Initial Stay Time and Work-Rest Scheduling During Consecutive Heavy-Intensity Workdays in Hot Environments in Young and Older Males.

INTRODUCTION: This study examined initial stay times (IST), defined as the duration of uninterrupted work before core temperature approaches recommended safety limits (38.0°C, ~1.0°C above baseline), and the efficacy of work-rest cycles during repeated heavy work across consecutive workdays in young and older males. METHODS: Twenty-one active, non-heat-acclimated males (10 young: 22 [20-25] y; 11 older: 67 [63-68] y) completed a 1.5-day work protocol wearing coveralls (~1 Clo). Participants performed treadmill walking at ~260 W·m-2 in 26°C WBGT during morning (AM1) and afternoon (PM1) bouts on Day 1, separated by a 1-h lunch break, followed by a morning bout (AM2) on Day 2. RESULTS: After reaching IST, ACGIH-recommended work-rest cycles (30-min work/30-min rest) were implemented for up to 240 min while core temperature was monitored. IST was shorter during PM1 than AM1, reflecting elevated baseline core temperatures entering the afternoon work period (median [Q1, Q3]: AM1: 61 [44, 75] min; PM1: 29 [24, 34] min). Following overnight recovery, IST returned to morning baseline values on Day 2 (AM2: 62 [47, 111] min). Age did not significantly influence IST across work periods. After reaching IST, prescribed work-rest cycles maintained core temperature near safety limits (≤ 38.1°C) across work periods, although older males exhibited slightly higher peak values. CONCLUSION: We show that the duration of the initial work bout before reaching recommended safety limits is reduced during afternoon work but returns to baseline after overnight recovery. Once implemented, prescribed work-rest cycles effectively regulate thermal strain across work periods in young and older males.