Francis D. Reardon

Calorimetric measurement of postexercise net heat loss and residual body heat storage.

PURPOSE Previous studies have shown a rapid reduction in postexercise local sweating and blood flow despite elevated core temperatures. However, local heat loss responses do not illustrate how much whole-body heat dissipation is reduced, and core temperature measurements do not accurately represent the magnitude of residual body heat storage. Whole-body evaporative (H(E)) and dry (H(D)) heat loss as well as changes in body heat content (DeltaH(b)) were measured using simultaneous direct whole-body and indirect calorimetry. METHODS Eight participants cycled for 60 min at an external work rate of 70 W followed by 60 min of recovery in a calorimeter at 30 degrees C and 30% relative humidity. Core temperature was measured in the esophagus (T(es)), rectum (T(re)), and aural canal (T(au)). Regional muscle temperature was measured in the vastus lateralis (T(vl)), triceps brachii (T(tb)), and upper trapezius (T(ut)). RESULTS After 60 min of exercise, average DeltaH(b) was +273 +/- 57 kJ, paralleled by increases in T(es), T(re), and T(au) of 0.84 +/- 0.49, 0.67 +/- 0.36, and 0.83 +/- 0.53 degrees C, respectively, and increases in T(vl), T(tb), and T(ut) of 2.43 +/- 0.60, 2.20 +/- 0.64, and 0.80 +/- 0.20 degrees C, respectively. After a 10-min recovery, metabolic heat production returned to pre-exercise levels, and H(E) was only 22.9 +/- 6.9% of the end-exercise value despite elevations in all core temperatures. After a 60-min recovery, DeltaH(b) was +129 +/- 58 kJ paralleled by elevations of T(es) = 0.19 +/- 0.13 degrees C, T(re) = 0.20 +/- 0.03 degrees C, T(au) = 0.18 +/- 0.04 degrees C, Tvl = 1.00 +/- 0.43 degrees C, T(tb) = 0.92 +/- 0.46 degrees C, and T(ut) = 0.31 +/- 0.27 degrees C. Despite this, H(E) returned to preexercise levels. Only minimal changes in H(D) occurred throughout. CONCLUSION We confirm a rapid reduction in postexercise whole-body heat dissipation by evaporation despite elevated core temperatures. Consequently, only 53% of the heat stored during 60 min of exercise was dissipated after 60 min of recovery, with the majority of residual heat stored in muscle tissue.

A Field Evaluation of the Physiological Demands of Miners in Canada's Deep Mechanized Mines

This study was conducted to evaluate the physical/mechanical characteristics of typical selected mining tasks and the energy expenditure required for their performance. The study comprised two phases designed to monitor and record the typical activities that miners perform and to measure the metabolic energy expenditure and thermal responses during the performance of these activities under a non-heat stress environmental condition (ambient air temperature of 25.8°C and 61% relative humidity with a wet bulb globe temperature (WBGT) of 22.0°C). Six common mining jobs were evaluated in 36 miners: (1) production drilling (jumbo drill) (n = 3), (2) production ore transportation (load-haul dump vehicle) (n = 4), (3) manual bolting (n = 9), (4) manual shotcrete (wet/dry) (n = 3), (5) general services (n = 8) and, (6) conventional mining (long-hole drill) (n = 9). The time/motion analysis involved the on-site monitoring, video recording, and mechanical characterization of the different jobs. During the second trial, continuous measurement of oxygen consumption was performed with a portable metabolic system. Core (ingestible capsule) and skin temperatures (dermal patches) were recorded continuously using a wireless integrated physiological monitoring system. We found that general services and manual bolting demonstrated the highest mean energy expenditure (331 ± 98 and 290 ± 95 W, respectively) as well as the highest peak work rates (513 and 529 W, respectively). In contrast, the lowest mean rate of energy expenditure was measured in conventional mining (221 ± 44 W) and manual shotcrete (187 ± 77 W) with a corresponding peak rate of 295 and 276 W, respectively. The low rate of energy expenditure recorded for manual shotcrete was paralleled by the lowest work to rest ratio (1.8:1). While we found that production drilling had a moderate rate of energy expenditure (271 ± 11 W), it was associated with the highest work to rest ratio (6.7:1) Despite the large inter-variability in energy expenditure and work intervals among jobs, only small differences in average core temperature (average ranged between 37.20 ± 0.22 to 37.42 ± 0.18°C) were measured. We found a high level of variability in the duration and intensity of tasks performed within each mining job. This was paralleled by a large variation in the work to rest allocation and mean energy expenditure over the course of the work shift.

Heat Balance and Cumulative Heat Storage during Intermittent Bouts of Exercise

PURPOSE The aim of this study was to investigate heat balance during thermal transients caused by successive exercise bouts. Whole-body heat loss (H x L) and changes in body heat content (Delta Hb) were measured using simultaneous direct whole-body and indirect calorimetry. METHODS Ten participants performed three successive bouts of 30-min cycling (Ex1, Ex2, and Ex3) at a constant rate of heat production of approximately 500 W, each separated by 15-min rest (R1, R2, and R3) at 30 degrees C. RESULTS Despite identical rates of heat production during exercise, the time constant (tau) of the exponential increase in H x L was greater in Ex1 (tau = 12.3 +/- 2.3 min) relative to both Ex2 (tau = 7.2 +/- 1.6 min) and Ex3 (tau = 7.1 +/- 1.6 min) (P < 0.05). Delta Hb during Ex1 (256 +/- 76 kJ) was greater than during Ex2 (135 +/- 60 kJ) and Ex3 (124 +/- 78 kJ) (P < 0.05). During recovery bouts, heat production was the same, and the tau of the exponential decrease in H L was the same during R1 (tau = 6.5 +/- 1.1 min), R2 (tau = 5.9 +/- 1.3 min), and R3 (tau = 6.0 +/- 1.2 min). Delta Hb during R1 (-82 +/- 48 kJ), R2 (-91 +/- 48 kJ), and R3 (-88 +/- 54 kJ) were the same. The cumulative Delta Hb was consequently greater at the end of Ex2 and Ex3 relative to the end of Ex1 (P < 0.05). Likewise, cumulative Delta Hb was greater at the end of R2 and R3 relative to R1 (P < 0.05). CONCLUSION The proportional decrease in the amount of heat stored in the successive exercise bouts is the result of an enhanced rate of heat dissipation during exercise and not due to a higher rate of heat loss in the recovery period. Despite a greater thermal drive with repeated exercise, the decline in the rate of total heat loss during successive recovery bouts was the same.

Hyperthermia modifies the nonthermal contribution to postexercise heat loss responses.

PURPOSE This study investigated the nonthermoregulatory control of cutaneous vascular conductance (CVC) and sweating during recovery from exercise-induced hyperthermia as well as possible sex-related differences in these responses. Two hypotheses were tested in this study: 1) active and passive recovery would be more effective in attenuating the fall in mean arterial pressure (MAP) than inactive recovery, but CVC and sweat rate responses would be similar between all recovery modes; and 2) the magnitude of the change in postexercise heat loss and hemodynamic responses between recovery modes would be similar between sexes. METHODS Nine males and nine females were rendered hyperthermic (esophageal temperature = 39.5 degrees C) by exercise, followed by 60 min of 1) active, 2) inactive, and 3) passive recovery. CVC, sweat rate, and MAP were recorded at baseline, after 2, 5, 12, and 20 min, and at every 10 min until the end of recovery. RESULTS MAP was elevated above inactive recovery by 6 +/- 2 and 4 +/- 1 mm Hg for active and passive recovery, respectively (P < 0.001). No differences were observed between modes during the initial 10 min of recovery for CVC and 50 min of recovery for sweat rate. However, relative to inactive recovery CVC and sweat rate were subsequently greater by 16.2 +/- 5.8% of CVCpeak and 0.28 +/- 0.04 mg.min.cm, respectively, during active recovery, and by 11.6 +/- 2.9% of CVCpeak and 0.23 +/- 0.03 mg.min.cm, respectively, during passive recovery. CONCLUSION We conclude that in the presence of a greater thermal drive associated with hyperthermia, the influence of nonthermal input on postexercise heat loss responses is still observed. However, thermal control predominates over nonthermal factors in the first 10 min of recovery for CVC and for up to 50 min postexercise for sweating. Sex did not influence the effect of recovery mode on any variable.

The effect of ambient temperature and exercise intensity on post-exercise thermal homeostasis

Abstract We have previously demonstrated a prolonged (65 min or longer) elevated plateau of esophageal temperature (Tes) (0.5–0.6°C above pre-exercise values) in humans following heavy dynamic exercise (70% maximal oxygen consumption, V˙O2max) at a thermoneutral temperature (Ta) of 29°C. The elevated Tes value was equal to the threshold Tes at which active skin vasodilation was initiated during exercise (Thdil). A subsequent observation, i.e., that successive exercise/recovery cycles (performed at progressively increasing pre-exercise Tes levels) produced parallel increases of Thdil and the post-exercise Tes, further supports a physiological relationship between these two variables. However, since all of these tests have been conducted at the same Ta (29°C) and exercise intensity (70% V˙O2max) it is possible that the relationship is limited to a narrow range of Ta/exercise intensity conditions. Therefore, five male subjects completed 18 min of treadmill exercise followed by 20 min of recovery in the following Ta/exercise intensity conditions: (1) cool with light exercise, Ta = 20°C, 45% V˙O2max (CL); (2) temperature with heavy exercise, Ta = 24°C, 75% O2 max (TH); (3) warm with heavy exercise, Ta = 29°C, 75% V˙O2max (WH); and (4) hot with light exercise, Ta = 40°C, 45% V˙O2max (HL). An abrupt decrease in the forearm-to-finger temperature gradient (Tfa −Tfi) was used to identify the Thdil during exercise. Mean pre-exercise Tes values were 36.80, 36.60, 36.72, and 37.20°C for CL, TH, WH, and HL conditions respectively. Tes increased during exercise, and end post-exercise fell to stable values of 37.13, 37.19, 37.29, and 37.55°C for CL, TH, WH, and HL trials respectively. Each plateau value was significantly higher than pre-exercise values (P < 0.05). Correspondingly, Thdil values (i.e., 37.20, 37.23, 37.37, and 37.48°C for CL, TH, WH, and HL) were comparable to the post-exercise Tes values for each condition. The relationship between Thdil and post-exercise Tes remained intact in all Ta/exercise intensity conditions, providing further evidence that the relationship between these two variables is physiological and not coincidental.

Human heat balance during postexercise recovery: separating metabolic and nonthermal effects

Previous studies report greater postexercise heat loss responses during active recovery relative to inactive recovery despite similar core temperatures between conditions. Differences have been ascribed to nonthermal factors influencing heat loss response control since elevations in metabolism during active recovery are assumed to be insufficient to change core temperature and modify heat loss responses. However, from a heat balance perspective, different rates of total heat loss with corresponding rates of metabolism are possible at any core temperature. Seven male volunteers cycled at 75% of Vo(2peak) in the Snellen whole body air calorimeter regulated at 25.0 degrees C, 30% relative humidity (RH), for 15 min followed by 30 min of active (AR) or inactive (IR) recovery. Relative to IR, a greater rate of metabolic heat production (M - W) during AR was paralleled by a greater rate of total heat loss (H(L)) and a greater local sweat rate, despite similar esophageal temperatures between conditions. At end-recovery, rate of body heat storage, that is, [(M - W) - H(L)] approached zero similarly in both conditions, with M - W and H(L) elevated during AR by 91 +/- 26 W and 93 +/- 25 W, respectively. Despite a higher M - W during AR, change in body heat content from calorimetry was similar between conditions due to a slower relative decrease in H(L) during AR, suggesting an influence of nonthermal factors. In conclusion, different levels of heat loss are possible at similar core temperatures during recovery modes of different metabolic rates. Evidence for nonthermal influences upon heat loss responses must therefore be sought after accounting for differences in heat production.

Menstrual cycle and oral contraceptive use do not modify postexercise heat loss responses

It is unknown whether menstrual cycle or oral contraceptive (OC) use influences nonthermal control of postexercise heat loss responses. We evaluated the effect of menstrual cycle and OC use on the activation of heat loss responses during a passive heating protocol performed pre- and postexercise. Women without OC (n = 8) underwent pre- and postexercise passive heating during the early follicular phase (FP) and midluteal phase (LP). Women with OC (n = 8) underwent testing during the active pill consumption (high exogenous hormone phase, HH) and placebo (low exogenous hormone phase, LH) weeks. After a 60-min habituation at 26 degrees C, subjects donned a liquid conditioned suit. Mean skin temperature was clamped at approximately 32.5 degrees C for approximately 15 min and then gradually increased, and the absolute esophageal temperature at which the onset of forearm vasodilation (Th(vd)) and upper back sweating (Th(sw)) were noted. Subjects then cycled for 30 min at 75% Vo(2 peak) followed by a 15-min seated recovery. A second passive heating was then performed to establish postexercise values for Th(vd) and Th(sw). Between 2 and 15 min postexercise, mean arterial pressure (MAP) remained significantly below baseline (P < 0.05) by 10 +/- 1 and 11 +/- 1 mmHg for the FP/LH and LP/HH, respectively. MAP was not different between cycle phases. During LP/HH, Th(vd) was 0.16 +/- 0.24 degrees C greater than FP/LH preexercise (P = 0.020) and 0.15 +/- 0.23 degrees C greater than FP/LH postexercise (P = 0.017). During LP/HH, Th(sw) was 0.17 +/- 0.23 degrees C greater than FP/LH preexercise (P = 0.016) and 0.18 +/- 0.16 degrees C greater than FP/LH postexercise (P = 0.001). Postexercise thresholds were significantly greater (P < or = 0.001) than preexercise during both FP/LH (Th(vd), 0.22 +/- 0.03 degrees C; Th(sw), 0.13 +/- 0.03 degrees C) and LP/HH (Th(vd), 0.21 +/- 0.03 degrees C; Th(sw), 0.14 +/- 0.03 degrees C); however, the effect of exercise was similar between LP/HH and FP/LH. No effect of OC use was observed. We conclude that neither menstrual cycle nor OC use modifies the magnitude of the postexercise elevation in Th(vd) and Th(sw).

Can supine recovery mitigate the exercise intensity dependent attenuation of post-exercise heat loss responses?

Cutaneous vascular conductance (CVC) and sweat rate are subject to non-thermal baroreflex-mediated attenuation post-exercise. Various recovery modalities have been effective in attenuating these decreases in CVC and sweat rate post-exercise. However, the interaction of recovery posture and preceding exercise intensity on post-exercise thermoregulation remains unresolved. We evaluated the combined effect of supine recovery and exercise intensity on post-exercise cardiovascular and thermal responses relative to an upright seated posture. Seven females performed 15 min of cycling ergometry at low- (LIE, 55% maximal oxygen consumption) or high-(HIE, 85% maximal oxygen consumption) intensity followed by 60 min of recovery in either an upright seated or supine posture. Esophageal temperature, CVC, sweat rate, cardiac output, stroke volume, heart rate, total peripheral resistance, and mean arterial pressure (MAP) were measured at baseline, at end-exercise, and at 2, 5, 12, 20, and every 10 min thereafter until the end of recovery. MAP and stroke volume were maintained during supine recovery to a greater extent relative to an upright seated recovery following HIE (p <or= 0.05) and were paralleled by an elevated CVC and sweat rate response (p <or= 0.05). A significantly lower esophageal temperature was subsequently observed when supine throughout recovery (p <or= 0.05). Although we observed a reflex bradycardia and increased stoke volume with supine recovery following LIE, no differences were observed for MAP, CVC, sweat rate or esophageal temperature. Supine recovery attenuates the post-exercise reductions in MAP, CVC, and sweat rate in a manner dependent directly on exercise intensity. This effect is likely attributable to a non-thermal baroreceptor mechanism.

Estimating changes in volume-weighted mean body temperature using thermometry with an individualized correction factor

This study investigated whether the estimation error of volume-weighted mean body temperature (DeltaT(b)) using changes in core and skin temperature can be accounted for using personal and environmental parameters. Whole body calorimetry was used to directly measure DeltaT(b) in an Experimental group (EG) of 36 participants (24 males, 12 females) and a Validation group (VG) of 20 (9 males, 11 females) throughout 90 min of cycle ergometry at 40 degrees C, 30% relative humidity (RH) (n = 9 EG, 5 VG); 30 degrees C, 30% RH (n = 9 EG, 5 VG); 30 degrees C, 60% RH (n = 9 EG, 5 VG); and 24 degrees C, 30% RH (n = 9 EG, 5 VG). The core of the two-compartment thermometry model was represented by rectal temperature and the shell by a 12-point mean skin temperature (DeltaT(sk)). The estimation error (X(0)) between DeltaT(b) from calorimetry and DeltaT(b) from thermometry using core/shell weightings of 0.66/0.34, 0.79/0.21, and 0.90/0.10 was calculated after 30, 60, and 90 min of exercise, respectively. The association between X(0) and the individual variation in metabolic heat production (M - W), body surface area (BSA), body fat percentage (%fat), and body surface area-to-mass ratio (BSA/BM) as well as differences in environmental conditions (Oxford index) in the EG data were assessed using stepwise linear regression. At all time points and with all core/shell weightings tested, M - W, BSA, and Oxford index independently correlated significantly with the residual variance in X(0), but %fat and BSA/BM did not. The subsequent regression models were used to predict the thermometric estimation error (X(0_pred)) for each individual in the VG. The value estimated for X(0_pred) was then added to the DeltaT(b) estimated using the two-compartment thermometry models yielding an adjusted estimation (DeltaT(b)_(adj)) for the individuals in the VG. When comparing DeltaT(b)_(adj) to the DeltaT(b) derived from calorimetry in the VG, the best performing model used a core/shell weighting of 0.66/0.34 describing 74%, 84%, and 82% of the variation observed in DeltaT(b) from calorimetry after 30, 60, and 90 min, respectively.