Assessment of Initial Stay Time and Work-Rest Scheduling Over Consecutive Moderate-Intensity Workdays in Hot...

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.

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.

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.

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.