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.O2max]). 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.O2max. Eight people (5 women; (mean ± SD) age = 25 ± 5 y; body mass = 74.8 ± 11.6 kg; V.O2max = 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.O2max was measured after 120 min. On a separate day, V.O2max 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.O2max 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.

Work–rest regimens for work in hot environments: A scoping review

“…53 The number of participants included in the experimental studies varied between 1 and 115 (median: 10), for a total of 642 participants (representing an average of 18 ± 25 participants per study). Only six studies 35,40,45,49,58,66 included females (total of 38, 6%) (Figure 3B). No studies focusing only on females were retrieved.…”

“…No studies focusing only on females were retrieved. The menstrual cycle was either controlled, 35,40,58,66 uncontrolled 45 or nothing was specified about it. 49 Nine studies (26%) included heat-acclimatized participants, 18 (51%) were conducted on nonheat-acclimatized individuals and eight (23%) did not mention the acclimatization status of the participants (Figure 3C).…”

“…Seven of the 35 experimental studies (20%) implemented a work-rest regimen based on ACGIH Threshold Limit Values, [26][27][28]39,59,60,62 none used the Action Limit Values, four (11%) used the NIOSH recommendations 35,41,45,58 and one (3%) used recommendations provided by the Australian Army 40 (Figure 4A). It is worth mentioning that seven of the studies that used Threshold Limit Values did so in participants who were not heat acclimatized 26,28,39,59 or of unknown acclimatization status.…”

“…*One study reported data collected on workers from four countries across two continents, Europe and Asia. 16 studies that used NIOSH recommendations, two specifically mentioned using the Recommended Alert Limits on unacclimatized participants, 45,58 whereas the other did not mention whether they used the Recommended Alert Limits or the Recommended Exposure Limits but investigated heat-acclimatized 41 Thirty-one experimental studies (89%) focused on physical tasks only (Figure 5A), with 30 studies using endurance tasks and one study using a psychomotor task (one-dimensional vertical compensatory tracking task). 41 One experimental study (3%) 57 investigated mental tasks only (a vertical visual vigilance task), one (3%) 63 investigated endurance and mental tasks sequentially, and two (6%) 16,36 field studies investigated occupational tasks with high cognitive and physical demand (included in "physical and mental, concurrently").…”

“…Of the 33 experimental studies that investigated endurance tasks or a combination of endurance and mental tasks, 25 (76%) used a single-task such as walking, 35,37,40,43,45,47,49,50,52,[59][60][61][63][64][65] cycling, 26-28,39,42,51,62,66 or block stepping. 44,48 The remaining eight studies (24%) incorporated specific occupational tasks related to agriculture, construction, and tourism, 16 welding, 36 repetitive box lifting, 54 walking with load carrying, 49,55,56 walking with arm curls, 58 or walking with shoveling. 53 Of the 33 experimental studies that investigated endurance tasks or a combination of endurance and mental tasks, three (9%) used a self-paced strategy (one reported the associated metabolic rate measured using second-by-second time-motion or realtime analysis 16 while the other two did not report intensity data 36,54 ); the remaining 30 (91%) used a fixed-pace strategy (Figure 5B).…”