HEART RATE DYNAMICS DURING AND AFTER SIMULATED FIRE GROUND TASKS: EFFECTS OF PHYSICAL FITNESS AND TRAINING

Firefighting is a strenuous occupation that increases the risk of cardiovascular events. Greater levels of physical fitness and training improve firefighters’ occupational performance, but little is known whether they are related to lower physiological stress and recovery from fire ground tasks. Heart rate, heart rate recovery and heart rate variability have been used to evaluate physical stress in association with exercise and fire ground tasks. However, there is a paucity of research evaluating the effects of physical fitness and training on heart rate dynamics during a simulated fire ground test (SFGT). Therefore, the primary purposes of this study were to evaluate the relationships between heart rate dynamics during a SFGT and to determine the effects of physical characteristics, fitness and physical training on these measures. The secondary purpose was to assess the relationship between maximal pace SFGT time and heart rate responses from a standardized pace SFGT. This information will help to understand the relationship between occupational performance and level of physical stress associated with work on the fire ground. Twenty-one firefighter academy recruits (Age = 28.4 ± 4.0 yr; Height = 177.1 ± 6.9 cm; Body mass = 88.3 ± 15.4 kg) participated in this cross-sectional and longitudinal study. The subjects completed a battery of physical fitness tests, including a 1.5-mile run, maximal pull-ups, push-ups, sit-ups, and isometric plank hold. In addition, the subjects completed a standardized pace SFGT that consisted of typical fire ground tasks performed in succession, without recovery (i.e., high-rise pack carry, hose drag, equipment carry, ladder raise, forcible entry, search, and victim rescue tasks). Heart rate variability was measured pre- and post-SFGT, and heart rate and 60 s heart rate recovery were measured during and post-SFGT, respectively. After a 10 wk physical training intervention, composed of approximately four physical training sessions per week, the measurements were repeated. A subsample of the original cohort (n = 11) also completed a maximal pace SFGT where their completion time was used as a measure of work capacity. Independent variables for this study included the physical and fitness test measures, physical training and maximal pace SFGT completion time. Dependent variables for this study were mean heart rate reserve during the SFGT (HRRes), difference between resting and mean heart rate during SFGT (HRSFGT-Rest), 60-second heart rate recovery (HRR60), and the difference between resting and post-SFGT root mean square of standard deviation between consecutive heart beats in logarithmic scale (LnRMSSDPost-Rest) measured with standardized pace SFGT. Independent and dependent variables were obtained at baseline and after physical training. Pearson r correlation coefficient was used to evaluate associations between outcome measures. Dependent samples t-test was used to compare differences in outcome measures at baseline and following physical training. Linear regression was used to evaluate the association between independent and dependent variables with standardized pace SFGT at baseline. Linear regression was used to assess the relationship between maximal pace SFGT time and outcome measures during the standardized pace SFGT. The level of significance was set as p ≤ 0.05 for all statistical analyses. In the standardized pace SFGT there was a positive correlation between HRSFGT-Rest and HRRes (R = .79, p \u3c .01). LnRMSSDPre-Post was positively correlated with HRRes (R = .47; p \u3c .05) HRSFGT-Rest (R = .70; p = .01) and HRPost-Rest (R = .84; p \u3c .01). There was also a significant inverse correlation between HRPost-Rest and HRR60 (R = -.46; p \u3c .05). Greater height and fat-free mass were favorably associated with HRSFGT-Res (R2 = .57; p = .02), HRRes (R2 = .55; p = .003) and HRPost-Rest (R2 = .59; p = .02). Height was also associated with attenuated depression of LnRMSSDRest-Post. After accounting for the effect of other independent variables in the multiple linear regression models, height was the only significant predictor for HRSFGT-Rest (β = -.90), HRRes (β = -.64), HRPost-Rest (β = -.76) and LnRMSSDRest-Post (β = -.06). A significant decrease in HRRes (80.1 ± 6.7% vs. 76.0 ± 6.7%; p \u3c 0.001) and increase in HRR60 (29.4 ± 8.3 b∙min-1 vs. 37.8 ± 9.0 b∙min-1; p \u3c .001) were observed after the physical training intervention. There was a significant correlation between maximal pace SFGT time and the standardized pace SFGT HRR60 (R = -0.70; p = 0.02), but not with other dependent variables. In summary, these findings indicate that greater cardiovascular demand during fire ground tasks is related to greater physical stress and lower parasympathetic activity during recovery. Greater height and fat-free mass as well as physical training are associated with lower physiological stress and accelerated recovery from fire ground tasks. Support for relationships between work capacity and heart rate dynamics during a standardized submaximal pace SFGT is limited, indicating that independent SFGT conditions may be necessary to provide work capacity and health information, respectively