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# On-duty resting heart rate variability at the fire station is related to cardiorespiratory fitness and physical activity levels in firefighters

Philémon Marcel-Millet, PhD¹,²,³ https://orcid.org/0000-0002-3557-611X & Philippe
Gendron, PhD¹

¹Département des sciences de l'activité physique, Université du Québec à Trois-Rivières,
Trois-Rivières, Québec

²Unité de Physiologie de l'Exercice et des Activités en Conditions Extrêmes, Institut de
Recherche Biomédicale des Armées, Brétigny-sur-Orge, France

³Laboratoire Culture Sport Santé Société, Université de Franche-Comté, Besançon, France

**Corresponding author:** Philippe Gendron, PhD. Université du Québec à Trois-Rivières, 3351 boul. des Forges, Trois-Rivières, QC, G8Z 4M3. Tel: 1-819-376-5011 #3789. Email: philippe.gendron@uqtr.ca

**Contributing author's email:** philemon.marcel.millet@gmail.com

**Conflicts of interest:** None declared.

**Sources of funding:** None declared (the authors have received no funding from the following organizations: National Institutes of Health (NIH); Wellcome Trust; Howard Hughes Medical Institute (HHMI); and others)
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**Acknowledgments:** The authors thank all the firefighters who participated in this study and the fire departments for the support. PG contributed to the conception and design of the work and to the acquisition of data. PMM and PG contributed to the analysis/interpretation of the data. PMM drafted the work and PG revised it critically for important intellectual content. Data are not available.

The authors adhere to the STROBE Guidelines.

**Ethical approval:** The research design was examined and approved by the Research Ethics Committee of the Université du Québec à Trois-Rivières (CER-16-228-07.17) and conformed to the recommendations of the Declaration of Helsinki. All participants provided written informed consent.
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ABSTRACT

**Objective:** To measure the association between the physical activity (PA) levels and cardiorespiratory fitness on the firefighters' cardiac autonomic function at rest at the fire station.

**Methods:** Anthropometry, heart rate variability (HRV), fitness and PA levels were assessed at the fire station in 98 career Québec male firefighters.

**Results:** Partial correlations, adjusted for age, waist circumference and perceived stress, revealed that heart rate and several HRV indices were significantly correlated to estimated maximal oxygen consumption and to high intensity PA but not to moderate intensity PA.

**Conclusion:** These results suggest that both cardiorespiratory fitness and high-intensity physical training are related to the cardiac autonomic function of firefighters. Therefore, fire department should promote PA programs to improve physical fitness among firefighters.

**Key Words:** HRV, cardiac autonomic function; parasympathetic tone; firefighters' health; physical training.
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Learning outcomes:

- After reviewing this article, readers should be able to identify the links between cardiorespiratory fitness and cardiac autonomic function in male firefighters.

- After reviewing this article, readers should be able to differentiate the influence that exercise intensity has on cardiac autonomic function.
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INTRODUCTION

Firefighting is one of the most demanding occupational activities. Fire-tasks place high
constraint on the body of firefighters, due to the physical work, personal protective
equipment, dehydration, psychological stress and environmental factors (heat, smoke, etc.),
that lead to vascular, cardiac, and hemostatic disruptions¹. These cardiovascular stresses can
trigger a fatal or non-fatal cardiovascular event¹, particularly in firefighters who already have
an underlying cardiovascular disease²,³ and/or cardiovascular risk factors such as obesity,
hypertension and smoking⁴⁻⁶. Cardiovascular events are the leading cause of line-of-duty
death among firefighters⁷,⁸ and approximately 17 to 25 nonfatal cardiac events occur for
every fatal event⁹. It is not surprising, since several studies showed that a significant
proportion of firefighters have cardiovascular disease risk factors¹⁰⁻¹⁴.

There is strong evidence linking the practice of physical activity or cardiorespiratory fitness
with cardiovascular disease prevention and risk factors management in the general
population<sup>15-17</sup>. In the firefighters' population, several studies showed that physical activity
or cardiorespiratory fitness levels are negatively associated with cardiovascular disease risk
factors such as obesity, hypertension, dyslipidemia and metabolic syndrome<sup>18-21</sup>.
Additionally, the evaluation of the effectiveness of physical exercise interventions in fire
departments revealed positive effects of health promotion programs including exercise
prescription on firefighter health and fitness<sup>19,22-24</sup>. For these reasons, fire departments are
strongly encouraged to implement health and fitness programs for their personnel<sup>1,25</sup>.

The activities of firefighters throughout their shifts require constant physiological
adjustments, particularly in the cardiovascular system<sup>1,26</sup>. Cardiac autonomic function is
widely involved in the control of cardiovascular homeostasis, through the activity of
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sympathetic and parasympathetic pathways. It is known that an imbalance of the cardiac autonomic function is associated with an increased risk of cardiovascular diseases and mortality<sup>27-29</sup>. Thus, an alteration of the cardiac autonomic function is a mechanism to be integrated into explanatory models of firefighter vulnerability. Cardiac autonomic modulations can be investigated non-invasively by analyzing heart rate variability<sup>30</sup>. Heart rate variability is the beat-to-beat variation of the RR interval. Some heart rate variability indices almost exclusively reflect cardiac vagal modulations while others reflect modulations of both sympathetic and parasympathetic pathways<sup>30,31</sup>.

In the field of firefighters, several studies used heart rate variability to investigate the influence of a 24-hour shift<sup>32</sup>, a night shift<sup>33</sup>, or a firefighting activity<sup>34,35</sup> on cardiac autonomic function. While using different methodological approaches (long- and short-term analysis), these studies revealed a strong temporary depressions in heart rate variability during emergencies<sup>32,34</sup>. In the general population, high cardiorespiratory fitness and/or levels of physical activity have been shown to improve resting cardiac autonomic function<sup>36-38</sup>. The only study that investigated the relation between cardiorespiratory physical fitness and cardiac autonomic function at rest in firefighters found similar results<sup>39</sup>. In this study, firefighters with better cardiorespiratory fitness were found to have better global and vagal-related heart rate variability indices than firefighters with low cardiorespiratory fitness.

Although cardiac autonomic function is an important element to consider for firefighters' health, it has still been little studied worldwide in this population. Moreover, to the best of our knowledge, the relationship between physical activity and autonomic function has never been studied in firefighters. Therefore, the objective of the present study was to evaluate the relation between the firefighters' cardiac autonomic function and their physical activity and
their cardiorespiratory fitness levels. We hypothesized that cardiac autonomic function, measured at rest at the fire station, would be enhanced among firefighters with the highest physical fitness and activity level.

# METHODS

## Ethical approval

The research design was examined and approved by the Research Ethics Committee of the institution (CER-16-228-07.17) and conformed to the recommendations of the Declaration of Helsinki. All participants provided written informed consent. This study adheres to the reporting guidelines provided by STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) (Supplemental Digital Content, http://links.lww.com/JOM/B487).⁴⁰

## Participants

The data presented herein are secondary outcomes of a previous study that analyzed the association between firefighters' physical training at the fire station and cardiovascular health indicators¹⁹. In the current cross-sectional study, measures of heart rate variability at the fire station taken in November and December 2017 are reported for 98 career male firefighters from five Québec fire departments.

## Study design

First, a physical health assessment (anthropometric and physiological measurements) was carried out at the fire station between 07:00 a.m. and 11:00 p.m. during a 10, 12 or 24 h regular work shift. After weight, height, and waist circumference measurements, resting heart rate was measured while participants sat for 10 minutes in a quiet room at a comfortable ambient temperature and were instructed not to speak. However, they knew that the health
assessment could be interrupted by any emergency call. Thus, participants were in a resting condition while being ready to attend to an emergency call at any moment. Finally, participants completed an online questionnaire in the hours following the physical health assessment.

## Anthropometric measurements

Weight was measured with a Tanita weight scale (Body Composition Analyzer BF-350, Japan), height with a measuring tape (Seca, Germany) and waist circumference based on the 2013 guidelines of the Canadian Society for Exercise Physiology⁴¹ with a measuring tape (Seca, Germany).

## Heart rate variability

The participants were equipped with a RS800CX Polar® chest belt (Kempele, Finland) to continuously record the series of RR intervals. Then, RR intervals were extracted using Polar ProTrainer 5 software and analyzed using Kubios software (Kubios HRV Analysis 2.0; Bio-signal Analysis and Medical Imaging Group at the Department of Applied Physics, University of Kuopio, Kuopio, Finland). RR intervals were analyzed in accordance with international recommendations between minutes 5 to 10 of the resting period³⁰. Abnormal beats correction level was adjusted individually using Kubios software correction option. The data were visually inspected. If artifacts were identified, the lowest threshold of Kubios software allowing correction of abnormal beats was selected. The selected variables for the time domain analysis were mean RR interval (RR), the standard deviation of R-R intervals (SDRR), the coefficient of variation of R-R intervals (CVRR), the root mean square of successive differences of RR interval (RMSSD) and the number of successive RR intervals differing by more than 50 ms divided by the total number of RR intervals (pRR50). In the
frequency domain, heart rate variability was analyzed using spectral analysis of the intervals
from the fast Fourier transform, through the high frequency (HF, 0.15–0.40 Hz), the low
frequency (LF, 0.04–0.15 Hz) bands and the ratio between the LF and HF bands (LF/HF).
RMSSD, HF, and pRR50 reflect cardiac parasympathetic modulations, whereas heart rate,
RR, CVRR, SDRR, LF and LF/HF reflect both cardiac vagal and sympathetic
modulations³⁰,³¹.

Online questionnaire

Age, gender, smoking status, psychological stress, estimated maximal oxygen consumption (VO2max) and physical activity level were collected with the online questionnaire (SurveyMonkey®). Participants were considered smokers if they were current smokers or had quit within the previous 6 months⁴². The 14-item French version of the Perceived Stress Scale was used to evaluate the firefighters’ psychological stress⁴³,⁴⁴. VO2max was estimated thanks to the Huet questionnaire⁴⁵ that provides a valid estimation of VO2max compared to laboratory direct measures in an adult population. Finally, the off- and on-duty level of physical activity was assessed from the number and duration of self-reported periods of exercise for a typical week. A distinction was made between moderate- (between 3 to 5.9 metabolic equivalent of task [METs]) and high-intensity exercise (≥ 6 METs) according to the American College of Sports Medicine (ACSM) criteria⁴². For example, moderate intensity exercise included resistance training, brisk walking, household tasks of moderate and vigorous effort. High intensity exercise included jogging, bicycling, running, soccer, circuit training and ice hockey. Moderate and high intensity activities were identified based on an illustrative compendium⁴⁶. Thus, the physical activity parameters included weekly moderate-intensity, high-intensity, and total exercise duration.
## Statistical analysis

All data were expressed as mean ± standard deviation. Pearson's correlation coefficients (r) were used to determine the relationship between cardiorespiratory fitness or physical activity level and heart rate variability indices. Relationship between these variables were also determined using partial correlations for statistical adjustment for covariates (age, waist circumference, perceived stress). Non-normal distributions were transformed to near-normality before analysis (distributions were transformed to obtain an asymmetry coefficient ($g_1$) value as close as possible to 0 when $g_1$ value obtained from the raw data was lower to -0.4 or higher to 0.4). All statistical analyses and graphs were performed using JASP software version 0.16.3 (University of Amsterdam). Alpha level for statistical significance was set at 0.05.

## RESULTS

Participants' descriptive data are presented in Table 1. Sixty-four (64) participants were regular firefighters, 28 were lieutenants/captains and 6 were officers.

Perceived stress score (/40) was 29.8 ± 6.8. Resting values of heart rate and time- and frequency-domain heart rate variability indices were 66 ± 9 bpm (heart rate), 929.7 ± 130.9 ms (RR), 54.8 ± 22.6 ms (SDRR), 0.059 ± 0.024 (CVRR), 36.7 ± 20.5 ms (RMSSD), 16.8 ± 16.6% (pRR50), 597.4 ± 794.3 ms² (HF), 1595.6 ± 2213.1 ms² (LF) and 4.2 ± 5.3 (LF/HF).

The relationship between estimated $V̇O_{2max}$ and heart rate variability indices are presented in Table 2. Non-adjusted correlations revealed significant correlations between estimated $V̇O_{2max}$ and all heart rate variability indices (excepted for LF/HF ratio). After adjustment for
age, waist circumference, and perceived stress, all heart rate variability indices (excepted for CVRR and LF/HF ratio) remained significantly correlated with estimated VO2max.

The relationship between weekly total physical activity levels and heart rate variability indices are displayed in Table 3. Non-adjusted correlations revealed significant correlations between physical activity levels and all heart rate variability indices (excepted for LF/HF ratio). After adjustment for age, waist circumference, and perceived stress, only heart rate and RR remained significantly correlated to physical activity levels.

The relationship between weekly total high intensity exercise levels and heart rate variability indices are displayed in Table 4. Non-adjusted correlations revealed significant correlations between high intensity exercise levels and all heart rate variability indices (excepted for LF/HF ratio). After adjustment for age, waist circumference, and perceived stress, only heart rate, RR and pRR50 remained significantly correlated to high intensity exercise levels.

The relationship between weekly total moderate-intensity exercise levels and heart rate and heart rate variability indices are displayed in Table 5. Non-adjusted correlations revealed significant correlations between moderate-intensity exercise levels and heart rate, RR, RMSSD, pRR50 and HF. After adjustment for age, waist circumference, and perceived stress, none of the heart rate variability indices remained significantly correlated.

DISCUSSION

The purpose of the present study was to evaluate the relation between the firefighters' cardiac autonomic function and their physical activity or their cardiorespiratory fitness levels. The main results showed that estimated VO2max remained significantly correlated with heart rate
variability indices after adjustment for covariates. However, regarding physical activity levels, only high intensity physical activity remained significantly correlated to some heart rate variability indices after adjustment for covariates, unlike moderate intensity physical activity. These results suggest that both cardiorespiratory fitness and high-intensity physical training are associated to the cardiac autonomic function of firefighters.

The resting heart rate variability values measured in the current study were for the most part higher than the values previously reported by Porto et al.³⁹ in Brazilian firefighters (supine and standing position) or those of Shin et al.⁴⁷ in Korean firefighters (sitting position). However, they were within the range of normative values described in the general population⁴⁸,⁴⁹. Given the interindividual variability and the influence of many factors such as the position, environment, age or ethnicity of individuals, the comparison of heart rate variability between studies is difficult and must be done cautiously⁴⁹. Nevertheless, our results suggest that, contrary to the findings in previous studies³⁹,⁴⁷, the cardiac autonomic function of the firefighters participating in our study did not seem to differ from the general population. It has been suggested that the chronic stress related to the profession of firefighter may unbalance the autonomic function, with a chronic arousal of the sympathetic system and/or a low parasympathetic activity³⁹,⁴⁷. The discrepancies between the findings of the literature and our values suggest that the Québec firefighters assessed in our study do not seem to experience comparable chronic stress. It is reasonable to expect that different working conditions (working hours, management, medical follow-up, etc.) or missions (number and nature of interventions) between countries can influence the chronic stress of firefighters.
One of the main findings of the present study was the association between cardiorespiratory
fitness and cardiac autonomic function. We observed significant and strong correlations
between VO2max and heart rate variability, which remained significant after adjustment for
age, waist circumference and perceived stress. Therefore, our results showed that a higher
cardiorespiratory fitness was associated with a better cardiac autonomic function related to
both overall autonomic function (i.e., RR, SDRR and LF) and vagal activity (i.e., RMSSD,
pRR50 and HF). These findings are consistent with the results reported in firefighters' 39 and in
the general population 36,38. Porto et al. 39 compared two groups of firefighters differentiated by
an estimated VO2max above or below 12 METs. Similar to our results, firefighters with a
greater cardiorespiratory fitness had a higher cardiac parasympathetic tone and a lower
cardiac sympathetic activity. Therefore, cardiorespiratory fitness is positively associated with
the firefighters' cardiac autonomic function.

Our results also showed a significant correlation between heart rate variability and total
physical activity levels. However, the model adjusted for the covariates was only significant
for HR and RR. A more detailed analysis showed that the amount of high intensity physical
activity remained significantly correlated after adjusting for covariates (HR, RR and pRR50),
unlike moderate intensity which was not significant. These findings were comparable to
previously reported results<sup>17,37,50</sup> which indicated that high intensity training have a greater
effect on autonomic function than moderate intensity training. It suggests that physical
activity could elicit changes in firefighters' cardiac autonomic activity, provided the exercise
is performed at a high enough intensity. However, we used the ACSM criteria to define
moderate and high intensity exercise. Thus, we considered physical activities more intense
than 6 METs<sup>42</sup> as high intensity. In reality, 6 METs represents a relatively low exercise
intensity, especially for a healthy and non-aging person. Indeed, an oxygen consumption of
21 mlO₂·kg⁻¹·min⁻¹ (6 METs) corresponds to a relative intensity of only 44% of the estimated ˙VO₂max of the participants in the current study, which is easily below the first ventilatory threshold in healthy subjects⁵¹. Therefore, a wide variety of physical activities fit this criterion and may have a beneficial effect on firefighters’ cardiac autonomic function. It has been hypothesized that physical activity induces both autonomic adaptations (vagal and sympathetic modulations) and cardiac intrinsic electrophysiological adaptations, which can explain our present findings⁵². However, we cannot exclude the hypothesis that the improvement in heart rate variability was caused by the increase in ˙VO₂max following physical training rather than physical activity itself. This is what Buchheit and Gindre³⁸ had shown, by isolating the influence of both physical activity and ˙VO₂max on resting heart rate variability. It is plausible that both elements work together to improve heart rate variability. In any case, physical activities with an intensity greater than 6 METs are effective for both heart rate variability and ˙VO₂max improvement and should therefore be encouraged in the fire service for firefighters’ safety and health.

The present study has some limitations that must be considered when interpreting the findings. First, ˙VO₂max and physical activity measurements were based on questionnaires, which could have affected the accuracy of the results. However, these measures have been validated⁴⁵ or supported by the literature⁴²,⁴⁶ and have been used previously with firefighters¹¹,¹²,¹⁹. Furthermore, the heart rate variability measurements were carried out at the fire station, which could have generated stress likely to influence the results. However, Porto et al.³⁹ found no differences between firefighters’ heart rate variability measured in the fire station or in the laboratory. Finally, we regret the absence of women in our study, which prevents generalization of the results to women.
In conclusion, the results of the present study confirmed the importance of promoting the practice of physical activity and improving physical fitness among firefighters. Indeed, a greater level of high intensity physical activity and physical fitness improves the firefighters' cardiac autonomic function. This improvement in autonomic function could have positive implications for the firefighters' health, including a protective effect against ventricular fibrillation⁵³. This last point could be particularly beneficial for the firefighters' health, since the majority of line-of-duty deaths from a cardiovascular event is usually triggered by arrhythmias¹.
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<table><caption>Table 1. Descriptive data of participants</caption><thead><tr><th>Variable</th><th>Mean ± SD</th><th>Median</th><th>Range</th></tr></thead><tbody><tr><td>Age (years)</td><td>40 ± 9</td><td>40</td><td>24-60</td></tr><tr><td>Body weight (kg)</td><td>85.9 ± 9.6</td><td>84.2</td><td>66.7-116.8</td></tr><tr><td>Body mass index (kg/m<sup>2</sup>)</td><td>27.7 ± 2.9</td><td>27.5</td><td>21.8-38.5</td></tr><tr><td>Waist circumference (cm)</td><td>92.7 ± 8.2</td><td>90.4</td><td>76.1-114.4</td></tr><tr><td>Estimated VO<sub>2max</sub> (mlO<sub>2</sub>·kg<sup>-1</sup>·min<sup>-1</sup>)</td><td>47.4 ± 7.2</td><td>48.1</td><td>28.7-62.4</td></tr><tr><td>Total PA at moderate intensity (min/week)</td><td>259 ± 239</td><td>180</td><td>0-1270</td></tr><tr><td>Total PA at high intensity (min/week)</td><td>154 ± 146</td><td>120</td><td>0-670</td></tr><tr><td>Total PA (min/week)</td><td>413 ± 306</td><td>338</td><td>30-1435</td></tr></tbody></table>

PA: physical activity, VO₂max: maximal oxygen consumption.
<table><caption>Table 2. Correlations and partial correlations between estimated VO₂max and the dependent variables.</caption><thead><tr><th rowspan="2"></th><th colspan="4">Adjusted</th></tr><tr><th>r</th><th>P</th><th>r</th><th>P</th></tr></thead><tbody><tr><td>Heart rate</td><td>-0.399</td><td>&lt; 0.001</td><td>-0.375</td><td>&lt; 0.001</td></tr><tr><td>RR</td><td>0.386</td><td>&lt; 0.001</td><td>0.365</td><td>&lt; 0.001</td></tr><tr><td>SDRR</td><td>0.516</td><td>&lt; 0.001</td><td>0.289</td><td>0.005</td></tr><tr><td>CVRR</td><td>0.418</td><td>&lt; 0.001</td><td>0.166</td><td>0.108</td></tr><tr><td>RMSSD</td><td>0.496</td><td>&lt; 0.001</td><td>0.260</td><td>0.011</td></tr><tr><td>pRR50</td><td>0.567</td><td>&lt; 0.001</td><td>0.315</td><td>0.002</td></tr><tr><td>LF</td><td>-0.422</td><td>&lt; 0.001</td><td>-0.255</td><td>0.013</td></tr><tr><td>HF</td><td>0.504</td><td>&lt; 0.001</td><td>0.212</td><td>0.039</td></tr><tr><td>LF/HF</td><td>-0.127</td><td>0.214</td><td>0.074</td><td>0.478</td></tr></tbody></table>

Partial correlations were adjusted for age, waist circumference, perceived stress. CVRR: the coefficient of variation of R-R intervals; HF: high frequency power band; LF: low frequency power band; pRR50: the number of successive RR intervals differing by more than 50 ms divided by the total number of RR intervals; RR: mean RR interval; SDRR: standard deviation of R-R intervals; RMSSD: root mean square of successive differences of RR interval; VO₂max: maximal oxygen consumption.
<table><caption>Table 3. Correlations and partial correlations between weekly total physical activity levels and the dependent variables.</caption><thead><tr><th rowspan="2"></th><th colspan="4">Adjusted</th></tr><tr><th>r</th><th>P</th><th>r</th><th>P</th></tr></thead><tbody><tr><td>Heart rate</td><td>-0.330</td><td>&lt; 0.001</td><td>-0.231</td><td>0.024</td></tr><tr><td>RR</td><td>0.319</td><td>&lt; 0.001</td><td>0.226</td><td>0.028</td></tr><tr><td>SDRR</td><td>0.242</td><td>0.016</td><td>0.113</td><td>0.275</td></tr><tr><td>CVRR</td><td>0.152</td><td>0.135</td><td>0.039</td><td>0.708</td></tr><tr><td>RMSSD</td><td>0.270</td><td>0.007</td><td>0.142</td><td>0.169</td></tr><tr><td>pRR50</td><td>0.315</td><td>0.002</td><td>0.175</td><td>0.090</td></tr><tr><td>LF</td><td>-0.219</td><td>0.030</td><td>-0.106</td><td>0.305</td></tr><tr><td>HF</td><td>0.290</td><td>0.004</td><td>0.170</td><td>0.100</td></tr><tr><td>LF/HF</td><td>-0.070</td><td>0.496</td><td>-0.023</td><td>0.826</td></tr></tbody></table>

Partial correlations were adjusted for age, waist circumference, perceived stress. CVRR: the coefficient of variation of R-R intervals; HF: high frequency power band; LF: low frequency power band; pRR50: the number of successive RR intervals differing by more than 50 ms divided by the total number of RR intervals; RR: mean RR interval; SDRR: standard deviation of R-R intervals; RMSSD: root mean square of successive differences of RR interval.
<table><caption>Table 4. Correlations and partial correlations between weekly total high intensity physical activity exercise levels and the dependent variables.</caption><thead><tr><th rowspan="2"></th><th colspan="4">Adjusted</th></tr><tr><th>r</th><th>P</th><th>r</th><th>P</th></tr></thead><tbody><tr><td>Heart rate</td><td>-0.307</td><td>0.002</td><td>-0.215</td><td>0.036</td></tr><tr><td>RR</td><td>0.303</td><td>0.002</td><td>0.214</td><td>0.038</td></tr><tr><td>SDRR</td><td>0.291</td><td>0.004</td><td>0.148</td><td>0.153</td></tr><tr><td>CVRR</td><td>0.207</td><td>0.041</td><td>0.076</td><td>0.464</td></tr><tr><td>RMSSD</td><td>0.291</td><td>0.004</td><td>0.155</td><td>0.133</td></tr><tr><td>pRR50</td><td>0.402</td><td>&lt; 0.001</td><td>0.265</td><td>0.0109</td></tr><tr><td>LF</td><td>-0.290</td><td>0.004</td><td>-0.174</td><td>0.091</td></tr><tr><td>HF</td><td>0.295</td><td>0.003</td><td>0.149</td><td>0.149</td></tr><tr><td>LF/HF</td><td>0.008</td><td>0.940</td><td>0.073</td><td>0.480</td></tr></tbody></table>

Partial correlations were adjusted for age, waist circumference, perceived stress. CVRR: the coefficient of variation of R-R intervals; HF: high frequency power band; LF: low frequency power band; pRR50: the number of successive RR intervals differing by more than 50 ms divided by the total number of RR intervals; RR: mean RR interval; SDRR: standard deviation of R-R intervals; RMSSD: root mean square of successive differences of RR interval.
<table><caption>Table 5. Correlations and partial correlations between weekly total moderate intensity physical activity levels and the dependent variables.</caption><thead><tr><th></th><th>r</th><th>P</th><th>r</th><th>P</th></tr></thead><tbody><tr><td>Heart rate</td><td>-0.252</td><td>0.012</td><td>-0.174</td><td>0.092</td></tr><tr><td>RR</td><td>0.245</td><td>0.015</td><td>0.174</td><td>0.091</td></tr><tr><td>SDRR</td><td>0.133</td><td>0.193</td><td>0.042</td><td>0.688</td></tr><tr><td>CVRR</td><td>0.060</td><td>0.556</td><td>-0.016</td><td>0.875</td></tr><tr><td>RMSSD</td><td>0.202</td><td>0.046</td><td>0.112</td><td>0.282</td></tr><tr><td>pRR50</td><td>0.190</td><td>0.061</td><td>0.088</td><td>0.395</td></tr><tr><td>LF</td><td>-0.091</td><td>0.375</td><td>-0.007</td><td>0.949</td></tr><tr><td>HF</td><td>0.220</td><td>0.029</td><td>0.143</td><td>0.166</td></tr><tr><td>LF/HF</td><td>-0.142</td><td>0.162</td><td>-0.117</td><td>0.258</td></tr></tbody></table>

Partial correlations were adjusted for age, waist circumference, perceived stress. CVRR: the coefficient of variation of R-R intervals; HF: high frequency power band; LF: low frequency power band; pRR50: the number of successive RR intervals differing by more than 50 ms divided by the total number of RR intervals; RR: mean RR interval; SDRR: standard deviation of R-R intervals; RMSSD: root mean square of successive differences of RR interval.
STROBE Statement—Checklist of items that should be included in reports of cross-sectional studies

<table><thead><tr><th></th><th>Item No</th><th>Recommendation</th><th>Page No</th></tr></thead><tbody><tr><td rowspan="2">Title and abstract</td><td rowspan="2">1</td><td>(a) Indicate the study's design with a commonly used term in the title or the abstract</td><td>NA</td></tr><tr><td>(b) Provide in the abstract an informative and balanced summary of what was done and what was found</td><td>1</td></tr><tr><td colspan="4"><strong>Introduction</strong></td></tr><tr><td>Background/rationale</td><td>2</td><td>Explain the scientific background and rationale for the investigation being reported</td><td>2</td></tr><tr><td>Objectives</td><td>3</td><td>State specific objectives, including any prespecified hypotheses</td><td>3-4</td></tr><tr><td colspan="4"><strong>Methods</strong></td></tr><tr><td>Study design</td><td>4</td><td>Present key elements of study design early in the paper</td><td>4</td></tr><tr><td>Setting</td><td>5</td><td>Describe the setting, locations, and relevant dates, including periods of recruitment, exposure, follow-up, and data collection</td><td>4</td></tr><tr><td>Participants</td><td>6</td><td>(a) Give the eligibility criteria, and the sources and methods of selection of participants</td><td>4</td></tr><tr><td>Variables</td><td>7</td><td>Clearly define all outcomes, exposures, predictors, potential confounders, and effect modifiers. Give diagnostic criteria, if applicable</td><td>5-6</td></tr><tr><td>Data sources/ measurement</td><td>8*</td><td>For each variable of interest, give sources of data and details of methods of assessment (measurement). Describe comparability of assessment methods if there is more than one group</td><td>5-6</td></tr><tr><td>Bias</td><td>9</td><td>Describe any efforts to address potential sources of bias</td><td>4-5-6</td></tr><tr><td>Study size</td><td>10</td><td>Explain how the study size was arrived at</td><td>4</td></tr><tr><td>Quantitative variables</td><td>11</td><td>Explain how quantitative variables were handled in the analyses. If applicable, describe which groupings were chosen and why</td><td>6</td></tr><tr><td rowspan="3">Statistical methods</td><td rowspan="3">12</td><td>(a) Describe all statistical methods, including those used to control for confounding</td><td>6</td></tr><tr><td>(b) Describe any methods used to examine subgroups and interactions</td><td>NA</td></tr><tr><td>(c) Explain how missing data were addressed</td><td>NA</td></tr><tr><td></td><td></td><td>(d) If applicable, describe analytical methods taking account of</td><td>NA</td></tr></tbody></table>
<table><tr><td></td><td>sampling strategy</td><td></td></tr><tr><td></td><td>(e) Describe any sensitivity analyses</td><td>NA</td></tr></table>

## Results

<table><tr><td rowspan="3">Participants</td><td>13*</td><td>(a) Report numbers of individuals at each stage of study—eg numbers potentially eligible, examined for eligibility, confirmed eligible, included in the study, completing follow-up, and analysed</td><td>7</td></tr><tr><td></td><td>(b) Give reasons for non-participation at each stage</td><td>NA</td></tr><tr><td></td><td>(c) Consider use of a flow diagram</td><td>NA</td></tr><tr><td rowspan="2">Descriptive data</td><td>14*</td><td>(a) Give characteristics of study participants (eg demographic, clinical, social) and information on exposures and potential confounders</td><td>7 and Table 1</td></tr><tr><td></td><td>(b) Indicate number of participants with missing data for each variable of interest</td><td>NA</td></tr><tr><td>Outcome data</td><td>15*</td><td>Report numbers of outcome events or summary measures</td><td>NA</td></tr><tr><td rowspan="3">Main results</td><td>16</td><td>(a) Give unadjusted estimates and, if applicable, confounder-adjusted estimates and their precision (eg, 95% confidence interval). Make clear which confounders were adjusted for and why they were included</td><td>7-8</td></tr><tr><td></td><td>(b) Report category boundaries when continuous variables were categorized</td><td>NA</td></tr><tr><td></td><td>(c) If relevant, consider translating estimates of relative risk into absolute risk for a meaningful time period</td><td>NA</td></tr><tr><td>Other analyses</td><td>17</td><td>Report other analyses done—eg analyses of subgroups and interactions, and sensitivity analyses</td><td>7-8</td></tr></table>

## Discussion

<table><tr><td>Key results</td><td>18</td><td>Summarise key results with reference to study objectives</td><td>8</td></tr><tr><td>Limitations</td><td>19</td><td>Discuss limitations of the study, taking into account sources of potential bias or imprecision. Discuss both direction and magnitude of any potential bias</td><td>10-11</td></tr><tr><td>Interpretation</td><td>20</td><td>Give a cautious overall interpretation of results considering objectives, limitations, multiplicity of analyses, results from similar studies, and other relevant evidence</td><td>8-9-10</td></tr><tr><td>Generalisability</td><td>21</td><td>Discuss the generalisability (external validity) of the study results</td><td>8-9-10</td></tr></table>

## Other information
<table><tr><td>Funding</td><td>22 Give the source of funding and the role of the funders for the present study and, if applicable, for the original study on which the present article is based</td><td>Title page</td></tr></table>

*Give information separately for exposed and unexposed groups.

**Note:** An Explanation and Elaboration article discusses each checklist item and gives methodological background and published examples of transparent reporting. The STROBE checklist is best used in conjunction with this article (freely available on the Web sites of PLoS Medicine at http://www.plosmedicine.org/, Annals of Internal Medicine at http://www.annals.org/, and Epidemiology at http://www.epidem.com/). Information on the STROBE Initiative is available at www.strobe-statement.org.
# Is the cardiac autonomic function of firefighters associated to their physical activity and their cardiorespiratory fitness levels?

Heart rate variability, estimated maximal oxygen consumption and physical activity levels were assessed in 98 career firefighters.

<figure><img src="image_1.png" /></figure>

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The firefighters' heart rate variability is significantly correlated with:

- The estimated maximal oxygen consumption

- The high intensity physical activity levels

<figure><img src="image_4.png" /></figure>

The results suggest that both cardiorespiratory fitness and high-intensity physical training are related to the cardiac autonomic function of firefighters.

**On-duty Resting Heart Rate Variability At The Fire Station Is Related To Cardiorespiratory Fitness And Physical Activity Levels In Firefighters.**
Philémon Marcel-Millet, PhD; Philippe Gendron, PhD

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@JOEMOnline

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@JOEMJournal

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Journal of Occupational and Environmental Medicine