(48) VO2MAX PREDICTS CHANGES IN BODY MASS DURING 75 MINUTES OF MODERATE INTENSITY CYCLING IN THE HEAT

Individuals with greater aerobic fitness may experience enhanced thermoregulation through a variety of mechanisms such as increased plasma volume, earlier sweating onset, and higher sweat rates. Further, inverse relationships between maximal oxygen consumption (VO₂max) and economy have been observed. These factors, in combination with external heat stress, may contribute to altered changes in physiological stress and hydration status between individuals of varying fitness during prolonged exercise.

PURPOSE
The purpose of this study was to determine the relationship between VO₂max and changes in body mass over 75 minutes of cycling in a hot environment at a power output that elicited 65%VO₂max.  

METHODS
Sixteen healthy adults (n = 8 women, n = 8 men; M_VO2max = 43.5 ± 4.9 ml/kg/min) who were able to achieve a VO₂max of at least 60% of their age and gender matched normative values completed the study. On three separate days, body mass (BM) and core body temperature (CBT) were recorded before and after a 75 minute cycling bout in a heat chamber (30-32℃ and 45-50% relative humidity) at a power output corresponding to 65% of their VO₂max. Pearson-product moment correlations were used to assess relationships between VO₂max and mean percent changes in BM (M_DBM) across the three trials, as well as between VO₂max and mean percent changes in core body temperature (M_DCB) and between M_DCB and M_DBM. An alpha level of 0.05 was used to determine statistical significance. 

RESULTS
A significant inverse correlation between VO₂max and M_DBM (r = -0.86, p < 0.0001) was observed. On average, M_DCB increased 3.16 ± 2.20%, but no significant relationship was found between VO₂max and M_DCB (r = 0.25, p = 0.35). On average, M_DBM decreased 0.94% ± 0.33%, but no significant relationship was found between M_DCB and M_DBM (r = -0.13, p = 0.63).

CONCLUSIONS
The strong inverse linear relationship between VO₂max and M_DBM indicates that fitter individuals, as estimated by their VO₂max, may be more likely to experience greater losses in BM during moderate-intensity exercise in the heat. While thermoregulatory mechanisms may play a role in these changes, we found no significant relationship between VO₂max and M_DCB in this study, demonstrating that a higher VO₂max did not equate to improved thermoregulation. Furthermore, the absence of a significant relationship between M_DCB and M_DBM would imply that changes in CBT were not responsible for differences in BM changes. Subsequently, it should be considered that exercise at a workload that elicits a given percent of  VO₂max may lead to a greater rise in VO₂ and relative stress in those with worse efficiency, which was not assessed in this study. These physiological changes may result in greater body water losses without improved thermoregulation.

PRACTICAL APPLICATIONS
Perhaps exercise intensity prescriptions based on a snapshot of an individual’s economy (e.g. fixed mechanical workloads derived from unstable physiological or internal intensities) may result in a high degree of variability in relative stress during prolonged exercise, particularly when coupled with environmental heat stress. Future research should further investigate methods of prescribing relative workloads based on capacity measurements rather than power measurements (e.g. time to exhaustion at VO₂max power) during prolonged aerobic exercise and should explore the effects of these methods on changes in economy and thermal power.

 
 

Acknowledgements: None