Victor Candas

Hydration during exercise: Effects on thermal and cardiovascular adjustments

SummaryFive young unacclimatised subjects were exposed for 4 h at 34‡ C (10‡ C dew-point temperature and 0.6 m · s−1 air velocity), while exercising on a bicycle ergometer: 25 min work — 5 min rest cycles for 2 hours followed by 20 min work — 10 min rest cycles for two further hours. 5 experimental sessions were carried out: one without rehydration (NO FLUID) resulting in 3.1% mean loss of body weight (δ Mb), and four sessions with 20‡ C fluid ingestion of spring water (WATER), hypotonic (HYPO), isotonic (ISO) and hypertonic (HYPER) solutions to study the effects of fluid osmolarity on rehydration. Mean final rehydration (±SE) after fluid intake was 82.2% (±1.2). Heart rate was higher in NO FLUID while no difference among conditions was found in either δ Mb or hourly sweat rates. Sweating sensitivity was lowest in the dehydration condition, and highest in the WATER one. Modifications in plasma volume and osmolarity demonstrated that NO FLUID induced hyperosmotic hypovolemia, ISO rehydration rapidly led to plasma isoosmotic hypervolemia, while WATER led to slightly hypoosmotic normovolemia.It is concluded that adequate rehydration through ingestion of isotonic electrolyte-sucrose solution, although in quantities much smaller than evaporative heat loss, rapidly restored and expanded plasma volume. While osmolarity influenced sweating sensitivity, the plasma volume changes (δ PV) within the range −6%⩽δ PV⩽+4% had little effect on temperature adjustments in our conditions.

Ambient temperature and human sleep

34 Walker, J.M., Floyd, T.C., Fein, G., Cavness, C.0 Lualhati, R., and Feinberg, I., Effects of exercise on sleep. J. appl. Physiol., Respirat. envir. Exercise Physiol. 44 (1978) 945-951. 35 Webb, W.B., and Agnew, H.W., Effects on performance of high and low energy expenditure during sleep deprivation. Percept. Motor Skills 37 (1973) 5li 514. 36 Williams, R. L., Karacan, I., and Hursch, C., EEG of human sleep. Clinical applications, 169 p. John Wiley and Son, New York 1974. 37 Zepelin, H., and Rechtschaffen, A., Mammalian sleep, longevity and energy conservation. Brain Behav. Evol. 10 (1974) 425-470. 38 Zimmerman, J., Stoyva, J., and Metcalf, D., Distorted visual feed425

Vascular fluid shifts and endocrine responses to exercise in the heat. Effect of rehydration.

SummaryThis study examines the relationships between vascular changes and endocrine responses to prolonged exercise in the heat, associated with dehydration and rehydration by fluids of different osmolarity. Five subjects were exposed, in a 34‡ C environment for 4 h of intermittent exercise on a cycle ergometer at 85±12 Watts (SD). Fluid regulatory hormones and cortisol were analysed in 3 experimental sessions: one without any fluid supplement (NO FLUID), and two with progressive rehydration, either by spring water (WATER) or isotonic solution (ISO), given after 70 min of exercise. Results were expressed in terms of differences between the mean values observed at the end of the exercise and the first hour values taken as references.Dehydration (NO FLUID) elicited a 4.0±0.8% (SE) decrease in plasma volume (PV) and an increase in osmolarity (8.4±3.1 mosmol · l−1). Concomitantly, plasma aldosterone (PA), renin activity (PRA), arginin vasopressin (AVP) and cortisol (PC) levels increased greatly in response to exercise in the heat (PA: 37.2+-10.8 ng. 100 ml−1; PRA: 13.4±2.5 ng · ml−1 · h−1; AVP: 3.8±1.3 pg · ml−1; PC: 12.2±2.7 Μ g · 100 ml−1). Rehydration with water led to decreased osmolarity (−8.2±2.1 mosmol · l−1) with no significant changes in PV. With ISO, PV increased by 6.0±1.3% and the decrease in osmolarity was −5.8±1.8 mosmol · l−1. With both modes of rehydration, the increases in PRA, AVP and cortisol were blunted; only ISO prevented the rise in PA.These data indicate that prolonged exercise in moderate heat is extremely effective in increasing cortisol and fluid-electrolyte regulatory hormones in dehydrated subjects. Progressive rehydration with water or isotonic solution, in the absence of osmotic and volemic stimuli, prevents the hormonal increases.

Pumping effects on thermal insulation of clothing worn by human subjects

This study was undertaken in order to analyse the importance of the pumping effect on clothings thermal insulation. To enhance differences in heat exchanges due to the pumping effect, two sets of condition were fixed, minimizing either the convective or the radiative heat transfers. The results showed that: (i) the clothing insulation determined on a manikin, even if he is moving, is larger than the resultant clothing insulation for living subjects; (ii) the insulation is not the same for radiant heat or cold as for convective heat or cold;(iii) the pumping effect can increase or decrease the resultant clothing insulation; (iv) the clothing insulation is smaller in warmer conditions thanin cooler ones; (v) it becomes necessary to make a definite distinction between several kinds of clothing insulation; intrinsic or basic insulation against radiation and convection; effective insulation against radiation and convection taking into account only the heat flowing through the clothing fabric; resultant insula...

Enhanced sensitivity to echo cues in blind subjects

Many studies have reported that blind people compensate for their visual deficit by sharpening auditory processes. Here we compare the sensitivity to echo cues between blind and sighted subjects. In the first experiment, the blind subjects were more accurate than the sighted subjects in localizing an object on the basis of echo cues. To ensure that enhanced echolocalization abilities were not only due to the fact that blind individuals are more used to consciously paying attention to echo cues and are more familiar with this kind of tasks than sighted subjects, we tested both groups of subjects in a simple azimuthal localization task of auditory stimuli. In this second experiment, we evaluated the influence of irrelevant echo signals on auditory localization by placing the subjects and the sound sources at different positions in a sound reverberant room. Results revealed that blind subjects exhibit a higher sensitivity to echo signals than sighted subjects.

Thermoregulatory effects of three different types of head cooling in humans during a mild hyperthermia

Abstract Seven healthy young men participated in six trials with three different types of local cooling [cool air breathing (CAB), face skin cooling (FaC), and combined cooling (CoC)] in a warm environment for 90 min while either resting (operative temperature: T0 = 40°C, dew point temperature: Tdp = 15°C, air velocity: va = 0.3 m·s−1) or exercising on a cycle ergometer with an external work load of 90 W (T0 = 36°C, Tdp = 15°C, va = 0.3 m·s−1). Cool air (10°C) arrived at the entry point of the hood and/or the mask at a ventilation rate of 12 m · s−1. Oesophageal temperature was not affected by any kind of cooling, while tympanic temperature was decreased at rest by both FaC and CoC [respectively −0.15 (0.06) and −0.09 (0.03)°C, P ≤ 0.05]. Mean skin temperature was decreased by FaC and CoC at rest [respectively −0.31 (0.07) and −0.27 (0.09)°C, P ≤ 0.05] and during exercise [respectively −0.64 (0.15) and −1.04 (0.22)°C, P ≤ 0.01]. CAB had no effect on skin temperatures. CoC and FaC reduced head skin temperature during both rest and work (P < 0.001) with no effect on the skin temperature of the rest of the body, except under CoC with exercise (P < 0.05). CAB did not influence local sweating. FaC, however, decreased the more profuse sweat rates (P ≤ 0.05) at rest, while CoC decreased all sweating rates at rest (P ≤ 0.05) and only the back, head and leg sweating rates during exercise (P ≤ 0.05). These results suggest that head skin cooling causes a reduction in heat strain, while CAB does not. This beneficial influence does not, however, appear to be the result of selective brain cooling. Tympanic temperature seems to be a good index of the core thermal inputs to the hypothalamic regulatory system, since variations in that parameter were associated with similarly directed variations in the sweating outputs.

Oral temperature as an index of core temperature during heat transients

SummaryRectal (Tre), oral (Tor) and oesophageal (Tes) temperatures were measured in five exercising subjects exposed for two hours to five conditions (1) a steady condition (WR) involving a constant work load (50 W) at a constant air temperature (Ta=36.5‡ C); (2) air temperature variations (δTa) between 28‡ C and 45‡ C and (3) between 23‡ C and 50‡ C at constant work load (50 W); (4) and (5) to work load variations (δW) between 25 W and 75 W at a constant Ta (=36.5‡ C). Oral temperature recordings were taken sublingually and were either continuous or discontinuous. When discontinuous, the time needed for Tor to stabilize after the mouth opening was taken into account. The respective reliability of Tor and Tre as estimates of Tes were compared in each condition. Results showed that the resting (Tor−Tes) difference (+0.12‡ C) was barely modified after two hours of exposure, whereas Tre overestimated Tes by 0.2‡ C to 0.4‡ C depending on the condition. The Tor variations were highly correlated with Tes variations under steady condition and under air temperature variations. In these conditions, Tor represented the best estimate of Tes. Under work-load variations, Tor was less closely related to Tes than was Tre. It is suggested that the relative inertia of Tor to step changes in exercise intensity could be ascribed to work induced variations in mouth blood flow.

Cortisol as a sensitive index of heat-intolerance.

The relationship between plasma cortisol levels, core temperatures, heat storage and the appearance of subjective manifestations of heat intolerance were investigated in two groups of 4 and 3 men, respectively. The first group underwent 4 randomized experimental sessions: a control session and three heat-exposure sessions (A: 48 degrees C, 34 Torr, 80 min; B: 55 degrees, 15 Torr, 120 min; C: 48 degrees C, 15 Torr, 180 min). During control periods, the subjects were maintained in a thermoneutral climate (28 degrees C, 10.5 Torr). The second group of subjects was studied before and after five successive daily exposures to hot and humid conditions (D: 43 degrees C, 32 Torr, 165 min). Signs of subjective discomfort in experiments A and B were accompanied by an increase in cortisol values over control day values, and this increase began at a mean rectal temperature of about 38 degrees C. Condition C was well tolerated, the plasma cortisol remained at basal levels and mean rectal temperature averaged 38 degrees C at the end of the exposure. Repeated exposures in condition D improved heat tolerance with a lesser effect on plasma cortisol levels and a lower body temperature at the end of exposure. Circulating cortisol is shown to be a very sensitive index of heat stress heralding the onset of poor tolerance of severe climates. The relation between concomitant levels of cortisol and rectal temperatures, in fit men, is affected by additional factors (hypotension, uncomfortable posture). These reduce tolerance time and are not reflected by body temperatures, but by rapidly increasing cortisol levels.

Heating and cooling stimulations during SWS and REM sleep in man

Abstract 1. 1.|The thermal response to heating and cooling transients was compared in man, during slow-wave sleep (SWS) and during rapid-eye-movement (REM) sleep. 2. 2.|Results showed that decreases in air temperature affected these sleep stages more frequently than did the air temperature increases and that stage changes that occurred during thermal stimulation were more frequent in REM sleep than in SWS. 3. 3.|Thermosensitivity was maintained during SWS as well as during REM sleep.

Time-on-task effect in pseudoneglect

Neurologically normal subjects systematically misbisect space during visual line-bisection or similar tasks, generally erring to the left of the veridical center when bisecting horizontal lines, a phenomenon referred to as pseudoneglect. This phenomenon is usually interpreted as enhanced attention toward the left hemispace resulting in an overestimation of the leftward extent of a line. While most studies have examined the role of attention in spatial bias using spatial cueing methods in bisection tasks, Manly et al. (Neuropsychologia 43(12):1721–1728, 2005) proposed an original paradigm in which the participants’ alertness was diminished by sleep deprivation or prolonged execution of a line-bisection task. The authors reported a significant rightward shift in attention related to declining alertness, but they did not control eye movements and, consequently, modifications of scanning and fixation strategies with fatigue cannot be ruled out in their study. Here we examine whether a diminution in alertness induced by a 60-min-long Landmark task would diminish (or even reverse) this attentional bias, when eye movements are absent. Participants performed a forced-choice judgment about the location of a transaction mark in relation to the veridical center of a horizontal line. The results confirmed a significant decrease in the leftward bias over the course of the session but, in contrast to the findings of Manly et al. (2005), we did not observe a reverse bias from the left to the right hemispace. The results are discussed within the context of the hemisphere-activation model.