Cardiovascular drift and maximal oxygen uptake during heat stress in women
During prolonged constant-rate exercise, heart rate and stroke volume progressively increase and decrease, respectively, characterizing cardiovascular (CV) drift. CV drift is greater when driven by hyperthermia and generally results in proportional decreases in maximal oxygen uptake (V̇O2max). Less is known about CV drift and decrements in V̇O2max in women because nearly all studies on this topic focused on men. This dissertation determined the effects of hormonal status, fitness level, and sudomotor function on CV drift and V̇O2max in women. In 3 separate studies, CV drift was measured during 45 min of cycling in 35 °C, immediately followed by measurement of V̇O2max. V̇O2max also was measured after 15 min in a separate trial to assess changes in V̇O2max over the same time interval that CV drift occurred. Study 1 compared follicular (FP) and luteal phases (LP) of the menstrual cycle during exercise at 60% V̇O2max. Resting and exercise core temperatures (Tre) were higher in LP, but increases during exercise (ΔTre) were similar to FP, so the CV drift/V̇O2max relationship was not modulated by phase. Study 2 compared high-fit (HI) and low-fit (LO) women during exercise at 60% V̇O2max (REL) and 500 W of metabolic heat production (FIXED). During REL, heat production and ΔTre were significantly greater in HI versus LO, as were magnitudes of CV drift and decrements in V̇O2max. During FIXED, heat production, ΔTre, CV drift, and V̇O2max were similar between groups. Study 3 compared women to men during exercise at 500 W of metabolic heat production. For women, sweating plateaued and accelerated ΔTre compared to men, but differences in CV drift and V̇O2max were not statistically discernible between sexes. In conclusion, the relationship between CV drift and V̇O2max during heat stress does not change across the menstrual cycle and is not affected by fitness level, independent of metabolic heat production. The relationship is similar between men and women during 45 min of exercise at the same, relatively high load.