Firefighters’ cardiovascular health and fitness: An observation of adaptations that occur during firefighter training academies

1 Introduction

According to the National Fire Protection Association [1], approximately 345,950 career firefighters are employed within the United States (U.S.) to serve over 318 million citizens across the nation. While on the job, firefighters must regularly execute a range of physically and mentally demanding tasks such as disaster assistance, fire prevention, and hazardous material response [2]. A recent report from the Federal Emergency Management Agency [3] revealed that 30 career firefighters incurred fatal injuries while on-duty in 2012 alone, 16 of which were caused by the stress and overexertion associated with on-the-job performance, and 13 of which were specifically classified as heart attacks. Sudden cardiac events have remained the leading cause of firefighter fatality in the U.S. for over 30 years [4, 5], yet reports indicate that only 30% of U.S. fire departments have preventive programs in place to maintain firefighter cardiovascular health and fitness [6]. According to Perroni et al. [7], “An adequate financial investment focused on assessment and increase of physical capabilities of firefighters could effectively reduce the health risks linked to emergencies.” In an attempt to minimize the persistent risk of sudden cardiac death among firefighters, leaders of national agencies such as FEMA continue to solicit calls for assistance to other researchers and health care professionals to improve the cardiovascular health and fitness offirefighters [3, 8].

It has been consistently reported in the literature that firefighters possessing high levels of cardiovascular fitness are at reduced risk of incurring a sudden cardiovascular event while performing vigorous activity on the job [8–10]. Despite this evidence, very little research has been conducted to examine the effects of the fitness training already implemented within the 10–26 weeks of training typically implemented/undertaken at U.S. firefighter training academies [11, 12]. Research has revealed that firefighter recruits experience an improvement in cardiovascular fitness from pre-training to post-training [13], and that firefighter recruits graduate with cardiovascular fitness levels at or slightly above the minimum standards established for performing live fire suppression tasks [14–16]. Based on the results above [13], fire departments should continue to implement mandatory exercise programs among new hires to establish minimum standards of cardiovascular fitness. Although evidence exists to support the contention that firefighter recruits’ cardiovascular fitness may improve from pre- to post- training [13], no research to date has been conducted to monitor and understand the changes or adaptations that occur across multiple time points during firefighter training academies. Since physiological training adaptations can occur in less than 16 weeks, such evidence-based knowledge could be utilized to modify the structure of training programs so as to maximize trainingefficiency (e.g., periodization) and elicit training gains beyond that which are already achieved.

Over the past few decades, several methods have been established to assess and monitor individuals’ cardiovascular fitness, results of which are typically reported in terms of maximal oxygen uptake (VO_2max). Perhaps the most common methods for cardiovascular fitness assessment in both the popularand academic realms are the treadmill and cycle ergometer maximal and submaximal VO_2max tests. Although the treadmill and cycle ergometer tests provide the most accurate prediction of an individual’s VO_2max, these tests often require expensive equipment, can take anywhere from 20–40 minutes to administer, and require administration by a trained professional. In light of these considerations as well as the limitations in resources available to most fire departments, researchers and fire organizations have utilized assessments such as the Forestry Step Test to assess cardiovascular fitness among firefighters. The Forestry Step Test, a submaximal test initially developed by Sharkey [17, 18] to predict the VO_2max of wildland firefighters, is a method of assessment that requires minimal and inexpensive equipment, takes less than 10 minutes to complete from start to finish, and can be administered without the direct supervision of a trained professional. In addition to the acquisition of VO_2max data, another benefit of utilizingthe Forestry Step Test is that heart rate recovery (ΔHR) data can be easily obtained during the administration of the test.

Previous research has indicated that ΔHR, or the rate at which one’s heart rate returns to resting values following a submaximal or maximal bout of exercise, is linked to both cardiovascular fitness [19, 20] and mortality [21–23]. Although ΔHR has been assessed and discussed in the firefighting literature as an indicator of cardiovascular strain [24], ΔHR has yet to be assessed or discussed in the literature as an indicator of firefighters’ cardiovascular fitness. Furthermore, no research to date has examined changes in ΔHR among firefighter recruits that may occur duringfirefighter training academies.

Given the paucity of research conducted to understand the phasic changes in firefighter recruits’ cardiovascular fitness over the course of the firefighter training academies, as well as the lack of literature on ΔHR, there is a need to concurrently monitor firefighter recruits’ VO_2max and ΔHR across multiple time points during the training academies. As such, the purposes of the current study were to: (a) describe changes observed in firefighter recruits’ estimated VO_2max across the duration of the training academy program, (b) describe changes observed in firefighter recruits’ ΔHR across the duration of the training academy program, and (c) determine the relationship between firefighter recruits’ estimated VO_2max and ΔHR across the duration of the training academy program. Changes observed in firefighter recruits’ body weight and body fat percentage as well as relationships between VO_2max, ΔHR, body weight, and body fat percentage were examined and reported to provide context for interpretation of study results.

2 Methods

3 Results

4 Discussion

Results of the current study indicated that throughout the training academy program: (a) firefighter recruits experienced significant improvements in estimated VO_2max; (b) firefighter recruits experienced significant improvements in ΔHR; and (c) estimated VO_2max was not significantly correlated with ΔHR at T1, T2, or T3. Furthermore, while attendingthe firefighter training academies, firefighter recruits experienced: No significant gains in estimated VO_2max or ΔHR after the first eight weeks of training, no significant loss in body weight until the last eight weeks of training, and no significant loss in body fat percentage after the first eight weeks of training. The findings of the current study, limitations of the current study, and directions for future research will be addressed throughout the next paragraphs.

It has been suggested that the minimum VO_2max necessary to complete fire suppression tasks on the job ranges between 40.3 and 45.0 mL/kg/min[14, 30–32]. During their 16 weeks at the academy, firefighter recruits in the current study achieved estimated VO_2max values above these suggested minimum standards, thereby demonstrating the practical effectiveness of the traditional academy program in terms of producing desirable fitness outcomes. Findings of the current study are also consistent with previous literature which suggests that short periods of resistance and/or endurance training (i.e., 12–16 weeks) can lead to significant improvements in cardiovascular fitness (i.e., VO_2max) among previously unfit or physically inactive young adults [33, 34]. In their study comparing the outcomes of high-intensity and traditional exercise interventions, Nybo and colleagues determined that 12 weeks of high-intensity interval running yielded a 12–16% increase in VO_2max among untrained, young adults. Other research has demonstrated that a 16-week periodized training program to improve overall fitness levels of firefighter recruits yielded a 28% improvement in VO_2max (35±7 to 45±6 mL/kg/min) as measured using a submaximal cycle ergometer test [13]. The sample of firefighter recruits in the current study experienced a 16.5% gain in mean estimated VO_2max during the first eight weeks of training, but lost 1.3% of that gain in the last eight weeks of training. A limitation of the current study is that a submaximal step test was used to estimate VO_2max values, as VO_2max testing with a treadmill or cycle ergometer was not possible in the available testing environment. Future research should be conducted to confirm the adaptations observed in the current study with those among a larger sample, utilizing a laboratory-based method to obtain VO_2max values.

Until the current study, no research had been cond-ucted to monitor the observed changes in ΔHR during fire training academies. Similar to the observed improvements in estimated VO_2max, the firefighter recruits in the study experienced a 12.8% improvement in mean ΔHR within the first eight weeks of training, and lost 2.8% of that gain over the next eight weeks of training. These findings are consistent with previous literature which suggests that strength and endurance trained athletes experience a more rapid ΔHR following exercise than sedentary subjects [35–39] and that post-exercise ΔHR may be accelerated after approximately eight weeks of moderate-intensity training [40, 41]. Future research should be conducted to: (a) confirm the ΔHR adaptations observed in the current study in larger samples of firefighter recruits; (b) explore the clinical relevance of ΔHR for long-term health, fitness, and performance; and (c) determine the link between or independence of ΔHR and VO_2max.

Since no relationship was identified between estimated VO_2max and ΔHR, it is possible that VO_2max and ΔHR represent two independent factors of cardiovascular health and/or fitness. Given that the step test used to predict maximal oxygen uptake in the current study was based on a 15-second post-exercise ΔHR measure, it is also possible that different calculations of ΔHR (e.g., 15-seconds post-exercise, 30-seconds post-exercise, 60-seconds post-exercise, etc.) or different mathematical components of ΔHR (e.g., slope of heart rate decline) may each be more or less related to estimations of VO_2max. Furthermore, and although the findings were not directly connected to a stated purpose of the current study, since ΔHR in the current study was only related to body weight at T1 and was not related to body fat percentage at any time points, future research could be conducted to better understand the role of muscle mass and/or muscular strength as it relates to ΔHR.

As the participants in the current study started to experience a small loss of training gains in both estimated VO_2max and ΔHR during the last eight weeks of the training academy, and since only 30% of U.S. fire departments have a preventive program in place to protect the cardiovascular fitness of firefighters after graduation [6], potential detraining patterns among firefighter recruits and graduates must be considered. Research suggests that detraining periods of just 3–6 weeks can result in 6–14% reductions in VO_2max [42], and post-exercise ΔHR improvements achieved during eight weeks of training can be lost in just four weeks of detraining [40]. Other researchers have suggested that overall fitness can regress by 50% duringa 4–12 week period of detraining [43], and that endurance capacity can vary substantially within a period of detraining without measurable changes in VO_2max [44]. Within the tactical domain, a recent study conducted by Liguori, Krebsbach, and Schuna Jr. [45] demonstrated that young, male Army Reserve Officers’ Training Corps (ROTC) cadets experienced a 4.3 –7.1% reduction in VO_2max during their3-month summer break, findings which were consistent with those observed in research on sport athletes [46]. Given the strong links between cardiovascular factors such as VO_2max and ΔHR and mortality [21–23], research is warranted to monitor the fitness levels of firefighter recruit graduates across multipletime points, examine the potential consequences of detraining among active duty firefighters, and establish the efficacy of interventions to prevent the loss of training gains achieved during fire training academies.

5 Conclusion

The current study is the first of its kind to describe the VO_2max and ΔHR adaptations observed during fire training academies across multiple time points. The collective findings of the current study suggest that while fire training academies are effective in yielding cardiovascular fitness gains over a 16-week period, there may be opportunities to modify programs targeting the last eight weeks in order to facilitate continuation, or at minimum, retention of gains. Furthermore, results of the current study indicate that VO_2max and ΔHR may represent two distinct but equally important factors of cardiovascular health and fitness, warranting the consideration of differentmodalities of training to best develop each factor. Much research is still needed to understand the clinical relevance of ΔHR in the contexts of long-term health and performance among firefighters.

Conflict of interest

The authors of the current study report that there are no conflicts of interest.