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.