The purpose of this study was to compare heart rate recovery (HRR) after a maximal treadmill (MAX-TM) and three-minute all-out (3MT) test between firefighters (FF) and a control (CON) group. Nine male CON and nine male FF participants completed height (m), weight (kg), body fat percent (BF%), normalized handgrip (GRIP_NORM, kg/kg), and MAX-TM with direct gas analysis to capture aerobic capacity (VO_2PEAK, mL/kg/min). A shuttle-sprint 3MT was used to measure critical velocity (CV, m/s) and D′ (m). Non-linear models determined HR decay (HRR_τ), HR asymptote (HR_∞), and HR amplitude (HR_amp). Two-way GROUP (FF vs. CON) by TEST (MAX-TM vs. 3MT) repeated measures ANOVAs indicated a significant TEST (F = 7.004, p = 0.018) effect on HR_amp. When divided by VO_2PEAK classification (FITNESS), a significant TEST effect was observed (F = 7.661, p = 0.014) on HR_amp. VO_2PEAK was significantly related to CV (r = 0.583, p = 0.011), GRIP_NORM (r = 0.668, p = 0.002), and BF% (r = −0.890, p < 0.001). Complete autonomic nervous system recovery may depend on the intensity of task demands and cardiorespiratory fitness.

Background: The purpose was to determine the ventilatory and metabolic demands in percentage-based heart rate (HR) zones in active-duty firefighters. Methods: Male career firefighters (n = 48, 38.17 ± 9.02 years, 1.79 ± 0.05 m, 88.27 ± 12.50 kg) completed a maximal treadmill test while wearing chest strap monitors to measure physiological responses corresponding to Zone 1 (50–59%), Zone 2 (60–69%), Zone 3 (70–79%), Zone 4 (80–89%), and Zone 5 (90–100%) based on age-predicted maximal HR. Aerobic capacity (VO_2PEAK, mL·kg⁻¹·min⁻¹), average minute ventilation (V_E, L·min⁻¹), and respiratory exchange ratio (RER) in each zone was measured via indirect calorimetry. Linear mixed models determined significant differences in V_E, RER, and time in zone (min). Results: Significant relationships emerged between VO_2PEAK and average RER in Zone 5 (r = −0.33) and time in Zone 3 (r = 0.45), Zone 4 (r = 0.41), and Zone 5 (r = 0.41). A significant HR zone effect emerged in V_E (F = 516.01, p < 0.001) indicating that V_E increased as zone intensity increased. After controlling for VO_2PEAK, a significant HR zone effect emerged in RER (F = 11.90, p < 0.001), indicating that average RER increased as zone intensity increased. No HR zone effect was found for time in zone (F = 1.18 p = 0.332) after controlling for VO_2PEAK. Conclusions: A practical cardiovascular workload measure, such as percentage-based HR zones determined from treadmill testing, have distinct ventilatory and metabolic responses. Higher aerobic capacity is related to greater time spent working in higher HR zones.