Key result
Long-term heat acclimatisation resulted in diminished autonomic excitability compared to short-term acclimatisation, evidenced by a 9.9% increase in logRMSSD and an 18.6% decrease in logLF:HF.
Why the study?
Does structured natural heat acclimatisation alter heart rate variability and plasma nephrines during heat tolerance tests in healthy male military personnel?
Observational (n=13)
No
Does structured natural heat acclimatisation alter heart rate variability and plasma nephrines during heat tolerance tests in healthy male military personnel?
p-value: p=<0.05
Heart rate variability and plasma nephrines demonstrate ongoing autonomic nervous system excitability during the transition from short-term to long-term heat acclimatisation, suggesting HRV may aid in real-time assessment of readiness for activity in the heat.
PURPOSE: Heat adaptation (HA) is critical to performance and health in a hot environment. Transition from short-term heat acclimatisation (STHA) to long-term heat acclimatisation (LTHA) is characterised by decreased autonomic disturbance and increased protection from thermal injury. A standard heat tolerance test (HTT) is recommended for validating exercise performance status, but any role in distinguishing STHA from LTHA is unreported. The aims of this study were to (1) define performance status by serial HTT during structured natural HA, (2) evaluate surrogate markers of autonomic activation, including heart rate variability (HRV), in relation to HA status. METHODS: peak) during STHA (Day 2, 6 and 9) and LTHA (Day 23). Core temperature (Tc) and heart rate (HR) were measured every 5 min. Sampling for HRV indices (RMSSD, LF:HF) and sympathoadrenal blood measures (cortisol, nephrines) was undertaken before and after (POST) each HTT. RESULTS: Significant (P < 0.05) interactions existed for Tc, logLF:HF, cortisol and nephrines (two-way ANOVA; HTT by Day). Relative to LTHA, POST results differed significantly for Tc (Day 2, 6 and 9), HR (Day 2), logRMSSD (Day 2 and Day 6), logLF:HF (Day 2 and Day 6), cortisol (Day 2) and nephrines (Day 2 and Day 9). POST differences in HRV (Day 6 vs. 23) were + 9.9% (logRMSSD) and - 18.6% (logLF:HF). CONCLUSIONS: Early reductions in HR and cortisol characterised STHA, whereas LTHA showed diminished excitability by Tc, HRV and nephrine measures. Measurement of HRV may have potential to aid real-time assessment of readiness for activity in the heat.
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Stacey et al. (2017) conducted an observational in Heat acclimatisation (n=13). Structured natural heat acclimatisation vs. Short-term heat acclimatisation (Day 2/6) was evaluated on Autonomic activation markers (HRV, cortisol, nephrines) and physiological strain (core temperature, heart rate) post-heat tolerance test (p=<0.05). Long-term heat acclimatisation resulted in diminished autonomic excitability compared to short-term acclimatisation, evidenced by a 9.9% increase in logRMSSD and an 18.6% decrease in logLF:HF.
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