Cardiovascular drift—a progressive increase in heart rate (HR) and decrease in stroke volume (SV) during prolonged exercise—is exacerbated by heat stress and thermal strain, and often accompanied by a decrease in work capacity (indexed as maximal oxygen uptake [$\stackrel{.}{\mathrm{V}}$O_2max]). To attenuate physiological strain during work in the heat, use of work:rest ratios is recommended by the National Institute for Occupational Safety and Health. The purpose of this study was to test the hypothesis that during moderate work in hot conditions, utilizing the recommended 45:15 min work:rest ratio would result in cardiovascular drift ‘accumulating’ over consecutive work:rest cycles and accompanying decrements in $\stackrel{.}{\mathrm{V}}$O_2max. Eight people (5 women; (mean ± SD) age = 25 ± 5 y; body mass = 74.8 ± 11.6 kg; $\stackrel{.}{\mathrm{V}}$O_2max = 42.9 ± 5.6 mL·kg⁻¹·min⁻¹) performed 120 min of simulated moderate work (201–300 kcal·h⁻¹) in hot conditions (indoor wet-bulb globe temperature = 29.0 ± 0.6 °C). Participants completed two 45:15 min work:rest cycles. Cardiovascular drift was evaluated at 15 and 45 min of each work bout; $\stackrel{.}{\mathrm{V}}$O_2max was measured after 120 min. On a separate day, $\stackrel{.}{\mathrm{V}}$O_2max was measured after 15 min under identical conditions for comparison before and after cardiovascular drift occurred. HR increased 16.7% (18 ± 9 beats·min⁻¹, p = 0.004) and SV decreased 16.9% (−12.3 ± 5.9 mL, p = 0.003) between 15 and 105 min, but $\stackrel{.}{\mathrm{V}}$O_2max was unaffected after 120 min (p = 0.14). Core body temperature increased 0.5 ± 0.2 °C (p = 0.006) over 2 h. Recommended work:rest ratios preserved work capacity but did not prevent the accumulation of cardiovascular and thermal strain.

This study examined the trajectory of heat exposure and core body temperature (T_c) across the workday into off-work hours among agricultural workers at risk of heat-related illness. Workday heat strain was common, and a significant proportion of workers experienced higher off-work T_c compared with their workday T_c. Survey and environmental data collected from workers provide insights into the off-work practices and conditions of the home environment that could influence the recovery of workers.

Introduction: Higher work rates may be sustainable when maintaining target rating of perceived exertion (RPE) compared to maintaining target heart rate (THR) during high-intensity interval training (HIIT) exercise in hot conditions, but may also result in greater thermal strain and cardiovascular drift, as well as greater decrements in maximal oxygen uptake (V˙O_2max).Purpose: To test the hypotheses that maintaining target RPE compared to THR during HIIT in the heat results in 1) smaller work rate adjustments, 2) greater thermal and cardiovascular strain, and 3) larger decreases in V˙O_2max.Methods: Eight adults (4 women) completed a graded exercise test on a cycle ergometer in 22°C and then 4 cycling trials in 35°C, consisting of an 8-min warm-up at 70% maximal heart rate (HR_max) or 12 RPE followed by 1 (15_HR and 15_RPE) or 5 (43_HR and 43_RPE) rounds of HIIT (1 round = 4 min work at 90% HR_max or 17 RPE and 3 min recovery at 70% HR_max or 12 RPE) totaling 15 min or 43 min of exercise, respectively. Each trial ended with a GXT to measure V˙O_2max.Results: In the 43-min trials work rate decreased from the first to the fifth work interval in both conditions, but by a non-significant, yet moderately larger (ES = 0.53) amount during 43_HR (46 ± 29 W) compared to 43_RPE (30 ± 28 W). From the first to fifth work interval HR increased over time by 12 b⋅min^–1 in 43_RPE (p < 0.001), but did not increase during 43_HR (p = 0.36). Rectal temperature increases were not different between conditions (43_HR = 0.7°C, p < 0.001; 43_RPE = 0.8°C, p < 0.001). V˙O_2max decreased 15.6% (ES = 0.41) between 15_RPE and 43_RPE (p = 0.005), but it was not different over time during the HR-based trials [6.5%, ES = 0.16 (α adjusted for multiple comparisons = 0.0125) p = 0.03].Conclusion: Maintaining target RPE and THR require considerable declines in work rate during HIIT in the heat, with ∼53% larger declines needed to maintain THR. The mitigation of cardiovascular drift in the THR trial may have contributed to the preservation of V˙O_2max.

Heart rate drifts upward over time during interval exercise and during exercise in hot conditions. As such, work rate must be lowered to maintain target heart rate. The purpose was to characterize acute work rate adjustments during high-intensity interval training based on target heart rate. Seven humans (three females) completed five study visits: a graded exercise test on a cycle ergometer to measure maximal heart rate (HR_max) in ∼22 °C and four trials performed in ∼22 °C (TEMP) or ∼35 °C (HOT), consisting of an 8 min warm-up at 70% HR_max followed by one (15_TEMP and 15_HOT) or five (43_TEMP and 43_HOT) rounds of high-intensity interval training (one round = 4 min work at 90% HR_max and 3 min recovery at 70% HR_max) totaling 15 min or 43 min of exercise, respectively. Work rate was lowered 33 ± 20 W ( p = 0.005) in 43_TEMP and 56 ± 30 W ( p = 0.003) in 43_HOT between the first and fifth work intervals. Thermal strain (0.2 °C higher rectal temperature, p = 0.01) and cardiovascular strain (6 beats·min^–1 larger increase in heart rate from first to fifth recovery interval, p = 0.01) were greater in 43_HOT versus 43_TEMP. Using target heart rate during high-intensity interval training may reduce the training stimulus, especially in hot environments, but it may also limit thermal strain and enable participants to complete the prescribed workout despite the heat.

Background: Maintaining euhydration is beneficial for health, safety, and physical performance [1]; however, it may also improve subjective feelings [2, 3]. Objective: The aim of this study was to evaluate the relationship between changes in self-reported thirst and alertness in people undergoing changes in drinking water volume. Methods: Subjects (mean ± SD) (n = 115, 59 males, 32 ± 10 years; 24.6 ± 4.4 kg·m⁻²) visited the lab 3 times over 10 days: V1, a baseline visit prior to participants were drinking ad libitum; V2, following 3 days of fluid restriction (1 L·d⁻¹, 250 mL was consumed in the morning prior to the visit); and V3, the morning following a prescribed increase in water intake. The increase in water intake at V3 varied by group assignment: control group (CON) maintained 250 mL, while LOW and HIGH groups (n = 45 each) consumed 496 ± 82 mL and 878 ± 125 mL, respectively. At each visit, subjects indicated on an open-ended visual analog scale (VAS) how thirsty and alert they felt and were measured in millimeters (mm). Four, two-way ANOVAs (group × visit) for change in thirst and alertness between V1–V2 and V2–V3 were completed. A repeated-measures correlation (r_rm) procedure was completed for change in alertness and thirst from V1 to V2 and V2 to V3 [4]. The study was approved by the University of Wyoming’s Institutional Review Board (protocol #20160524EJ01208), and all subjects provided written informed consent. Results: Groups were similar at baseline (V1) for fluid intake, thirst, and alertness (all p ≥ 0.17). Fluid restriction (V2) resulted in a main effect of visit for thirst and alertness (both p < 0.01), with no main effect of group. Thirst increased (35 ± 35 mm) and alertness decreased (−19 ± 31 mm) from V1 to V2. The prescribed increase in water intake (V3) revealed a significant interaction of visit and group for thirst and alertness (both p < 0.01) (Table 1). Independent-samples t tests with a Bonferroni correction revealed that HIGH reduced thirst (−38 ± 37 mm) and increased alertness (18 ± 25 mm), while no change was observed for LOW (thirst, −7 ± 37 mm; alertness −1 ± 24 mm) and CON (thirst, −6 ± 23 mm; alertness 0 ± 23 mm; all p < 0.01) (Fig. 1). There was no difference between LOW and CON (both p > 0.92). Repeated-measures correlation analysis revealed an inverse relationship between change in alertness and thirst (r_rm [114] = −0.53, 95% CI [−0.65, −0.38], p < 0.01). Conclusion: A reduction in water intake resulted in an increase in thirst and decrease in alertness. Following 3 days of fluid restriction, 750–1,000 mL of water intake was needed to decrease thirst and increase alertness. Overall, an inverse relationship was observed between self-reported thirst and alertness.

The extent to which groundskeepers experience thermal and cardiovascular strain, dehydration, and accompanying declines in kidney function during work in hot-humid conditions is unknown. Demographics, thermal and cardiovascular strain, hydration, and kidney function were assessed in 20 groundskeepers (18 men; mean ± SD, age = 37.9 ± 8.4 years, body mass index = 31.5 ± 7.5 kg⋅m⁻²) during work on two summer days. Before (AM) and after (PM) the work shift, resting blood pressure (BP) and heart rate (HR) were measured and urine and blood samples were collected. During the work shift, fluid intake was recorded, gastrointestinal temperature ( T_GI) was recorded, and HR was recorded continuously. In 45% of participants, AM BP > 130/80 mm Hg on Day 1 and Day 2. Peak HR and T_GI achieved across both days were 143 ± 15 b·min⁻¹ and 37.7 ± 0.3 °C, respectively, and average peak wet bulb globe temperature was 39.1 ± 3.5 °C. On both days, urine specific gravity ( U_SG = 1.021 ± 01) and urine color ( U_COL = 6 ± 1) did not change across the work shift (all P > 0.28), but most subjects began with concentrated urine (62.5% U_SG ≥ 1.020 and 95% U_COL ≥ 4). Total fluid intake = 2.3 ± 1.6 L during work and consisted of 70% water, 25% sugar-sweetened beverages, 5% other. For six subjects, the average estimated glomerular filtration rate (eGFR) at AM was ≤ 60 mL·min⁻¹·1.73 m⁻². Five subjects had SCr increases from AM to PM ≥ 0.3 mg·dL⁻¹, signifying an acute kidney injury (AKI). While hyperthermia was not prevalent, subjects began and ended the workday with concentrated urine. Hypertension, obesity, and low water intake may have contributed to the overall low kidney function and AKIs observed.