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Heat Loss Is Impaired in Older Men on the Day after Prolonged Work in the Heat

NOTLEY, SEAN R.; MEADE, ROBERT D.; D’SOUZA, ANDREW W.; FRIESEN, BRIAN J.; KENNY, GLEN P.

Medicine & Science in Sports & Exercise: September 2018 - Volume 50 - Issue 9 - p 1859–1867
doi: 10.1249/MSS.0000000000001643
APPLIED SCIENCES
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Purpose Prolonged work in the heat may exacerbate the rise in core temperature on the next work day, especially in older workers who display impairments in whole-body heat loss that increase body heat storage and core temperature relative to young adults during heat stress. We therefore evaluated whether whole-body heat loss in older adults was impaired on the day after prolonged work in the heat.

Methods Whole-body heat exchange and heat storage were assessed in nine older (53–64 yr) men during three 30-min bouts of semirecumbent cycling at fixed rates of metabolic heat production (150 [Ex1], 200 [Ex2], 250 W·m−2 [Ex3]), each separated by 15-min recovery, in hot-dry conditions (40°C, 20% relative humidity), immediately before (day 1), and on the day after (day 2) a prolonged, work simulation (~7.5 h) involving moderate-intensity intermittent exercise in hot-dry conditions (38°C, 34% relative humidity). Total heat loss (evaporative ± dry heat exchange) and metabolic heat production were measured using direct and indirect calorimetry, respectively. Body heat storage was quantified as the temporal summation of heat production and loss.

Results Total heat loss (mean ± SD) during Ex1 did not differ between days 1 and 2 (151 ± 15 and 147 ± 14 W·m−2, respectively; P = 0.27), but was attenuated on day 2 during Ex2 (181 ± 15 W·m−2) and Ex3 (218 ± 16 W·m−2) relative to day 1 (192 ± 14 and 230 ± 19 W·m−2, respectively; both P < 0.01). Consequently, body heat storage throughout the protocol on day 2 (276 ± 114 kJ) was 31% greater than on day 1 (191 ± 87 kJ; P < 0.01).

Conclusions Prolonged work in the heat causes next-day impairments in whole-body heat loss, which exacerbate heat storage and may elevate the risk of heat injury on the following day in older workers.

Human and Environmental Physiology Research Unit, School of Human Kinetics, University of Ottawa, Ottawa, CANADA

Address for correspondence: Dr. Glen P. Kenny, Ph.D., University of Ottawa, School of Human Kinetics, 125 University, Room 367, Montpetit Hall, Ottawa, Ontario, Canada K1N 6N5; E-mail: gkenny@uottawa.ca.

Submitted for publication February 2018.

Accepted for publication April 2018.

© 2018 American College of Sports Medicine