Discrepancy between microcellular stress and macrophysiological responses in humid heat: A systematic review.

PURPOSE: This systematic review evaluates cross-level physiological responses to humid-heat stress and proposes the "Stress Decoupling" framework to explain the dissociation between microcellular signals and macro-functional outputs.

METHODS

Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, five databases were searched through December 2025. A two-tier synthesis integrated studies reporting both micro-biochemical (e.g., heat shock protein 72 (HSP72), interleukin-6 (IL-6) and macrophysiological (e.g., heart rate (HR), rating of perceived exertion (RPE)) markers, alongside evidence on performance thresholds and adaptive interventions.

RESULTS

Sixteen studies met the inclusion criteria. Synthesis revealed a non-linear, humidity-mediated response pattern, identifying 60% relative humidity (RH) as a pivotal physiological inflection point. At this threshold, microcellular stress markers (particularly plasma free HSP72) exhibited a disproportionate surge despite constant external workloads, while macrophysiological indicators remained relatively stable or linear-a phase termed the "Decoupling Window" regulated by the central governor system. However, under extreme conditions (e.g., 41 °C, 98% RH), this buffer collapses into "Accelerated Synchronization," where subjective fatigue and peripheral metabolic strain converge, forcing systemic failure.

CONCLUSIONS

Humid-heat stress triggers a hierarchical defense mechanism where cellular alarms precede systemic functional decline. The 60% RH threshold serves as a critical early-warning boundary. Understanding this decoupling window is essential for refining safety protocols and optimizing performance interventions in elite sports and occupational settings confronted with escalating thermal challenges.