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

Objectives: To derive an empirical model for the impact of aerobic fitness (maximal oxygen consumption; V̇O _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 V̇O _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. V̇O _2max also modulated the relationship between sweat rate and body temperature changes to increasing WBGT. Including individual V̇O _2max data in the PWC prediction model increased the predicting power by 4%.
Conclusions: Incorporating individual V̇O _2max improved the predictive power of the heat stress index WBGT for Physical Work Capacity in the heat. The largest impact of V̇O _2max on PWC was observed at a WBGT between 25 and 35 °C.
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