Longitudinal Assessment of Morning Hemodynamics, Hydration, and Perceived Stress in Fire Academy Trainees

Introduction: Fire academy training exposes firefighter (FF) recruits to substantial physical and psychological demands. The purpose of this study was to characterize weekly patterns in morning resting hemodynamics, hydration, and perceived stress (PS) across a 6-month fire and emergency medical technician (EMT) academy and to determine whether the sequence of training (EMT-first vs fire-first) influenced trajectories. Methods: Thirty-four FF recruits from two concurrently enrolled cohorts completed academy curricula in opposite sequences. Class A ( n = 17) completed EMT training before fire training, whereas Class B ( n = 17) completed fire training first. Daily morning measures included bodyweight, urine color (hydration), resting heart rate, respiratory rate, systolic and diastolic blood pressure (BP), and PS. Data were analyzed using linear mixed-effects models, nonlinear growth models, and phase-based comparisons (EMT vs fire), with random intercepts for individuals. Results: Morning measures demonstrated significant ( P < 0.05) week-to-week variability. Early training weeks and high-demand fire training periods elicited the greatest fluctuations in the measures. In Class A, significant temporal effects were observed for urine concentration, resting heart rate, respiratory rate, PS, and systolic BP (all P < 0.05), whereas Class B exhibited significant week effects primarily for PS and BP. Growth models indicated that nonlinear models fit PS and BP better than linear models ( P < 0.05), with high conditional R 2 values (0.48–0.99) reflecting substantial interindividual variability. Phase-based analyses revealed class × phase interactions for PS ( P = 0.001) and systolic BP ( P = 0.015). During fire training, Class A reported lower PS ( P = 0.038) and exhibited a reduction in systolic BP from EMT to fire training ( P < 0.001), whereas Class B showed no comparable phase-related change. Conclusion: Fire academy morning resting hemodynamics, hydration, and PS exhibited dynamic, nonlinear, and individualized patterns across academy training. The training sequence may contribute to distinct response patterns. These findings highlight the value of morning monitoring to identify periods or individuals exhibiting persistently elevated measures.