in humans immersed in time-varying water baths is described. The model is readily adaptable to other exposure conditions and control assumptions. Blood flow appears explicitly as a major factor in transferring heat from deep layers to the surface, and its distribution and magnitude at the surface is controlled in the model by the deviations of head core and local skin temperatures, as well as the rate of change of local skin temperature. The model results are compared with data from two water bath immersion experiments, one in which the temperature of the bath was periodically varied between 34 and 39°C, and in the other as a ramp-step function between 28 and 32°C. The resulting agreement demonstrates the feasibility of the control scheme, and also indicates ways in which distributed models of thermoregulation can be improved. The difficult inverse problem of solving the model equations for blood flow rates for a given temperature distribution is discussed, and some sample solutions are given. Extension to other exposure conditions is discussed.     

Diurnal cycle of mother-young contact in Norway rats

Mother rats maintained on a LD 12:12 photoperiod (lights on 0800 hrs) had longer contact bouts with their offspring during the day than during the night and maternal brain temperature peaked during the night. When the daily temperature cycle was suppressed by removal of adrenal and ovarian hormones, the daily maternal contact cycle was also suppressed. These data are consistent with a thermal model for the limitation of mother-young contact bout duration.     

Prediction of the thermoregulatory response for clothed immersion in cold water

Summary A multi-compartmental thermoregulatory model was applied to data of ten resting clothed males immersed for 3 h in water at 10 and 15°C. Clothing consisted of a dry suit and either a light or heavy undergarment, representing a total insulation of 0.15 (0.95) or 0.20 m²°CW⁻¹ (1.28 clo), respectively. Data were grouped according to low (<14%) and high (14 to 24%) body fat individuals. Mean decreases in rectal temperature ranged from 0.79 to 1.38°C, mean decreases in the mean weighted skin temperature ranged from 6.3 to 10.2°C, and mean increases in the metabolic rate ranged from 33.9 to 80.8 W. The model consists of eight segments, each representing a specific region of the body. Each segment is comprised of compartments representing the core, muscle, fat, skin, and clothing. Each compartment is assigned thermophysical values of heat conduction and heat capacitance, and with the exception of clothing, physiological values of blood flow and metabolic heat production. During cold exposure, responses are directed towards increased heat production in the form of shivering and heat conservation in the form of vasoconstriction and convective heat exchange at the vascular level. Agreement between the model predictions and the experimental observations was obtained by adjusting the parameters governing these responses. These adjusted parameters were 1) the onset of limb shivering with an exponential half-time of 30 min, 2) the fractional value of 0.5 for the convective heat exchange between the core compartments of the limbs and the blood flowing through these compartments, 3) the fractional contribution of trunk shivering to overall shivering, which ranged from 0.77 to 0.95, and 4) the onset of vasoconstriction with exponential half-times that ranged from 3 to 25 min. Steady state was predicted to occur within 4 h and a heat balance analysis indicated that the limbs were responsible for most of the body's heat loss while acquiring most of their own heat from the trunk through convective heat exchange with the central blood.     

Calorimetric analysis of the effect of drinking saline solution on whole-body sweating

Summary In a previous paper a technique has been described for analysing the sweating response to drinking calorimetrically. The response showed a thermal component which was a linear function with unit slope of the heat content of the ingested liquid.In this paper the analysis is extended to ingesting various volumes of liquid of varying salinity and temperatures. In the range 0–9 g NaCl per liter, the salinity of the ingested solution had no, effect on sweating. Drinking hypertonic saline markedly depressed sweating.Drinking itself provoked a sweating response. This component of the overall response was a linear function of the ingested volume irrespective of temperature and salinity. The proportionality factor was found to be 511 W. min/l (11.8 g/l). Calculations indicated that below a volume of 0.21 l no response would be elicited in the range of temperatures studied.Once the influence of salinity and volume had been established, average body temperature could be calculated calorimetrically from the observations. The temperature of the ingested liquid for which the thermal component of the sweating response is zero must be the average body temperature. For the subject studied, resting, and sweating at a steady rate of about 2 g/min, average body temperature was 36.7° C.This study was carried out during his tenure of a research fellowship from the Anglo-American Corporation of South Africa.