Author(s): SKINNER, B., PHILPOT, G. 1; SCHLADER, Z. 2; LUCAS, R 1., Institution: UNIVERSITY OF BIRMINGHAM, Country: UNITED KINGDOM, Abstract-ID: 2763

INTRODUCTION:
Measurement and monitoring of core body temperature (TCore) in field-based settings is fundamental to understanding the magnitude and consequences of excessive heat exposure. Rectal temperature (TR) and gastrointestinal temperature (TGI) are accurate and well-correlated indices of TCore in laboratory-based studies yet are either incompatible (TR) or have logistical and interpretive challenges (TGI) with use in the field. Subsequently, estimating TCore from continuous heart rate measures (TECT) is an attractive alternative. However, it is unclear how typical in situ responses to heat stress such as self-pacing, fluid intake and rest influence TGI and TECT measures. Therefore, this study aims to examine the influence of a rest and hydration intervention on TGI, TR, and TECT indices of TCore during self-paced work in the heat.
METHODS:
In a pseudo-randomised crossover order, 8 participants (23 ± 2 yrs; 5 male; VO2max: 45.7 ± 7.7 mL/kg/min) completed two 2-h cycling time trials in hot conditions (38°C, 30% relative humidity), one with a mandatory rest (REST; 10-min seated rest after 1-h, access to cool water [~8°C]) and one without (NO-REST; no mandatory rest, access to room temperature water [~33°C]). In both trials participants were asked to cycle as far as possible while maintaining an RPE of 12-14 and could rest at any time with water available by dismounting the bike. TCore indices were continuously measured via rectal, gastrointestinal (pill ingestion ~54 min prior by design) and estimated from continuous heart rate measures. Paired t-tests (REST vs. NO-REST) and two-way RM ANOVAs (trial [REST vs. NO-REST] x core [TR vs. TGI vs. TECT]) were performed.
RESULTS:
Distance covered was greater in NO-REST compared to REST (41.7 ± 6.1 km vs. 39.1 ± 5.0 km; p<.01), although mean power output when cycling was similar (NO-REST: 101 ± 37 W, REST: 99 ± 34 W; p=.20). Maximum TCore differed between TCore indices (core: p<.01), irrespective of trial (trial: p=.10; core x trial: p=.98). Maximum TECT was higher than TGI (+0.58 ± 0.44 °C; p<.01) and TR (+0.39 ± 0.40 °C; p=.02), but was similar between TGI and TR (p=.40). Average TCore differed between TCore indices (core: p<.01), irrespective of trial (trial: p=.10; core x trial: p=.29). Average TGI was lower than TR (−0.38 ± 0.37 °C; p=.05) and TECT (−0.69 ± 0.55 °C; p<.01), with no difference between TR and TECT (p=.12). Following drinking, TGI nadir was similar between trials (p=.13), decreasing below resting TGI by −0.7 ± 1.0 °C and −1.3 ± 1.2 °C during NO-REST and REST, respectively.
CONCLUSION:
Despite fluid consumption strongly influencing measures, maximal TCore appeared to be reliably captured via TGI. Conversely, while TECT overestimated maximal TCore, average TECT matched TR during exercise in the heat. This introduces interpretive considerations when conducting field-based heat strain assessments, as the assessment’s aim (i.e., cumulative strain vs. exertional heat stress risks) may determine the most appropriate measurement method.