Aerobic fitness as a parameter of importance for labour loss in the heat

Abstract

Objectives: To derive an empirical model for the impact of aerobic fitness (maximal oxygen consumption; VȮ 2max in mL•kg −1 •min −1 ) on physical work capacity (PWC) in the heat. Design: Prospective, repeated measures. Methods: Total work completed during 1 h of treadmill walking at a fixed heart rate of 130 b•min −1 was assessed in 19 young adult males across a variety of warm and hot climate types, characterised by wet-bulb globe temperatures (WBGT) ranging from 12 to 40°C. For data presentation and obtaining initial parameter estimates for modelling, participants were grouped into low (n = 6, 74 trials), moderate (n = 8, 76 trials), and high (n = 5, 29 trials) fitness, with group mean VȮ 2max 42, 52, and 64 mL•kg −1 •min −1 , respectively. For the heated conditions (WBGT 18 to 40°C), we calculated PWC% by expressing total energy expenditure (kJ above resting) in each trial relative to that achieved in a cool reference condition (WBGT = 12°C = 100% PWC). Results: The relative reduction in energy expenditure (PWC%) caused by heat was significantly smaller by up to 16% for the fit participants compared to those with lower aerobic capacity. VȮ 2max also modulated the relationship between sweat rate and body temperature changes to increasing WBGT. Including individual VȮ 2max data in the PWC prediction model increased the predicting power by 4%. Conclusions: Incorporating individual VȮ 2max improved the predictive power of the heat stress index WBGT for Physical Work Capacity in the heat. The largest impact of VȮ 2max on PWC was observed at a WBGT between 25 and 35°C.

Quantifying the impact of heat on human physical work capacity; part II: the observed interaction of air velocity with temperature, humidity, sweat rate, and clothing is not captured by most heat stress indices

“…We aimed to simulate this pacing strategy by fixing the working heart rate (and thus, cardiovascular strain) across all tested conditions of T a and relative humidity. This fixed heart rate protocol has been used previously to determine the effect of fans on light work output at 30 °C T a (Jay et al, 2019 ), and for modelling the impact of heat stress and physical work capacity (Foster et al, 2021a , b , c ). During physical work, skin blood flow requirements increase as a function of heat stress severity (Rowell, 1974 ), resulting in a progressively larger proportion of cardiac output being directed to the skin (to support heat loss) compared with muscle (to support physical work).…”

“…We defined PWC as “the maximum physical work output that can be reasonably expected from an individual performing moderate to heavy work over an entire shift” (Foster et al, 2021a ). Our group developed empirical models which precisely estimate PWC from a wide range of air temperature and relative humidity combinations (Foster et al, 2021a ), including correction factors which account for individual characteristics (fitness, age, and sex) (Foster et al, 2021c ) and solar irradiance (Foster et al, 2021b ). Another major task was to evaluate the impact of increasing wind speed on PWC, which can likely be beneficial or detrimental depending on the specific air temperature ( T a ) and humidity combination.…”

Quantifying the impact of heat on human physical work capacity; part III: the impact of solar radiation varies with air temperature, humidity, and clothing coverage

“…Since our recent review suggests that the effect of age and sex are of importance primarily at high heat loads (Foster et al, 2020), it is possible that these populations would show more severe reductions in PWC at the higher ranges of WBGT. We recently demonstrated that fitness level has its largest impact on PWC when WBGT is between 25 and 35°C (Foster et al, 2021c). As fitness has been identified as the main parameter explaining the impact of age (Havenith et al, 1995;Foster et al, 2020), the effect of age may show the same impact range in paced work.…”

Indicators to assess physiological heat strain – Part 3: Multi-country field evaluation and consensus recommendations

“…Previous studies evaluating whether TSIs reflect the physiological heat strain experienced by working people have reported higher correlations [ 9 , 14–16 , 18–21 ]. These studies were either well-controlled laboratory experiments [ 14 , 20 , 21 ] or small-scale field studies evaluating a group of workers at a single work site, wearing similar clothing, and performing similar jobs [ 9 , 15 , 16 , 18 , 19 ]. Therefore, it is likely that the 45% of the Delphi criteria that has not been addressed by even the best TSIs in our study is explained by differences that modify the heat strain response and the associated health outcomes.…”