W. L. Kenney

Relevance of individual characteristics for human heat stress response is dependent on exercise intensity and climate type

Abstract Multiple heterogeneous groups of subjects (both sexes and a wide range of maximal oxygen uptake V˙O2max, body mass, body surface area (AD),% body fat, and AD/mass coefficient) exercised on a cycle ergometer at a relative (%V˙O2max, REL) or an absolute (60 W) exercise intensity in a cool (CO 21°C, 50% relative humidity), warm humid (WH 35°C, 80%) and a hot dry (HD 45°C, 20%) environment. Rectal temperature (Tre) responses were analysed for the influence of the individuals characteristics, environment and exercise intensity. Exposures consisted of 30-min rest, followed by 60-min exercise. The Tre was negatively correlated with mass in all conditions. Body mass acted as a passive heat sink in all the conditions tested. While negatively correlated with V˙O2max and V˙O2max per kilogram body mass in most climates, Tre was positively correlated with V˙O2max and V˙O2max per kilogram body mass in the WH/REL condition. Thus, when evaporative heat loss was limited as in WH, the higher heat production of the fitter subjects in the REL trials determined Tre and not the greater efficiency for heat loss associated with high V˙O2max. Body fatness significantly affected Tre only in the CO condition, where, with low skin blood flows (measured as increases in forearm blood flow), the insulative effect of fat was pronounced. In the warmer environments, high skin blood flows offset the resistance offered by peripheral adipose tissue. Contrary to other studies, Tre was positively correlated with AD/mass coefficient for all conditions tested. For both exercise types used, being big (a high heat loss area and heat capacity) was apparently more beneficial from a heat strain standpoint than having a favourable AD/mass coefficient (high in small subjects). The total amount of variance in Tre responses which could be attributed to individual characteristics was dependent on the climate and the type of exercise. Though substantial for absolute exercise intensities (52%–58%) the variance explained in Tre differed markedly for relative intensities: 72% for the WH climate with its limited evaporative capacity, and only 10%–26% for the HD and CO climates. The results showed that individual characteristics play a significant role in determining the responses of body core temperature in all conditions tested, but their contribution was low for relative exercise intensities when evaporative heat loss was not restricted. This study demonstrated that effects of individual characteristics on human responses to heat stress cannot be interpreted without taking into consideration both the heat transfer properties of the environment and the metabolic heat production resulting from the exercise type and intensity chosen. Their impact varies substantially among conditions.

Age, splanchnic vasoconstriction, and heat stress during tilting

During upright tilting, blood is translocated to the dependent veins of the legs and compensatory circulatory adjustments are necessary to maintain arterial pressure. For examination of the effect of age on these responses, seven young (23 ± 1 yr) and seven older (70 ± 3 yr) men were head-up tilted to 60° in a thermoneutral condition and during passive heating with water-perfused suits. Measurements included heart rate (HR), cardiac output (Q˙c; acetylene rebreathing technique), central venous pressure (CVP), blood pressures, forearm blood flow (venous occlusion plethysmography), splanchnic and renal blood flows (indocyanine green and p-aminohippurate clearance), and esophageal and mean skin temperatures. In response to tilting in the thermoneutral condition, CVP and stroke volume decreased to a greater extent in the young men, but HR increased more, such that the fall in Q˙cwas similar between the two groups in the upright posture. The rise in splanchnic vascular resistance (SVR) was greater in the older men, but the young men increased forearm vascular resistance (FVR) to a greater extent than the older men. The fall inQ˙c during combined heat stress and tilting was greater in the young compared with older men. Only four of the young men versus six of the older men were able to finish the second tilt without becoming presyncopal. In summary, the older men relied on a greater increase in SVR to compensate for a reduced ability to constrict the skin and muscle circulations (as determined by changes in FVR) during head-up tilting.

A review of comparative responses of men and women to heat stress

Most of our present knowledge regarding human responses to thermal stress is primarily a result of research conducted on male subjects. Recently, as women have moved into the industrial workplace and forefront of athletic activity, attention has turned to comparative responses of men and women. Very limited research on preadolescent children suggests no physiological thermoregulatory sex differences except for a slightly higher sweat rate in lean boys as compared to lean girls of a similar age. Boys also tended to be more tolerant of higher temperatures. Current beliefs regarding men and women are: Women, as a population, are less tolerant to a given imposed heat stress; however, if cardiovascular fitness level, body size, and acclimation state are standardized, the differences tend to disappear; women have a lower sweat rate than men of equal fitness, size, and acclimation which is disadvantageous in hot-dry environments, but advantageous in hot-wet environments; and menstrual cycle effects are minimal. It is concluded that aerobic capacity, surface area-to-mass ratio, and state of acclimation are more important than sex in determining physiological responses to heat stress.

Rationale for a Personal Monitor for Heat Strain

Worker heat-stress exposures can be controlled for short periods above the threshold limit value (TLV) by self-assessment, if the worker can avoid overexposure based on excessive heart rate and/or excessive core temperature. A socially acceptable surrogate for core temperature and a measure of heart rate are objective measures that can increase the reliability of the self-assessment decision. This article describes a surface-mounted temperature sensor developed to indicate when rectal temperature reaches a safe limit. Protective criteria were established for temperature sensor alert limits. A fixed threshold for heart rate may cause premature alerts during bursts of activity and miss lower, but sustained, heart rates that represent significant physiological strain. For these reasons, heart rate criteria based on seven moving-time averages also were developed. The criteria are based on a relationship between heart rate and endurance time. The temperature sensor and heart rate criteria form the basis of a real-time personal monitor for heat strain.

Lactate and gas exchange responses to incremental and steady state running.

This study compared the oxygen uptake (VO2) and running velocity at which the lactate threshold (LT), the ventilatory threshold (VT), and the maximal lactate steady state (MSSLA), and the maximal VO2 steady state (MSSVO2) occurred in 11 trained male runners (mean age = 22.4 years, range 18-28 years). Each underwent an incremental treadmill test to exhaustion. The LT was defined by a systematic, continuous increase in arterialised venous blood lactate; the VT was determined by an abrupt rise in VE.VO2(-1) without an increase in VE.VCO2(-1). Each subject also completed a series of steady state treadmill runs of 20 minutes duration. The MSSLA was determined as the highest velocity and VO2 at which lactate concentration increased by less than 0.2 mmol.l-1 from minute 10 to minute 20. The MSSVO2 was determined as the highest velocity or VO2 at which a steady state in VO2 was not delayed for more than 3 minutes (with a steady state defined as VO2 within 0.2 l.min-1 of the average VO2 over the last 10 minutes of each test). Each subject also completed a 5 km time trial run to assess performance. No significant differences were found among the four variables expressed either as VO2 or velocity. Significant correlations were found between MSSLA and MSSVO2 (r = 0.74) expressed as VO2, and between MSSLA and MSSVO2 (r = 0.90), MSSVO2 and VT (r = 0.70) and MSSLA and VT (r = 0.67) expressed as velocity. A stepwise regression analysis found MSSLA (expressed as velocity) to be the best predictor of 5 km performance (r = 0.87). It was concluded that (a) MSSLA and MSSVO2 are closely related, and (b) MSSLA is a good predictor of performance and may be an important, objective measure of cardiorespiratory endurance capacity.

Variables predictive of performance in elite middle-distance runners.

The purpose of this study was to investigate possible factors which may account for differences in performance times within a closely-matched group (in terms of performance) of elite distance runners. The runners were training for competition in the 1984 Olympic Games in either the 5000 m or the 3000 m steeplechase events. Each runners best performance time (BPT) was obtained and a stepwise regression analysis was performed with the following independent variables: age, weight, % body fat, VO2 max, aerobic threshold (AerT), and anaerobic threshold (AnT). For the 5000 m-runners, a multiple correlation of age and AnT accounted for 77% of the variance (p less than 0.02); for the 3000 m steeplechase runners, body weight alone and body weight and AnT accounted for 94% (p less than .01) and 98% (p less than .05) of the variance, respectively. The results suggest that, among elite middle-distance runners, these parameters deserve attention as potential predictors of performance.

Simultaneous derivation of clothing-specific heat exchange coefficients.

Clothing adds resistance to heat exchange between the wearer and the environment. If clothing-specific heat exchange coefficients are known, a combined rational/empirical approach can be used to describe thermal exchange between clothed humans and the environment. However, during exercise these coefficients--typically calculated using thermal manikins--change, primarily due to wetting of the fabric during intense sweating and body movement. A procedure is described that allows for the simultaneous determination of both total insulation (IT) and resistance to water vapor permeation (Re) on exercising clothed subjects without the need to directly measure skin water vapor pressure or continuously weigh the subjects. Two tests are performed by each subject in each clothing ensemble. In one test, ambient water vapor pressure (Pa) is systematically increased in stepwise fashion while dry-bulb temperature (Tdb) is held constant; in the second test protocol Pa is held constant while Tdb is increased. Heat exchange data are collected at the time at which core temperature is forced out of equilibrium by the environment (according to the assumption that heat production is balanced by heat loss immediately prior to this critical environmental limit). Previous studies using similar approaches have typically estimated IT a priori and used this value in the subsequent derivation of Re for each clothing ensemble or condition tested. In the proposed method, IT and Re are derived from the solution of two simultaneous equations based on heat balance data from both tests. This paper describes and critiques this methodology via an error analysis, and compares the coefficients obtained with those from similar trials using other physiological and nonphysiological approaches.

Effect of age on renal blood flow during exercise

The present study examined the effect of age on the control of renal blood flow (RBF; PAH clearance) and renal vascular conductance (RVC=RBF/mean arterial pressure) during and after a bout of dynamic exercise in a warm environment. Six healthy fit older men (O; 67± 1 years) and 6 young men (Y; 24± 2 years) were matched for body size, adiposity, and maximal oxygen uptake (VO2max). Subjects exercised at ∼50% of V02max for 90 minutes in an environment of 30°C, 60% humidity on each of 4 consecutive days, with data collected on days 1 and 4. There was no effect of repeated days of exercise on RBF or RVC, despite a 4% expansion of blood volume in Y (<l°/o in O). On each day, resting RBF was significantly lower in O (e.g., Y=1127± 67, O=852± 114 mL/min on day 1; p<0.05). During exercise, Y decreased RBF to a significantly (p<0.05) greater extent [− 508 (− 45%) and − 365 (− 36%) mL/min on days 1 and 4, respectively] than the O [− 98 (− 12%) and − 83 (− 12%) mL/min]. RVC followed a similar pattern, decreasing by 52% and 37% during exercise for Y vs only 15% and 13% for O. The relationships between ARBF and HR and ΔRBF and plasma norepinephrine concentration were independent of age, implying similar sympathetic control during exercise. During recovery, RBF and RVC increased as expected in Y, but continued to decrease in O, falling significantly below exercise values (p<0.05). Compared to young men, fit healthy older men redistribute less blood flow away from the kidneys during dynamic exercise in the heat, an effect which appears to result from the existence of a smaller resting RBF rather than differential sympathetic control. On the other hand, chronological age seems to be associated with altered control of RBF and RVC during recovery from exercise. (Aging Clin. Exp. Res. 6: 293–302, 1994)

Age and cardiac output during cycle exercise in thermoneutral and warm environments.

To determine whether chronological age, independent of changes in aerobic capacity, alters cardiac output (Qc), the central hemodynamic responses to intermittent incremental cycle exercise were studied in two groups of men. Qc was measured at rest and during exercise at 35%, 60%, 75%, and 85% peak aerobic capacity (VO2peak) using a CO2 rebreathing method in seven trained older (65 +/- 2 yr) and eight normally active but untrained young men (26 +/- 1 yr) matched for VO2peak and anthropometric measures. Subjects were tested in both a thermoneutral (22 degrees C) and a warm (36 degrees C) environment to investigate possible differential cardiovascular responses to exercise in the heat. Only subjects with no history of pulmonary, cardiac, neuromuscular, or endocrine disease and a normal electrocardiogram were studied. The older men had significantly lower (P < 0.05) Qc relative to the younger men at intensities greater than 60% VO2peak in both environmental conditions. At these higher intensities, the older men had a significantly higher stroke volume (SV) and lower heart rate (HR) (P < 0.05). A higher arteriovenous oxygen difference ((a-v)O2)) compared with their younger counterparts enabled the older men to exercise at the same absolute intensity, most likely because of training induced changes in left-ventricular performance and oxygen extraction. The addition of an exogenous heat source did not alter the Qc response in either group of men; however, a higher HR (P < 0.05) and smaller SV (P > 0.05) were observed in the young men during exercise in the heat. This may reflect previously reported differences in the skin blood flow response of VO2peak-matched young and older men during exercise. It is suggested that endurance trained older men can enhance left-ventricular performance to augment SV, but not sufficiently to maintain Qc in light of an attenuated HR response during exercise at intensities above 60% VO2peak.

Readdressing Personal Cooling With Ice

An ice-based system of personal, non-restrictive cooling of workers exposed to high temperature work environments in nuclear power plants was evaluated. The garments were designed to be worn under the protective clothing donned for penetration into radiation areas. The cooling system consisted of direct body contact with small packets of frozen water enclosed in the pockets of a shirt in high ambient temperatures (55 degrees C) and moderate metabolic heat production (200-300 kcal/hr). Mean exposure time without cooling (control) was 52 min for workloads demanding 200 kcal/hr energy expenditure. A long garment with 7.2 kg of frozen water (LFWG) increased mean exposure time over the control by 242% (163% for the same garment with 6.2 kg of frozen water). A short version garment with 3.8 kg of frozen water (SFWG) increased the stay time by 115%. In field observations, the LFWG with 6.2 kg of frozen water improved stay time by 125%. The leveling off of the body temperatures and heart rates during the work both in the laboratory and in the field confirmed a reduction of heat strain associated with the use of these garments. Calculated heat balance equations of heat uptake by the ice predicted these results very closely. It was concluded that direct body contact with frozen water provided predictable adequate body cooling for the work and ambient conditions investigated in this study.