Sean R. Notley

Morphological dependency of cutaneous blood flow and sweating during compensable heat stress when heat-loss requirements are matched across participants

Human heat loss is thought, in part, to be morphologically related. It was therefore hypothesized that when heat-loss requirements and body temperatures were matched, that the mass-specific surface area alone could significantly explain both cutaneous vascular and sudomotor responses during compensable exercise. These thermoeffector responses were examined in 36 men with widely varying mass-specific surface areas (range, 232.3-292.7 cm(2)/kg), but of similar age, aerobic fitness, and adiposity. Subjects completed two trials under compensable conditions (28.1°C, 36.8% relative humidity), each involving rest (20 min) and steady-state cycling (45 min) at two matched metabolic heat-production rates (light, ∼135 W/m(2); moderate, ∼200 W/m(2)). Following equivalent mean body temperature changes, forearm blood flow and vascular conductance (r = 0.63 and r = 0.65) shared significant, positive associations with the mass-specific surface area during light work (P < 0.05), explaining ∼45% of the vasomotor variation. Conversely, during light and moderate work, whole body sweat rate, as well as local sweat rate and sudomotor sensitivity at three of four measured sites, revealed moderate, negative relationships with the mass-specific surface area (correlation coefficient range -0.37 to -0.73, P < 0.05). Moreover, those relationships could uniquely account for between 10 and 53% of those sweating responses (P < 0.05). Therefore, both thermoeffector responses displayed a significant morphological dependency in the presence of equivalent thermoafferent drive. Indeed, up to half of the interindividual variation in these effector responses could now be explained through morphological differences and the first principles governing heat transfer.

Variations in body morphology explain sex differences in thermoeffector function during compensable heat stress

What is the central question of this study? Can sex‐related differences in cutaneous vascular and sudomotor responses be explained primarily by variations in the ratio between body surface area and mass during compensable exercise that elicits equivalent heat‐loss requirements and mean body temperature changes across participants? What is the main finding and its importance? Mass‐specific surface area was a significant determinant of vasomotor and sudomotor responses in men and women, explaining 10–48% of the individual thermoeffector variance. Nonetheless, after accounting for changes in mean body temperature and morphological differences, sex explained only 5% of that inter‐individual variability. It was concluded that sex differences in thermoeffector function are morphologically dependent, but not sex dependent.

The effects of thoracic load carriage on maximal ambulatory work tolerance and acceptable work durations.

PurposeTorso loads restrict chest-wall movement and ventilation, particularly during heavy exercise. Therefore, the differential impact of load carriage and chest-wall restriction was investigated during progressive treadmill exercise. In addition, acceptable work durations were derived across a wide range of submaximal, loaded exercise intensities.MethodsTwelve males participated in two research phases. Phase 1: Three incremental treadmill tests until exhaustion [control (clothing only), load carriage (clothing plus 22-kg vest), and clothing with chest strapping]. Phase 2: Five steady-state exercise trials (clothing plus 22-kg vest) at intensities from 30 to 80xa0% of peak aerobic power to determine maximal acceptable work durations.ResultsMaximal work tolerance [control 17.21xa0min (±0.93); loaded 13.44xa0min (±0.68); strapped 17.00xa0min (±0.83)] and the mass-specific peak aerobic power [control 61.61xa0mLxa0kg−1xa0min−1 (±2.28); loaded 45.42xa0mLxa0kg−1xa0min−1 (±1.41); strapped 59.99xa0mLxa0kg−1xa0min−1 (±1.61)] were reduced only when loaded (Pxa0<xa00.05). Peak minute ventilation was retained, although loading and chest strapping reduced the breathing reserve. The lower and upper acceptable work duration derivations when working at the 30xa0% intensity were 133.40xa0min (±23.77) and 220.10xa0min (±48.69), but at 80xa0% intensity, both durations were reduced to <7xa0min.ConclusionThoracic loading significantly reduced exercise tolerance and the breathing reserve, but did not modify peak minute ventilation or the absolute peak aerobic power. Chest strapping, as used herein, exerted minimal impact. However, the projected maximal acceptable work durations were much less than derived using previously published methods.

The utility of heart rate and minute ventilation as predictors of whole-body metabolic rate during occupational simulations involving load carriage

The utility of cardiac and ventilatory predictors of metabolic rate derived under temperate and heated laboratory conditions was evaluated during three fire-fighting simulations (70-mm hose drag, Hazmat recovery, bushfire hose drag; N = 16 per simulation). The limits of agreement for cardiac (temperate: − 0.54 to 1.77; heated: − 1.39 to 0.80 l min− 1) and ventilatory surrogates (temperate: − 0.19 to 1.27; heated: − 0.26 to 1.16 l min− 1) revealed an over-estimation of oxygen consumption that exceeded the acceptable limits required by occupational physiologists (N = 25; ± 0.24 l min− 1). Although ventilatory predictions offered superior precision during low-intensity work (P < 0.05), a cardiac prediction was superior during more demanding work (P < 0.05). Deriving those equations under heated conditions failed to improve precision, with the exception of the cardiac surrogate during low-intensity work (P < 0.05). These observations imply that individualised prediction curves are necessary for valid estimations of metabolic demand in the field. Practitioner Summary: Cardiac and ventilatory surrogates are often used to approximate the metabolic demands of work. In this study, however, such predictions demonstrated unsatisfactory agreement with simultaneously measured values across three fire-fighting simulations. Therefore, individually calibrated equations appear necessary to obtain the level of predictive precision required by occupational physiologists.

Administration of prostacyclin modulates cutaneous blood flow but not sweating in young and older males: roles for nitric oxide and calcium‐activated potassium channels

In young adults, cyclooxygenase (COX) contributes to the heat loss responses of cutaneous vasodilatation and sweating, and this may be mediated by prostacyclin‐induced activation of nitric oxide synthase (NOS) and calcium‐activated potassium (KCa) channels. This prostacyclin‐induced response may be diminished in older relative to young adults because ageing is known to attenuate COX‐dependent heat loss responses. We observed that, although prostacyclin does not mediate sweating in young and older males, it does modulate cutaneous vasodilatation, although the magnitude of increase is similar between groups. We also found that, although NOS and KCa channels contribute to prostacyclin‐induced cutaneous vasodilatation in young males, these contributions are diminished in older males. Our findings provide new insight into the mechanisms governing heat loss responses and suggest that the age‐related diminished COX‐dependent heat loss responses reported in previous studies may be a result of the reduced COX‐derived production of prostanoids (e.g., prostacyclin) rather than the decreased sensitivity of prostanoid receptors.

The impact of thermal pre-conditioning on cutaneous vasomotor and shivering thresholds

The mean body temperature of resting, normothermic humans falls within the zone that separates the temperature thresholds for shivering and sweating; the vasomotor zone. Whilst these thresholds are often defined by their corresponding deep-body or mean body temperatures, it is well known that these are not set temperatures or points. Nevertheless, our knowledge concerning the factors that determine or modify these thresholds is imprecise. Therefore, the aim of this experiment was to investigate the effects of a deliberate modification of the pre-exposure mean body temperature on the subsequent vasomotor and shivering threshold temperatures. Mean body temperature was first displaced upwards, then slowly driven in the opposite direction, permitting the separate determination of these thermoeffector thresholds.

Individual differences in thermoeffector function in the heat: morphological variations help determine effector activation

It is possible that much of the inter-individual variability observed within human thermoeffector responses can be explained by differences in body morphology, specifically the ratio between surface area and mass. However, few have examined these relationships across a sufficiently wide range of body sizes, while controlling for the factors that can independently alter heat dissipation. This investigation was aimed at identifying the proportion of thermoeffector variability that could be explained on the basis of morphology within individuals of widely different surface-area-to-mass ratios, but of similar age, fitness and adiposity.

Is the dermatomal recruitment of sweating a physiological reality or a misinterpretation

A caudal-to-rostral (sympathetic dermatomal) recruitment pattern for human eccrine sweating was first described by Randall and Hertzman [1], and is widely accepted. Nevertheless, neither sudomotor activation nor sweat gland recruitment patterns were actually measured during that, or their subsequent supporting research. Instead, recruitment was derived using curves fitted to data obtained over several months, with data for separate skin regions not necessarily obtained from the same individuals. Since such data are ill-suited for drawing interpretations relative to sympathetic activation, and since Kuno [2] reported a simultaneous glandular activation across all skin regions, this hypothesis was revisited.

Cutaneous vasomotor adaptation following repeated, isothermal heat exposures: evidence of adaptation specificity

Unequivocal enhancement of cutaneous vasomotor function has yet to be demonstrated following heat acclimation, possibly because the adaptation stimulus was not sustained, or because thermoeffector function was not assessed at equivalent deep-body temperatures. Therefore, forearm and local cutaneous vascular conductances were evaluated during exercise eliciting matched deep-body temperatures (37.5 °C, 38.5 °C), before and after isothermal heat acclimation. Both indices increased (21% and 25%), confirming cutaneous vasomotor adaptation can occur, provided those experimental design specifications are satisfied.

Postural influences on sweating: exploring the effects of gravity and pressure

The distribution of thermal sweating is neither uniform nor does it commence simultaneously at all sites. One reason for this variability may be associated with gravitational influences. That is, localised and posture-dependent compression of tissues containing pressure-sensitive receptors is believed to inhibit sweating from the compressed and ipsilateral sites, whilst enhancing secretion from contralateral surfaces [1]. To evaluate the possibility that local sweat rates might be influenced by gravity, it is necessary to test subjects with and without gravitational loading. This can be achieved by using water immersion to simulate zero gravity, and this experimental model was used for this pilot investigation.