American journal of industrial medicine · 2026
Assessment of Initial Stay Time and Work-Rest Scheduling Over Consecutive Moderate-Intensity Workdays in Hot Environments in Young and Older Males and Females.
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Kenny Glen P, Wagar Katie E, Harris-Mostert Roberto C, Janetos Kristina-Marie T, McCormick James J, King Kelli E, Ioannou Leonidas G, Sigal Ronald J, Journeay W Shane, O'Connor Fergus K
Аннотация
INTRODUCTION: Initial stay time (IST), the duration of continuous work before core temperature reaches 38.0°C, has been characterized in young and older adults but not across consecutive work periods or in conjunction with standard work-rest cycles. We examined sex- and age-specific ISTs during moderate-intensity work in very warm conditions and evaluated the effectiveness of prescribed work-rest cycles.
METHODS: Thirty young (18-30 years, n = 15 females) and thirty older (50-69 years, n = 15 females) habitually active, non-heat-acclimatized adults completed a 1.5-day simulated work protocol while wearing coveralls ( ~ 1-Clo). On Day 1, participants completed morning (AM1) and afternoon (PM1) work bouts separated by a 1 h lunch recovery in a cooled environment (22°C); on Day 2, a morning bout (AM2) was completed. Work consisted of treadmill walking at ~200 W·m-2 in 26°C wet-bulb globe temperature until IST, followed by work-rest cycles starting with a 15-min rest and 45 min of work, for up to 240 min or volitional fatigue.
RESULTS: IST was shorter in PM1 than AM1 but did not differ between mornings. Females reached 38.0°C faster than males, with older females showing the shortest IST. Work-rest cycles maintained mean rectal temperature ≤38.3°C in >90% of bouts, with no sex- or age-related differences in cumulative time above 38.0°C. Cardiovascular strain was higher during PM1, while perceptual measures did not reliably track physiological strain.
CONCLUSION: Continuous work to IST was reduced during afternoon work following morning work but was restored the next morning across sex and age groups. When applied after IST, prescribed work-rest cycles effectively maintained core temperature near safety limits. These findings highlight the need to consider work timing and worker characteristics when planning work in hot environments.
METHODS: Thirty young (18-30 years, n = 15 females) and thirty older (50-69 years, n = 15 females) habitually active, non-heat-acclimatized adults completed a 1.5-day simulated work protocol while wearing coveralls ( ~ 1-Clo). On Day 1, participants completed morning (AM1) and afternoon (PM1) work bouts separated by a 1 h lunch recovery in a cooled environment (22°C); on Day 2, a morning bout (AM2) was completed. Work consisted of treadmill walking at ~200 W·m-2 in 26°C wet-bulb globe temperature until IST, followed by work-rest cycles starting with a 15-min rest and 45 min of work, for up to 240 min or volitional fatigue.
RESULTS: IST was shorter in PM1 than AM1 but did not differ between mornings. Females reached 38.0°C faster than males, with older females showing the shortest IST. Work-rest cycles maintained mean rectal temperature ≤38.3°C in >90% of bouts, with no sex- or age-related differences in cumulative time above 38.0°C. Cardiovascular strain was higher during PM1, while perceptual measures did not reliably track physiological strain.
CONCLUSION: Continuous work to IST was reduced during afternoon work following morning work but was restored the next morning across sex and age groups. When applied after IST, prescribed work-rest cycles effectively maintained core temperature near safety limits. These findings highlight the need to consider work timing and worker characteristics when planning work in hot environments.