Heart rate dynamics during a simulated fireground test: The influence of physical characteristics and fitness 1

Abstract

BACKGROUND:

Firefighting is a physiologically demanding occupation and there is a need to evaluate physical and fitness characteristics that are related to attenuated physiological stress during fireground tasks. Previous studies have not measured associations between heart rate responses during simulated fireground tasks with a standardized work rate.

OBJECTIVE:

The purpose of this study was to examine associations between heart rate during a standardized pace simulated fireground test (SFGT) and heart rate recovery and variability following the SFGT. In addition, this study sought to evaluate associations between heart rate measures versus physical and fitness characteristics in structural firefighter recruits.

METHODS:

Twenty-one fire academy recruits performed a standardized pace SFGT while mean heart rate reserve (HR_Res) during the SFGT, change in heart rate variability from rest to post-SFGT (LnRMSSD_Rest-Post), and 60-second post-SFGT heart rate recovery (HRR₆₀) were measured. Regression analyses were conducted between HR_Res, LnRMSSD_Rest-Post and HRR₆₀ and between heart rate measures versus physical and fitness characteristics while accounting for differences in SFGT completion time.

RESULTS:

HR_Res was associated with LnRMSSD_Rest-Post, but not HRR₆₀. Height and pull-ups explained most of the variance in HR_Res, height explained most of the variance in LnRMSSD_Rest-Post and push-ups and 1.5-mile run explained most of the variance in HRR₆₀.

CONCLUSION:

Greater cardiovascular stress during fireground tasks is associated with greater depression of post-SFGT heart rate variability, but not heart rate recovery. Physical and fitness characteristics are important to consider in relation to firefighters’ ability to cope with physical stress on the fireground.

Keywords

Cardiovascular system firefighters first responders physical functional performance workload

1 Introduction

Firefighting is a physically demanding occupation that induces significant physiological stress during fireground tasks [1]. Previous studies have shown that firefighters experience periodic events of high physiological stress during a shift, and many of these stressful experiences are on the fireground [2]. The high physiological demand, coupled with psychological and environmental stressors are associated with a greater risk of on-duty cardiac events [3]. Elevated cardiac stress occurs during and after fireground tasks [4, 5], which may partially explain a disproportionately greater prevalence of cardiovascular events during and after fireground events compared to all other times on-duty [3]. Previous studies utilizing bona fide and simulated fireground operations have demonstrated that firefighters perform fireground tasks at intensities ranging from light-to-vigorous levels and durations ranging from minutes to hours [5, 6]. The high variability in work duration and intensity of physiological stress indicates that firefighters are not only required to complete high intensity tasks effectively but also to sustain lower intensity tasks for extended time periods [6, 7]. Thus, there is a need to evaluate firefighters’ ability to physiologically cope with the rigorous demands of fireground tasks during and after occupational operations.

Observing heart rate during and following occupational tasks provides an assessment of firefighters’ cardiovascular stress and may indicate whether the work rate during the tasks is physiologically sustainable [4 , 9]. In addition, heart rate derived measures of parasympathetic nervous system activity after fireground tasks may provide further insights regarding firefighters’ ability to cope with occupational physical demands [9]. Previous research has indicated that post-exercise heart rate recovery and heart rate variability may be used to assess parasympathetic regulation of cardiac rhythm [10, 11]. Since greater parasympathetic modulation of heart rate provides protection from cardiovascular events [12], greater heart rate recovery and heart rate variability after fireground tasks reflect a more favorable physiological state during recovery for firefighters. Research has indicated that elevated heart rate and depressed heart rate variability occur during and after simulated fireground tasks, respectively [4 , 13–16]. Previous studies have also indicated that heart rate variability derived indices have indicated worse initial recovery after a shift performing rescue duties compared to a shift performing ambulance and emergency services [2]. However, it is unknown whether the magnitude of cardiovascular stress while performing fireground tasks acutely is associated with heart rate recovery and heart rate variability during short term recovery.

Previous studies demonstrated that greater exercise intensity, and thus cardiovascular demand, resulted in greater depression in heart rate variability [17 –19] and indicators of greater training load during running workouts have also been associated with lower post-exercise heart rate variability [20]. In addition, research on other tactical populations indicates that the magnitude of stress may relate to the greater depression of heart rate variability after the stressor has been removed [21]. However, there is a paucity in research assessing whether the magnitude of cardiovascular stress during fireground tasks results in greater depression in heart rate variability during initial recovery from the tasks. Similarly, heart rate recovery has been shown to vary due to differences in exercise intensity [17 , 22], but these differences have not been observed after performing fireground tasks. Therefore, it is unknown whether heart rate recovery is associated with the magnitude of cardiovascular stress accumulated during fireground tasks. Obtaining information about the association between indices of cardiovascular stress during versus after fireground tasks would be beneficial due to greater feasibility of conducting assessments on the field during recovery and for obtaining further information about the effects of the level of cardiovascular stress during fireground tasks on the cardiac parasympathetic activity during recovery. This information would also be insightful for a more comprehensive understanding of factors that may influence firefighters’ autonomic nervous system activity while recovering from fireground tasks.

Greater psychological resilience is associated with greater heart rate variability during different fireground tasks [23]. However, there is a lack of research evaluating the relationship of physical characteristics and fitness parameters on heart rate measures. Although attenuating cardiovascular stress during firefighting would be beneficial for the health and safety of firefighters, assessing the effects of associated factors (i.e., physical and fitness characteristics) with indices of cardiovascular stress should be conducted using methodological approaches that control over confounding variables. Evaluating differences in cardiovascular responses during fireground tasks between firefighters may require the tasks to be performed at a similar absolute work rate. Several previous studies have shown that firefighters who are taller and have greater neuromuscular and cardiovascular fitness are able to complete fireground tasks faster [24 –26]. However, differences in work rate between subjects in these studies confounds the comparison of relative cardiovascular stress between firefighters of various fitness levels. In support of this notion, Dennison et al. [27] reported that trained and untrained firefighters had similar cardiovascular responses to simulated fireground tasks although the trained firefighters completed the tasks faster. Marcel-Millet et al. (2020) also did not find differences in heart rate during and heart rate variability after simulated fireground tasks between more and less fit firefighters, but again more fit firefighters performed the tasks faster than the less fit firefighters. Thus, we propose that utilizing a standardized work rate during fireground tasks may be more appropriate for assessing differences in physiological stress between firefighters. Other researchers have evaluated physiological stress while allowing subjects to perform tasks at a self-selected pace [28, 29]. These studies demonstrated that greater maximal aerobic capacity was associated with lower relative physiological stress during the tasks [28, 29]. However, the authors did not report additional physical or fitness characteristics that may be associated with lower physiological stress during fireground tasks. These studies also did not assess any markers of physiological stress during recovery. Thus, further research is warranted to identify associations between physical and fitness characteristics versus markers of physiological stress during and after fireground tasks.

The purpose of this study was to evaluate associations between heart rate during a standardized pace simulated fireground test (SFGT) versus heart rate variability and heart rate recovery after the SFGT. Secondly, this study sought to evaluate associations between physical and fitness characteristics against these heart rate measures. It was hypothesized that greater magnitude of cardiovascular stress (i.e., heart rate) during the SFGT would be associated with a greater depression heart rate variability and slower heart rate recovery during recovery. It was also hypothesized that greater stature as well as greater cardiovascular and muscular fitness would be associated with favorable cardiovascular responses during and after the SFGT. These findings may help practitioners to evaluate heart rate measures after fireground tasks more appropriately and identify physical and fitness characteristics that are beneficial for reducing the occupational physical stress.

2 Materials and methods

2.1 Experimental design

This cross-sectional study evaluated associations between heart rate measures in response to a standardized pace SFGT. Specifically, mean heart rate reserve during (HR_Res), 60-second heart rate recovery after (HRR₆₀), and the change in heart rate variability from rest to after the SFGT (LnRMSSD_Rest-Post) were evaluated. In addition, associations between physical and fitness characteristics versus the above-mentioned heart rate measures were evaluated. Fitness characteristics were obtained from a fire academy administered Physical Fitness Ability Test (PFAT). The SFGT has been found to be a valid assessment for evaluating firefighters’ occupational physical ability [30] and heart rate measures have been previously evaluated in response to fireground tasks [13]. All procedures involving experiments on human subjects were done in accord with the ethical standards of the Committee on Human Experimentation of the institution (i.e., Institutional Review Board; IRB #: 45529; Date of approval: 8/28/2018) in which the experiments were done or in accord with the Declaration of Helsinki of 1964 and its later amendments or comparable ethical standards

2.2 Subjects

A convenience sample of 21 firefighter recruits (Males: N = 20; Female: N = 1) volunteered to participate in this study. Subjects’ physical characteristics are reported in Table 1. Subjects were metropolitan structural firefighter recruits who participated in the local fire department’s fire academy to become career firefighters. An informed consent was provided by the subjects and details of the study procedures and about their participation were reiterated verbally at the time of subject recruitment.

Table 1
Physical characteristics and fitness outcomes in 21 firefighter recruits

95% Confidence Interval

Variable Mean ± SD Lower Upper

Age (yr) 28 ± 4 27 30

Height (cm) 177.1 ± 6.9 174.0 180.3

Body mass (kg) 88.3 ± 15.4 81.3 95.3

Relative body fat (%) 20.7 ± 5.4 18.3 23.1

Fat-free mass (kg) 69.5 ± 8.7 65.5 73.4

Fat mass (kg) 18.8 ± 7.8 15.3 22.4

1.5-mile run (min) 12.9 ± 1.6 12.2 13.6

Push-ups (count) 44 ± 10 39 48

Pull-ups (count) 15 ± 9 11 19

Sit-ups (count) 50 ± 13 44 56

Plank time (min) 2.7 ± 0.9 2.3 3.1

				95% Confidence Interval
Age (yr)	28	±	4	27	30
Height (cm)	177.1	±	6.9	174.0	180.3
Body mass (kg)	88.3	±	15.4	81.3	95.3
Relative body fat (%)	20.7	±	5.4	18.3	23.1
Fat-free mass (kg)	69.5	±	8.7	65.5	73.4
Fat mass (kg)	18.8	±	7.8	15.3	22.4
1.5-mile run (min)	12.9	±	1.6	12.2	13.6
Push-ups (count)	44	±	10	39	48
Pull-ups (count)	15	±	9	11	19
Sit-ups (count)	50	±	13	44	56
Plank time (min)	2.7	±	0.9	2.3	3.1

A Physical Activity Readiness Questionnaire for Everyone (PAR-Q+) and medical history questionnaire were given to the subjects to exclude subjects who were diagnosed with cardiovascular, pulmonary, or other chronic diseases or pre-existing musculoskeletal injuries that would prevent them from safely participating in the research protocol. Subjects were cleared by their physician to participate in the academy training prior to volunteering for the study. Subjects were informed that they could withdraw from the study at any time without penalty or effect on their status in the academy.

2.3 Procedures

Two testing sessions were utilized in this study, which took place at the fire academy’s training center. The sessions were completed within two weeks of performing the PFAT. The first testing session included baseline anthropometric and body composition measurements. Standing height was taken without shoes using a portable stadiometer (Seca 213, Seca Corporation, CA) and body mass was taken in light clothing and without shoes using a digital scale (EB4074 C, Etekcity, CA). Body composition measurements were obtained with a tetrapolar bioelectric impedance (BIA) analyzer (Bodystat 1500, CA). Specifically, subjects laid supine while electrodes were placed on the wrist, hand, ankle, and foot. The manufacturer’s proprietary equation was used to estimate percent fat based on the subject’s age, sex, height, body mass, and single frequency (50 kHz) bioimpedance measures. Fat-free mass obtained from the BIA measurements was used to calculate absolute and relative body fat by subtracting fat-free mass from total body mass.

The second testing session consisted of subjects completing the SFGT and the associated measurements. Subjects’ hydration levels were measured via urine specific gravity (USG; PAL10 S, Atago, Tokyo, Japan) and were obtained 30 min prior to completing the SFGT. Specifically, the density of urine was measured against the density of distilled water and was reported as a ratio between them. Resting heart rate variability measurements were recorded indoors within 30 min of starting the SFGT. The measurements were taken with a digital two-lead electrocardiographic (ECG) chest monitor and associated analytics software (Omegawave Ltd, Espoo, Finland). Specifically, the subjects laid supine on a mat and avoided moving and talking and the measurement was initiated after the subjects’ heart rate had stabilized. Upon initiation of the measurement, the first 90 s were considered a rest period, and measurements taken during that time were not used for data processing. After the 90 s washout period, the device recorded time intervals between 100 R-waves for data processing. Subjects were advised not to eat or consume caffeine three hours prior to measurements and to abstain from exercise on the day of testing to minimize autonomic disturbances during the pre-SFGT measurement. After completion of the heart rate variability measurement, a heart rate monitor was placed on the subject’s chest (H10, Polar Inc., Finland), which was used for recording heart rate during and after the SFGT. The heart rate recording was initiated before the SFGT and completed within 5 min after the SFGT. Blood lactate was taken at rest prior to the SFGT and four minutes post-SFGT using a lactate analyzer (LactatePlus, Nova Biomedical, MA). The sample was obtained with a fingerstick technique and universal precautions for obtaining biological samples were utilized. Following the fingerstick, the first drop of blood was wiped away and the second drop of blood was used for analysis. The blood lactate analyzer’s accuracy was tested with low (1.0–1.6 mmol·L^-1) and high (4.0–5.4 mmol·L^–1) concentration control solutions. Rating of perceived exertion (RPE) and thermal sensation were obtained after the blood lactate measurement. A 0–10 (0 = rest; 10 = maximal exertion) category-ratio scale (CR-10) was used for RPE measurement, and the subject pointed to a number on a chart based on perception of effort before and 60 s after completing the SFGT. This scale has been used in previous studies to assess the level of exertion in occupational tasks [30 –32]. Subjective thermal sensations were recorded using a validated Omni Thermal Sensation Scale that utilized a 1–5 (1 = comfortable; 5 = Very hot) Likert type scale. The Omni scale has been found to be valid and reliable during exercise conditions [33]. Overall subjective feelings of anxiety were assessed using a 0–10 (0 = Not anxious at all; 10 = Extremely anxious) Likert type scale.

The SFGT was performed while wearing department issued personal protective equipment (PPE; approximate mass = 25 kg) including a helmet, a turnout coat and pants, gloves and boots. A self-contained breathing apparatus was worn during the SFGT, but its included mask was not worn for ventilation, as subjects were not trained on its use prior to testing. The SFGT was previously developed in collaboration with the academy’s training officers and the test was found to have excellent test-retest reliability (ICC = 0.93) [30]. The SFGT had been performed by a previous cohort of fire academy recruits with the same PPE at a maximal pace and resulted in a RPE of 6.3±1.5 (CR-10 scale), and mean heart rate of 84.3±5.0% of maximum [30]. The SFGT consisted of seven occupational tasks that included a stair climb, hose drag, equipment carry, ladder hoist, forcible entry, search, and victim rescue (Table 2). The time to complete each task and total time were paced based on findings from a previous study, which used a maximal pace protocol [30], where the time to complete the SFGT was set as more than 2 standard deviations slower (428 s) than a previously recorded mean time (331±39 s) with similar subjects. Pacing was used so that heart rate during the SFGT could be assessed between subjects to minimize potential variance due to differences in external work rate. Pacing of each task was facilitated by the primary investigator. The overall SFGT and specific task times were recorded using a smart phone stopwatch application (iPhone 6, Apple, San Francisco, CA).

Table 2
Simulated fireground test (SFGT) task descriptions and standardized completion times

Task Description Time to complete (s)

High-rise pack carry A dry hoseline packaged as a “highrise pack” (mass: 22.2 kg) was carried on the subjects’ outside shoulder up 4 flights of stairs (total of 68 steps) and placed the pack on the stairway landing. Subjects were allowed to skip as many steps as possible on the ascent and use handrails for balance. From the top level of the tower, subjects descended the stairs and were required to touch each step. 87.1

Hose drag Subjects proceeded 15.2 m and dragged a 30.48 m (4.45 cm diameter) hoseline by the nozzle for a distance of 25 m in a straight line. 41.2

Equipment carry Subjects proceeded 15.2 m and carried two 20 kg construction utility buckets 42 m. 70.6

Ladder raise Subjects proceeded 11.2 m to a structure wall and grasped a 4.27 m extension ladder and placed the ladder on the structure wall by grasping each rung of the ladder to lift it up against the wall. Then, subjects returned the ladder to the ground using the same technique. 25.4

Forcible entry The subject proceeded 4.4 m into a training structure and stepped onto Keiser Force Machine (Keiser Inc., Fresno, CA), which simulates breaching a structure with an axe. The device has a 72.5 kg steel I beam positioned on a tray between steel railings. The subject straddled the beam on the rails so that their feet were on both sides of the beam. The subject was required to strike the end of the I beam with a 4.1 kg sledgehammer (Trusty-Cook, Indianapolis, IN) until the beam traveled 1.5 m. 68.1

Search The subject proceeded 7 m inside the structure and up 17 stairs to the second floor and crawled on hands and knees along the perimeter of the room while the outside arm was in contact with the wall (i.e., right hand search). The task was completed when the subject had covered each side of the wall for a total distance of 35 m and proceeded down the same flight of stairs. 71.2

Victim carry The subject proceeded 15.6 m out of the structure to a concrete area and picked up a 73 kg, 1.83 m tall mannequin (Rescue Randy, Moore Medical, Chicago, IL) dressed in firefighter protective clothing by grasping the dummy underneath its arms and dragged it 27 m backwards. The SFGT course was completed when the mannequin completely crossed the finish line. 64.8

Total SFGT time 428.4

Task	Description	Time to complete (s)
High-rise pack carry	A dry hoseline packaged as a “highrise pack” (mass: 22.2 kg) was carried on the subjects’ outside shoulder up 4 flights of stairs (total of 68 steps) and placed the pack on the stairway landing. Subjects were allowed to skip as many steps as possible on the ascent and use handrails for balance. From the top level of the tower, subjects descended the stairs and were required to touch each step.	87.1
Hose drag	Subjects proceeded 15.2 m and dragged a 30.48 m (4.45 cm diameter) hoseline by the nozzle for a distance of 25 m in a straight line.	41.2
Equipment carry	Subjects proceeded 15.2 m and carried two 20 kg construction utility buckets 42 m.	70.6
Ladder raise	Subjects proceeded 11.2 m to a structure wall and grasped a 4.27 m extension ladder and placed the ladder on the structure wall by grasping each rung of the ladder to lift it up against the wall. Then, subjects returned the ladder to the ground using the same technique.	25.4
Forcible entry	The subject proceeded 4.4 m into a training structure and stepped onto Keiser Force Machine (Keiser Inc., Fresno, CA), which simulates breaching a structure with an axe. The device has a 72.5 kg steel I beam positioned on a tray between steel railings. The subject straddled the beam on the rails so that their feet were on both sides of the beam. The subject was required to strike the end of the I beam with a 4.1 kg sledgehammer (Trusty-Cook, Indianapolis, IN) until the beam traveled 1.5 m.	68.1
Search	The subject proceeded 7 m inside the structure and up 17 stairs to the second floor and crawled on hands and knees along the perimeter of the room while the outside arm was in contact with the wall (i.e., right hand search). The task was completed when the subject had covered each side of the wall for a total distance of 35 m and proceeded down the same flight of stairs.	71.2
Victim carry	The subject proceeded 15.6 m out of the structure to a concrete area and picked up a 73 kg, 1.83 m tall mannequin (Rescue Randy, Moore Medical, Chicago, IL) dressed in firefighter protective clothing by grasping the dummy underneath its arms and dragged it 27 m backwards. The SFGT course was completed when the mannequin completely crossed the finish line.	64.8
Total SFGT time	428.4

Immediately upon completion of the SFGT, subjects were instructed to sit down and avoid moving and talking to allow for a standardized heart rate recovery measurement. For the post-SFGT heart rate variability measurement, the subject walked approximately 50 m to an indoor testing location. Post-SFGT heart rate variability was measured 8 min after completing the SFGT. Previous studies have measured post-exercise heart rate variability at varying time points (minutes to hours) post-activity [34, 35]. For our study purposes, it was important to measure heart rate variability at a standardized time when the initial rapid recovery of heart rate had plateaued enough so that changes in heart rate during the measurements would not affect the heart rate variability measurement [36]. It has been observed that heart rate does not change at a rate that would affect recordings after the first few minutes after exercise or simulated firefighting [37, 38]. Heart rate was also observed during the heart rate variability assessment and confirmed that heart rate remained stable throughout the recording period. Weather conditions, including ambient temperature and relative humidity, at the time of the SFGT were obtained from an online weather forecast service [39].

Mean and peak heart rate (HR_Mean and HR_Peak) were obtained between the initiation and completion of the SFGT. Mean heart rate reserve (HR_Res) was calculated for the SFGT using the following equation:

${(HR}_{Mean} {- HR}_{Rest} {) / (HR}_{Max} {- HR}_{Rest}) [6]$

Resting heart rate (HR_Rest) was obtained from the pre-SFGT heart rate variability measurement and estimated maximum heart rate (HR_Max) was acquired by using an age-predicted equation [208 – (0.7 x age)] [40] or HR_Peak obtained from the SFGT, whichever value was greater.

Heart rate recovery was calculated by subtracting the 60 s post-SFGT heart rate from the heart rate measured at the end of the SFGT. Root mean square of the successive differences between R-waves (RMSSD) was used as the heart rate variability measurement. The utilized analytics software (Omegawave Ltd, Espoo, Finland) that calculated the heart rate variability metrics provided the RMSSD values in their raw form, which were then transformed into natural logarithmic scale values (LnRMSSD) to account for non-normal data distribution [41]. The software automatically removed artifacts and ectopic ventricular depolarization from the recording [42]. The Omegawave System complies with the recommended guidelines for heart rate variability measurement and analysis established by the Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology [10], and it has been previously validated [42]. According to the Task Force, LnRMSSD is interpreted as the time-domain derived index of cardiac rhythm modulation by the parasympathetic nervous system [10]. Therefore, LnRMSSD_Rest-Post was used to evaluate the change in parasympathetic nervous system’s modulation of cardiac rhythm where greater LnRMSSD_Rest-Post was interpreted as greater depression of parasympathetic modulation of heart rate and worse recovery.

The academy’s PFAT was a mandatory requirement of the fire academy training, and the fitness test outcomes derived from the PFAT were used to obtain subjects’ fitness measures. The tests included a 1.5-mile run, push-ups, prone plank, sit-ups, and pull-ups. All subjects performed the PFAT during the same session. The subjects were instructed to perform as many repetitions as possible within a 2 min duration and rest for the push-ups, sit-ups, and pull-ups tests where rest was allowed between repetitions. A swinging motion was allowed when performing the pull-ups. Subjects were instructed to utilize correct form by supporting their bodyweight with their toes and forearms during the plank, and they were allowed to correct their position one time. The 1.5-mile run was performed on an outdoor (non-track) course. All tests included in the PFAT were administered by the same fire academy training officer.

2.4 Statistical analysis

Descriptive statistics were calculated as mean±standard deviation (SD) with 95% confidence intervals (CI). Correlations between weather conditions and heart rate measures were analyzed by calculating Pearson Product Moment correlation coefficients. Dependent samples t-tests were performed to compare differences in subjective and physiological responses between resting versus SFGT conditions. Five subjects were not able to complete the SFGT within 1 SD of the cohort’s mean pace and independent samples t-tests were used to compare these subjects’ SFGT and task times, characteristics, and psychophysiological responses to the SFGT against the subjects who completed the SFGT within the paced time. For the first purpose of the study, linear regression analyses were conducted to evaluate associations between HR_Res versus LnRMSSD_Rest-Post and HRR₆₀. For the second purpose of this study, multiple linear regression (MLR) analyses (backward elimination method) were conducted to evaluate associations between physical and fitness characteristics versus HR_Res, HRR₆₀, and LnRMSSD_Rest-Post. Time to complete the SFGT was used as a forced covariate in the MLR analyses to account for its influence on the outcomes. Adjusted coefficient of determination (R²) and was used to evaluate the performance of each MLR model and only significantly associated physical and fitness measures were kept in each regression model. Standardized coefficients (β) and unadjusted R² were used to interpret outcomes within each model. The level of significance was set at p < 0.05 for all statistical analyses except when multiple t-tests were performed it was adjusted to p < 0.01 to reduce the likelihood of Type 1 error (Table 3).

Table 3
Physiological and subjective responses to a simulated fireground test (SFGT) in 21 firefighter recruits

95% Confidence Interval

Variable Mean ± SD Lower Upper

SFGT time (s) 440.6 ± 42.5 421.2 460.0

HR-_Rest 70.5 ± 7.9 67.1 73.9

HR_SFGT^* 166.0 ± 11.5 161.0 171.0

^†HR_Post^* 99.0 ± 9.0 95.1 103.0

HR_Peak (b·min^–1)^* 182 ± 11 177 187

HR_Res (%) 79.7 ± 6.7 76.5 82.9

HRR₆₀ (b·min^–1) 29 ± 8 26 33

RPE_Pre (1–10) 1 ± 1 0 1

RPE_Post (1–10)^* 7 ± 2 6 8

Thermal_Pre (1–5) 1 ± 1 1 2

Thermal_Post (1–5)^* 3 ± 1 3 4

BL_Pre (mmo·l^–1) 1.3 ± 0.6 1.0 1.6

BL_Post (mmo·l^–1)^* 10.0 ± 2.7 8.8 11.2

LnRMSSD_Rest (AU) 3.69 ± 0.66 3.39 3.99

^†LnRMSSD_Post (AU)^* 1.77 ± 0.62 1.49 2.06

				95% Confidence Interval
SFGT time (s)	440.6	±	42.5	421.2	460.0
HR-_Rest	70.5	±	7.9	67.1	73.9
HR_SFGT^*	166.0	±	11.5	161.0	171.0
^†HR_Post^*	99.0	±	9.0	95.1	103.0
HR_Peak (b·min^–1)^*	182	±	11	177	187
HR_Res (%)	79.7	±	6.7	76.5	82.9
HRR₆₀ (b·min^–1)	29	±	8	26	33
RPE_Pre (1–10)	1	±	1	0	1
RPE_Post (1–10)^*	7	±	2	6	8
Thermal_Pre (1–5)	1	±	1	1	2
Thermal_Post (1–5)^*	3	±	1	3	4
BL_Pre (mmo·l^–1)	1.3	±	0.6	1.0	1.6
BL_Post (mmo·l^–1)^*	10.0	±	2.7	8.8	11.2
LnRMSSD_Rest (AU)	3.69	±	0.66	3.39	3.99
^†LnRMSSD_Post (AU)^*	1.77	±	0.62	1.49	2.06

^†One missing value (n = 20); *Significantly different from resting value (p < 0;001); HR_Peak: Peak heart rate during SFGT; HR_Res: Mean heart rate reserve during SFGT; RPE: Rating of perceived exertion; Thermal: Thermal sensation; BL: Blood lactate; LnRMSSD: Root mean square of the successive differences between heart beats in natural logarithmic scale; AU: Arbitrary unit; Pre: Before SFGT; Post: After SFGT.

3 Results

Variable weather conditions between subjects were observed but they were not correlated with outcome measures (r = 0.004 –0.257, p = 0.26 –0.99). Mean heart rate at rest was significantly lower versus heart rate during and after the SFGT (p < 0.001) (Table 3). Post-SFGT measures of LnRMSSD, blood lactate, RPE, and thermal sensation were significantly different from resting values (p < 0.001) (Table 3). The mean SFGT completion time was 12.2 s slower than the standardized pacing time (428.4 vs. 440.6 s). Five subjects could not complete the SFGT within one standard deviation of the cohort’s mean time, which caused most of this discrepancy. These subjects’ total time (511.1±26.14 vs. 418.6±7.3 s; p < 0.001), hose drag (56.3±16.8 vs. 42.5±10.8 s; p = 0.042), search (84.0±7.4 vs. 74.2±5.0 s; p = 0.003) and victim carry (127.2±21.8 vs. 52.13±4.5 s; p = 0.001) tasks were significantly slower compared to the remainder of the cohort. Comparison of physical and fitness measures revealed that the subjects who completed the SFGT slower than pacing time had significantly less fat-free mass (61.6±6.1 vs. 71.9±7.9 kg, p = 0.016), but all other measures were similar. SFGT time was inversely correlated with HRR₆₀ (r = –0.53, p = 0.014) but not with HR_Res or LnRMSSD_Rest-Post. Correlation analyses revealed several correlations between physical and fitness measures (Table 4). Notably, greater height and fat-free mass were related to inferior fitness outcomes.

Table 4
Correlation coefficients (r) between physical characteristics and fitness outcomes in 21 firefighter recruits.

Variable Height Body mass % BF FFM

Push-ups –0.35 –0.43 –0.51* –0.32

Plank –0.42 –0.53* –0.46* –0.47*

Sit-ups –0.29 –0.47* –0.40 –0.42

Pull-ups –0.56 –0.61 –0.46* –0.59**

1.5-mi run 0.05 0.41 0.29 0.38

Variable	Height	Body mass	% BF	FFM
Push-ups	–0.35	–0.43	–0.51*	–0.32
Plank	–0.42	–0.53*	–0.46*	–0.47*
Sit-ups	–0.29	–0.47*	–0.40	–0.42
Pull-ups	–0.56**	–0.61**	–0.46*	–0.59**
1.5-mi run	0.05	0.41	0.29	0.38

**Correlation is significant at the 0.01 level (2-tailed). *Correlation is significant at the 0.05 level (2-tailed); 95% CI: Upper and lower bounds of 95% confidence interval; % BF: Relative body fat; FFM: Fat-free mass.

For the first purpose of the study, regression analyses indicated that HR_Res and LnRMSSD_Rest-Post were associated (r² = 0.22, p = 0.035) where a one SD greater HR_Res was associated with a 0.47 SD greater LnRMSSD_Rest-Post (Table 5). Conversely, HR_Res was not associated with HRR₆₀ (r² = 0.15, p = 0.088). For the second purpose of the study, height (β=–0.95; p < 0.001) and pull-ups (β= –0.38; p = 0.048) were associated with HR_Res (r² = 0.65; p < 0.001); height (β= –0.75; p < 0.001) was associated with LnRMSSD_Rest-Post (r² = 0.56, p < 0.001); and push-ups (β= –0.43; p = 0.016) and 1.5-mi run (β= –0.41; p = 0.028) were associated with HRR₆₀ (r² = 0.56, p = 0.003) while accounting for the effect of SFGT completion time. SFGT time was significantly associated only with HRR₆₀ where a one SD slower SFGT-time was associated with 0.43 SD lower (i.e., worse) HRR₆₀.

Table 5

Linear regression determining associations between heart rate reserve (HR_Res) versus LnRMSSD_Rest-Post and HRR₆₀ in 21 firefighter recruits.

Outcome variable	r ²	p	β	Lower –
				Upper 95% CI
LnRMSSD_Rest-Post	0.22	0.035	0.47	0.04 –0.91
HRR₆₀	0.15	0.088	–0.38	–0.83–0.06

r²: coefficient of determination; p: probability of no association between variables; β: standardized coefficient; Lower – Upper 95% CI: β-values with 95% confidence; LnRMSSD_Rest-Post: Post-SFGT LnRMSSD subtracted from resting LnRMSSD; HRR₆₀: Sixty second heart rate recovery post-SFGT.

Table 6

Multiple linear regression determining associations between physical and fitness characteristics versus HR_Res, LnRMSSD_Rest-Post and HRR₆₀ in 21 firefighter recruits

Response variable	r ²	Adjusted r²	p-value	Predictor variables (β)
HR_Res	0.65	0.58	<0.001	Height (–0.95; p < 0.001)Pull-ups (–0.38; p = 0.048)SFGT time (0.01; p = 0.970)
LnRMSSD_Rest-Post	0.56	0.51	<0.001	Height (–0.75; p < 0.001)SFGT time (0.10; p = 0.955)
HRR₆₀	0.56	0.48	0.003	Push-ups (–0.44; p = 0.016)1.5-mi run (–0.41; p = 0.028)SFGT time (–0.43; p = 0.020)

r² = coefficient of determination; p = significance test value for model; β= standardized coefficient; HR_Res: Mean heart rate reserve during SFGT; LnRMSSD_Rest-Post: Post-SFGT LnRMSSD subtracted from resting LnRMSSD; HRR₆₀: Sixty second heart rate recovery post-SFGT.

4 Discussion

The primary purpose of this study was to evaluate associations between heart rate during versus heart rate variability and recovery after a standardized pace SFGT and associations between physical and fitness characteristics and these heart rate measures. Greater heart rate during the SFGT was associated with greater depression of heart rate variability post-SFGT, but not heart rate recovery. Several, physical and fitness characteristics were associated with heart rate related outcomes during and after the SFGT.

Descriptive findings indicated that the SFGT resulted in significant physiological stress in the firefighter recruits. Specifically, heart rate during and blood lactate and RPE after the SFGT indicated substantial physiological stress during the fireground tasks (Table 3). The physiological responses were lower than those reported by Lesniak et al. [30], which was expected due to the paced as opposed to maximal nature of the SFGT in the current study. Mean HR_Res was 79.7±6.7% which was within the range reported by Bos et al. [6] during bona fide fireground tasks (30.3 –92.0%), although it was greater than the mean (58.4±19.1%). Thus, it can be concluded the standardized pace SFGT caused physiological demands representative of firefighting although the environmental and psychological stress could not be simulated using the current procedures.

A significant positive relationship was found between HR_Res during the SFGT and LnRMSSD_Rest-Post, which indicates that subjects who experienced greater magnitude of cardiovascular stress during fireground tasks also had greater depression in heart rate variability after the tasks and, thus, greater physiological stress (Table 5). Previous research has indicated that post-exercise heart rate variability may be used to evaluate exercise training load and our findings suggest that it may also be used to evaluate physiological stress after firefighting [20, 43]. Previous research on tactical populations has indicated that heart rate variability during and after tactical operations may be influenced by the magnitude of stress, but these findings have not been demonstrated after acute fireground tasks [21]. Our findings support the notion that the magnitude of cardiovascular stress during fireground tasks also influences heart rate variability, which is indicative of autonomic nervous system activity, also after the tasks. Because LnRMSSD is predominately a measure of parasympathetic nervous system modulation of heart rate [44], greater cardiovascular demand during fireground tasks may result in greater reduction in parasympathetic nervous system modulation of heart rate after the event. Supporting findings have been reported by Ebersole et al. [19] who used a laboratory-based exercise protocol where greater depression of LnRMSSD was observed from 30 s to 10 min after maximal exercise compared to submaximal exercise. In addition, Marcel-Millet and coworkers [13] reported that greater time spent in a heart rate zone between 86–95% of HR_max was followed by a more depressed LnRMSSD post-SFGT. Greater intensity during exercise has been reported to result in greater depression of heart rate variability following exercise [34, 35]. In contrast, greater exercise duration has not been found to affect heart rate variability [35, 45]. Thus, we suggest that greater physiological stress rather than time to complete the SFGT caused the greater depression of LnRMSSD in the subjects who could not maintain the standardized pace. This was supported also by our correlation analyses that indicated no significant relationship between SFGT time and LnRMSSD_Rest-Post.

Greater heart rate during the SFGT was not related to lower HRR₆₀. Lower HRR₆₀ has been reported with greater load carriage during fireground tasks and after a maximal versus submaximal exercise test was performed in previous studies [9, 19]. It is possible that HRR_60- could have been significantly affected by greater cardiovascular demands, if greater differences in relative intensity would have occurred. A near maximal work intensity may need to be reached for HRR_60- to be significantly depressed [19]. We have unpublished data that showed lower HRR₆₀ after a maximal versus submaximal pace SFGT. However, evidence from the current study does not support these previous findings and conjectures.

The multiple linear regression analyses indicated that greater height, pull-ups, push-ups and 1.5-mile time were favorable characteristics for the assessed heart rate related outcomes. Previous studies have indicated that favorable values in similar measures are associated with faster SFGT performance, but our results indicate that these measures are also beneficial for coping with the physiological stress associated with firefighting. Interestingly, taller subjects had inferior pull-up scores (Table 4) indicating that fitness tests that favor shorter individuals may not adequately evaluate firefighters’ ability to physiologically cope with the demands of firefighting. Previous studies suggest that physical fitness is related to performance while performing fireground tasks [24 –26]. However, physical performance during fireground tasks has also been positively related to height [46, 47] so it is important for practitioners to evaluate fitness parameters that have the greatest transfer to occupational performance and also predict lower physiological stress during fireground tasks. We hope that the results from the current study encourage practitioners to adopt fitness assessments that do not penalize firefighters who possess occupationally advantageous physical characteristics.

The characteristics that were associated with HR_Res (height and pull-ups) and LnRMSSD_Rest-Post (height) were different from those associated with HRR₆₀ (push-ups and 1.5-mile run). This finding along with the lack of association between HR_Res and HRR₆₀ indicates that heart rate recovery may be dependent on different factors compared to heart rate and heart rate variability. We suggest that instead of heart rate recovery being dependent on the accumulated stress (i.e., heart rate during the activity) during physical exertion, it may be related to the physical fitness status of the individual. In support of this notion, previous research has indicated that heart rate recovery is faster with greater cardiovascular fitness and improves after cardiovascular type training [48 –50]. Our results also demonstrated a relationship between 1.5-mile run, a proxy for cardiovascular fitness, and HRR₆₀, but not the other measures. Although we do not wish to conclude that greater cardiovascular fitness is not advantageous to reducing physical stress during fireground tasks, it may be a more important factor for heart rate recovery compared to our other outcome measures. However, this assertion requires further research before more conclusive statements can be made. In conclusion, physical, cardiovascular and muscular performance measures should be included in assessments relating to firefighters’ ability to cope with the physiological demands of fireground operations. Previous studies have indicated that more effective cooling strategies reduce cardiovascular stress during fireground tasks [51]. Our results indicate that greater outcomes in fitness tests are also associated with lower cardiovascular stress during fireground tasks as well as greater cardiac parasympathetic activity during recovery, both of which may protect firefighters from cardiovascular events. Thus, firefighters can be better prepared to physiologically cope with the rigors of firefighting by raising their physical fitness levels to meet their occupational demands.

Overall, this study employed a unique assessment to evaluate the effects of firefighting on physiological stress and autonomic nervous system activity, but some potential limitations should be addressed. The present study included a SFGT that was shorter in duration compared to some previously reported fireground events [6]. Longer fireground events may induce cardiovascular stress also due to cardiac fatigue and decreased plasma volume [5, 52], which likely did not affect the results of the present study. The results of the present study may be limited to short duration fireground work, which is less impacted by cardiac fatigue and fluid losses. However, feasibility of a long duration SFGT may limit its adoption for testing purposes and shorter tests appear to still provide valuable information about firefighters’ ability to cope with the physical demands of firefighting. It is possible that results from heart rate assessments with shorter duration SFGTs provide similar information about a firefighter’s ability to physiologically cope with firefighting as longer tests but this notion cannot currently be supported. Further research should evaluate the effect of fitness and physical training on physiological stress during longer duration firefighting. Another limiting factor of the current study is the use of a fairly homogenous group of firefighter cadets instead of a heterogenous group of career firefighters. The technical difficulty of the included tasks did not present problems for the subjects but it is unknown how older and less fit firefighters would cope with the demands of the administered SFGT. A larger heterogenous cohort of firefighters could be used to obtain further support for important physical and fitness characteristics for coping with the demands of firefighting. An additional limitation of this study is the use of a convenience sample instead of a randomized sample, which was chosen due to the limited access to a representative population who would be permitted on the testing facility and have the requisite familiarity to fireground tasks. Due to the use of a convenience sample, the results may be biased to individuals who would choose to participate in a research study and overall findings generalizability could have been affected. However, the results indicate that we were able to obtain a sample who experienced a wide range of responses. For instance, based off the standard deviation of heart rate reserve data, values around one standard deviation around the mean of the sample heart rate reserve were between 73.0–83.4%, which indicates a large range of responses in one of our main outcomes of interest. Thus, we are confident that the results may be extrapolated to the firefighter population as a whole.

Although our research design was meant to eliminate the effect of work rate during the SFGT by using a standardized pace that was slow enough to permit all subjects to complete the course in similar time [30], five subjects were not able to complete the course within that time. Thus, we had to account for the differences SFGT time within our statistical models. However, SFGT time was only associated with heart rate recovery where slower subjects also had inferior heart rate recovery. If slower subjects would have also had more favorable heart rate related outcomes, we would have been more concerned about their level of effort during the SFGT. Since the opposite occurred, this limitation does not appear to change our overall conclusion but indicates that unfavorable physical characteristics may both decrease the ability to complete strenuous fireground tasks and increase the physiological stress associated with them. However, better precision in the effects of the physical and fitness characteristics on physical stress due to firefighting may be attainable when all subjects complete the fireground tasks with more homogenous time.

5 Conclusion

Work on the fireground imposes significant demands on the cardiovascular, metabolic, and autonomic nervous systems during and after the tasks. Evaluating firefighters’ internal responses with a simulated fireground test using a standardized work rate may allow practitioners to compare firefighters’ abilities to cope with the physiological demands of firefighting. Our findings indicate that greater cardiovascular stress during fireground tasks carries over to greater depression of heart rate variability and parasympathetic activity during recovery. Physical and fitness characteristics that are related to physical performance during fireground tasks are also related to the magnitude of physiological stress experienced during and after fireground tasks. Thus, improving the physical fitness profiles of firefighters helps manage the physiological stress of firefighting. When firefighters’ physical and fitness characteristics are evaluated, practitioners should use fitness tests that account for differences in stature to help determine occupationally relevant fitness needs more appropriately.

Footnotes

Acknowledgments

The authors would like to thank the Lexington Fire Academy for providing access to the fireground equipment and testing site and all the support needed to complete this research project.

Conflict of interest

The authors declare that they have no conflict of interest.

Funding

Internal funding was provided through the University of Kentucky College of Education.

Ethics statement

Institutional ethics research committee approval was obtained for the study procedures. The study conformed to the provisions of the Declaration of Helsinki.

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				95% Confidence Interval
Variable	Mean	±	SD	Lower	Upper
Age (yr)	28	±	4	27	30
Height (cm)	177.1	±	6.9	174.0	180.3
Body mass (kg)	88.3	±	15.4	81.3	95.3
Relative body fat (%)	20.7	±	5.4	18.3	23.1
Fat-free mass (kg)	69.5	±	8.7	65.5	73.4
Fat mass (kg)	18.8	±	7.8	15.3	22.4
1.5-mile run (min)	12.9	±	1.6	12.2	13.6
Push-ups (count)	44	±	10	39	48
Pull-ups (count)	15	±	9	11	19
Sit-ups (count)	50	±	13	44	56
Plank time (min)	2.7	±	0.9	2.3	3.1