Effect of Work-to-Rest Cycles on Cardiovascular Strain and Maximal Oxygen Uptake during Heat Stress

Abstract

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 [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 V.O_2max. Eight people (5 women; (mean ± SD) age = 25 ± 5 y; body mass = 74.8 ± 11.6 kg; 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; V.O_2max was measured after 120 min. On a separate day, 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 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.

Keywords: cardiovascular drift; occupational health and safety; work capacity; work:rest ratios.

Figure 1

Procedures during the 120 min visit. Simulated work was achieved by 2.5 min of arm curls and 20 min of treadmill walking, repeated for a total of 45 min of work and followed by 15 min of rest. All baseline measurements were obtained before minute 0 except for hematological measures, which were taken before the participant entered the environmental chamber at minute −20. HR, heart rate; SV, stroke volume; GXT, graded exercise test.

Figure 2

Individual and mean ± SD percent change from minute 15 for heart rate (HR), stroke volume (SV), and maximal oxygen uptake ($\stackrel{.}{\mathrm{V}}$O_2max). Bars represent group means and symbols correspond to responses for each participant. * p < 0.05 compared to 15 min; † p < 0.05 compared to 75 min.

Figure 3

Individual and mean ± SD gastrointestinal temperature (T_gi) over two 45:15 min work:rest cycles. The solid line corresponds to mean T_gi and the symbols correspond to responses for each participant. Shaded areas represent rest breaks. N = 7 at all time points because of technical difficulties for one participant that resulted in missing data at 45, 75, and 105 min. * p < 0.05 compared to 15 min; † p < 0.05 compared to 45 min; ‡ p < 0.05 compared to 75 min; # p < 0.05 compared to baseline (0 min).

Figure 4

Individual and mean ± SD mean skin temperature (T_sk) over two 45:15 min work:rest cycles. The solid line corresponds to the average mean T_sk for the entire sample and the symbols correspond to responses for each participant. Shaded areas represent rest breaks. * p < 0.05 compared to 45 min. † p < 0.05 compared to 105 min.