It is well known that firefighters encounter many hazardous occupational conditions. In particular, strenuous duties expose firefighters to high levels of stress and physical demands. As a result, strenuous emergency duties have been associated with markedly higher risks of cardiovascular events that lead to death or disability compared with nonemergency duty (13,17,18). These data are in agreement with studies from the general population finding that, among unconditioned persons, stressful situations resulting in sympathetic and cardiovascular activation such as vigorous exercise and snow shoveling, episodes of anger, and even watching major sports matches can "trigger" acute heart disease events (4,23,33).
Accordingly, maintaining and improving the cardiorespiratory fitness (CRF) of firefighters by increasing their physical activity (PA) is a major priority for the US fire service (15,25). This is especially the case because cardiovascular disease (CVD) has accounted for 44% of roughly 100 annual on-duty deaths among firefighters between 1995 and 2004 (9). Moreover, 765 nonfatal on-duty cardiovascular events were reported in 2005 (20). Thus, the combination of fatal and nonfatal on-duty cardiovascular events affects close to 1 in 1000 US firefighters each year. Finally, CVD also results in many other off-duty events and disability retirements (13).
Although it is universally accepted by fire service organizations and fire department physicians that firefighters should achieve and maintain adequate levels of CRF, there is less consensus as to the minimum aerobic capacity required for the safe performance of structural firefighting. While on-duty, structural firefighters often have to wear 50 lb of personal protective equipment, carry between 20 and 40 lb of tools, climb ladders, advance heavy water-filled hoses, occasionally drag or carry victims, and perform other heavy labor. These activities have been estimated to require a maximal oxygen consumption of ≥44 mL·kg−1·min−1 or 12.5 METs (11), which, in energy expenditure terms, is comparable to activities such as running at 7.5 miles·h−1 (8 min·mile−1) or vigorous cross-country skiing (1). Thus, the National Fire Protection Association has suggested 12 METs (about 42 mL·kg−1·min−1 V˙O2) as the minimum exercise capacity required for the safe performance of firefighting (24). However, the National Fire Protection Association is not a governing body and can only make recommendations. Each department decides on issues of exercise, fitness, and health screening individually.
The physical demands of the job, elevated on-duty CVD risks, and the steadily increasing prevalence of obesity among firefighters have led to growing concerns regarding the physical fitness of US firefighters. Several representative studies have found that roughly 75% of all firefighters are overweight, with 40% classified as obese, including a third of new recruits (5,16,30,31). Moreover, a recent population-based cohort study of both career and volunteer firefighters demonstrated a prevalence of overweight and obesity exceeding that of the US general population (28). In addition, this investigation proved that the high obesity prevalence was not due to misclassification of numerous highly muscular firefighters. Contrary to common wisdom in the fire service, Poston et al. (28) found that obesity in the population-based firefighter cohort was even more prevalent when assessed by body fat rather than body mass index (BMI) and that misclassifying muscular firefighters as obese by using BMI occurred very infrequently.
Inadequate exercise has been hypothesized as a major contributing factor to the current situation. Infrequent PA and inadequate amounts of PA are considered commonplace in the fire service. Structural firefighters' work shifts often involve long sedentary periods. The majority of US fire departments do not mandate exercise, lack regular exercise regimens, and do not require the maintenance of discrete physical fitness parameters after hire. Furthermore, discretionary time for exercise may be largely consumed by overtime work and second jobs. Poston et al. (28) estimated maximal oxygen consumption (V˙O2max) using PA self-reports reflecting the past 30 d along with BMI, age, and gender and found that the majority of firefighters likely had substandard fitness. However, the study design did not allow them to corroborate these estimates with an exercise test to measure CRF. Thus, few, if any, representative studies have systematically examined the exercise habits of large firefighter populations and their relationship to CVD risk and CRF as measured by a maximal exercise test.
On the other hand, the benefits of increasing PA have been demonstrated in the general population. For example, the Finnish twin cohort study showed that aerobically conditioned exercisers and even occasional exercisers had decreased risk of all-cause mortality and coronary heart disease compared with those who were sedentary (19,21). Hu et al. (14) found that PA reduces the risk of cardiovascular mortality in both obese and nonobese individuals. In addition to its beneficial effects on CRF, regularly practiced, frequent PA of sufficient duration can promote weight loss and prevent weight gain (2). Moreover, PA independently ameliorates multiple cardiovascular risk factors (10).
PA can be defined and quantified in several ways. To assess energy expenditure, three PA dimensions are usually considered: (a) frequency-"the number of events of physical activity during a specific period," (b) duration-"time of participation in a single bout of activity," and (c) intensity-"physiological effort associated with participating in a specific type of physical activity" (32).
In the present study, we attempted to determine the most beneficial dimension of PA among structural firefighters: weekly frequency and session duration or session intensity, specifically focusing on aerobic exercise activity and each dimension's correlation with objectively measured CRF and CVD risk factors.
The study sample was obtained from the ongoing cohort: "Predicting Cardiovascular Risk and Fitness in Firefighters" (US Department of Homeland Security Award No. EMW-2006-FP-01493; S.N. Kales, HSPH). The participants were structural firefighters recruited from 10 career fire departments located in the states of Kansas and Missouri. All participants were at least 18 yr, had no restrictions on duty, and performed a maximal exercise treadmill test (ETT) as part of a periodic fire department medical surveillance examination. Firefighters taking the exercise test for an exit examination, the evaluation of symptoms, or for retirement/disability evaluation were excluded from enrollment.
Institutional review board approval was granted by the Harvard School of Public Health Human Subjects Committee and local institutional review boards as appropriate. All participants provided informed consent and then completed in person the same written health and lifestyle questionnaire in addition to undergoing their fire department's standard medical examination. In this cross-sectional study, we analyzed data from 527 male career firefighters who had completed PA self-reports and had available ETT and CVD risk factor data.
ASSESSMENT OF PHYSICAL ACTIVITY
Self-reports of PA were extracted from health and lifestyle questionnaire responses. Firefighters were given standardized written instructions to complete the multiple-choice survey regarding eating, health, exercise, sleep, and work habits as honestly and as best as they could. They were also informed that the completed questionnaires would be confidential and would not become part of their fire department or medical record. To assess PA, we analyzed the answers to three selected multiple-choice questions about involvement in sports and exercise activities from the lifestyle questionnaire. Each of the three questions represented a different PA dimension (32): 1) frequency-"Most weeks, I exercise…(include home/work/gym and elsewhere)"; 2) duration-"Most times that I do cardio or aerobic exercise (e.g., jogging, brisk walking, bike, treadmill), I do an average of…each session"; and 3) intensity-"Most times that I exercise, I sweat…on average each session." To increase the power of the analysis, the original six response choices for frequency and duration were grouped into three and four alternatives, respectively. The questions, response alternatives, and the respective frequency of responses are shown in Table 1.
In addition, a new variable was calculated to evaluate the combined effects of PA frequency and PA session duration as total weekly aerobic exercise. Each of the six original alternatives in the frequency and duration dimension questions received a numerical value equivalent to the middle point of the range of each response choice. Thus, a value of 2.5 times per week was assigned to the frequency response of 2-3 times per week, 3.5 for 3-4 times per week, and so on. The same methodology was applied to the duration responses: 22.5 min was assigned for 15-30 min, 37.5 min for durations of 30-45 min, and so on. Subsequently, the resulting frequency and duration values were multiplied to estimate total weekly exercise in minutes per week.
For the present study, we did not consider other PA domains such as occupational PA, PA during transportation and household, or domestic PA if participants did not classify these as exercise in response to the questionnaire.
Assessment of CRF.
CRF was determined from symptom-limited, maximal treadmill exercise testing with ECG monitoring and estimation of oxygen consumption (METs) after the Bruce or modified Bruce protocols. The participants were encouraged to continue exercise until volitional exhaustion, even after exceeding 85% of their maximum predicted HR (220 − age). On average, the cohort achieved 99.7% ± 6.5% of maximal age-predicted HR on these tests. During the exercise test, total treadmill time in seconds (ETTT), maximum METs achieved (maxMETs), and HR recovery at 1 min (HRR) were recorded. In addition, an "autonomic index" was calculated by dividing the resting HR (RHR) (taken at rest preceding the physical examination) by the HRR as an assessment of integrated autonomic nervous system function (RHR/HRR index) (22).
Assessment of cardiovascular risk factors
Height was measured in a standing position with a clinic stadiometer. Body weight was measured with bare feet and in light clothes on a calibrated scale. BMI was defined as weight in kilograms divided by the square of height in meters. The RHR and blood pressure were measured before the physical examination with the subject in the seated position and using an appropriately sized cuff for the blood pressure.
Venous blood samples were analyzed for total cholesterol (TC), HDL, LDL, triglycerides (TG), glucose, and high-sensitivity C-reactive protein (hs-CRP) using standardized methods.
Each of the PA dimensions was analyzed separately as a categorical variable (Table 1). The outcome measures were all continuous. Analyses were carried out using SAS 9.1. We used the ANOVA method to assess the difference in the mean values among three or more groups and general linear models to examine the effect of covariates on the different outcomes after adjusting for age, BMI, and smoking status. In the analyses involving the total minutes of exercise per week variable, we further adjusted for physical exercise intensity (sweat). For statistical significance, we considered P < 0.05 using two-tailed tests.
The mean ± SD age of the 527 participants was 37.2 ± 8.6 yr. The mean ± SD BMI was 29.3 ± 4.5 kg·m−2. Using the World Health Organization BMI categorization, 13.1% (n = 69) were of normal weight, 51.2% (n = 270) were overweight, and 35.7% (n = 188) were obese. Only 63 participants (12.0%) reported regular smoking. Almost half (49%) of the cohort exercised three or fewer times per week, and only 22% exercised five or more times weekly. Regarding total weekly aerobic exercise, 47% were estimated to be exceeding 90 min of weekly of moderately vigorous PA, whereas only 20% exceeded 150 min weekly. The cohort's mean ± SD measured exercise tolerance was 12.7 ± 1.6 METs. More than a third (37%) of the firefighters had CRF ≤12 METs.
The analysis of PA frequency, duration, and intensity in relation to the main outcomes is shown in Tables 2-4. Among the four CRF outcome variables, ETTT and maxMETs consistently showed significant differences along the three PA dimensions after adjusting for age, BMI, and smoking status (P < 0.001). The dose-response patterns of both variables as a function of BMI and each of the PA dimensions are summarized in Figure 1, where adverse associations with increasing BMI category across all PA dimensions for both CRF and CVD risk factors are readily apparent. Although HRR progressively increased along with PA frequency, the difference did not reach statistical significance. Conversely, HRR showed statistically significant differences along the duration (P < 0.001) and intensity dimensions (P = 0.005) despite showing dose-response inconsistencies within the lower range of each PA dimension. The RHR/HRR index was only significantly associated (P = 0.02) with the duration dimension. Beneficial effects were most evident when comparing those whose reported exercise duration was ≥30 min with those who exercised <30 min (P = 0.003).
With respect to CVD risk factors, the frequency dimension showed significant associations with HDL (P = 0.001), TG (P = 0.02), TC/HDL (P = 0.003), and fasting glucose (P = 0.005), whereas hs-CRP levels decreased progressively although the changes did not reach statistical significance (P = 0.07). Neither the duration nor intensity dimensions showed overall significant associations with measured CVD risk factors. However, the mean values of HDL (P = 0.02) and TG (P = 0.04) were significantly improved and TC/HDL was borderline improved (P = 0.05, NS) in participants that exercised at least 30 min per session compared with those who exercised <30 min per session. In addition, participants who reported sweating at least moderately had significantly lower mean values of TG (P = 0.03) and borderline lower TC/HDL (P = 0.06, NS) compared with those who reported light sweating or who "do not exercise often."
The associations between the variable that combines the effect of frequency and duration (total weekly exercise in minutes per week) and the main outcomes are shown in Table 5. Significant associations were found for all CRF indicators even after multivariate adjustment except for the HRR and RHR/HRR index in multivariate model 2.
Total weekly exercise was also positively associated with HDL (P = 0.003) and negatively associated with TG (P = 0.03), TC/HDL (P = 0.01) and hs-CRP (P = 0.03). Surprisingly, although there was also a borderline association with systolic blood pressure (SBP) (P = 0.05), the association is positive. Hence, for every minute increase in exercise per week, the SBP increased.
The present study on US firefighters strongly supports previously described associations between PA or physical exercise and CRF (27). The association with CRF was strong across all three measured PA dimensions, as well as with total weekly aerobic exercise (min·wk−1). Unfortunately, the investigation also documents that >75% of the career firefighters studied reported that they engage in <150 min of aerobic exercise on a weekly basis, the minimum recommended by the Centers for Disease Control and Prevention and US Preventive Services Task Force guidelines (3). This fact, coupled with the high prevalence of obesity, likely explains that 37% of our participants were unable to exceed a CRF of 12 METs, the minimum exercise capacity required for the safe performance of firefighting according to the National Fire Protection Association.
The frequency of PA and total weekly exercise also showed significant independent associations with multiple CVD risk factors, which imply that increasing these behaviors among firefighters would improve their fitness and risk profiles. We found that frequency of PA and total weekly exercise was associated strongly with HDL, TC/HDL, and TG but not LDL or TC. In a meta-analysis of cross-sectional studies, Durstine et al. (7) found similar results for PA frequency dimension as well as weekly exercise duration, including significant dose-response relationships between physical exercise and HDL increase, as well as TG decrease, but not TC or LDL.
Although there is a general consensus that PA has a direct role in the prevention of insulin resistance (12), it is not known which PA characteristic is most helpful to prevent diabetes. Because our study demonstrated a strong dose-response correlation between the frequency dimension and fasting glucose, we suggest that increasing the exercise frequency may have the strongest effect.
Interestingly and despite the numerous reports that associate exercise training with blood pressure reduction (8), our study did not find beneficial associations for any of the three dimensions evaluated. In fact, the present study found that the SBP increased along with increasing minutes of aerobic exercise per week, although the effect was not clinically significant (and only borderline statistically significant).
The use of sweating, as in our study, as a measure of PA intensity has been discussed in the literature (6,26). Because decreased physical fitness levels may mean that less exercise is needed for sweating, we expected this measure to have limitations as a measure of exercise intensity. In addition, the assessment of the activity intensity by sweating might be distorted by individual body size. In fact, 27% of our obese participants reported heavy sweating during physical exercise. Despite all these disadvantages, we still found strong associations between the intensity dimension and the objective measurements of CRF, although the decrease of TG and TC/HDL ratio was also significant and borderline significant, respectively, in subjects who reported moderate or heavy sweating.
The creation of a continuous variable estimating total weekly aerobic exercise in minutes per week allowed us to interpret the slope of various relationships in a linear regression model. The similar CVD risk factor patterns in association with total minutes of PA per week and with PA frequency suggest that the benefits of increased weekly PA duration are primarily mediated by the PA frequency.
Beyond the cross-sectional nature of the study, another limitation of the present study is the self-report instrument used to evaluate PA. Social desirability bias can lead to overreporting of PA (29). However, this would have biased our study toward negative results. Thus, our results may in fact be conservative if they were indeed affected by social desirability bias. Given that current exercise habits are easy to remember and quantify and the study regarded present, not past activity, recall bias should have been minimal.
Another limitation of the study is that we only measured PA considered by participants to encompass exercise training and we did not consider other activities of daily living that may enhance the energy expenditure. Nevertheless, not considering those other activities most likely would have biased our results toward the null hypothesis.
Our study has several important strengths. First, we had multiple objective measures of CRF and CVD risk against which to validate PA self-reports. Second, we had a large sample size. Third, our sample had similar demographic and anthropometric characteristics to those found in other epidemiologic studies of firefighters (5,16,31), suggesting that the overall results of the present study can be generalized to male career firefighters nationwide.
Although the beneficial effects of PA on CRF and CVD risk profiles were evident in our study across all BMI categories, on average, increasing BMI markedly limits the benefits achieved. In other words, obese firefighters are at a distinct disadvantage with respect to CRF and CVD risk profiles, and in most cases, despite similar levels of PA, they would also need to achieve weight loss to reach the same levels of CRF and CVD risk as lighter colleagues. These findings are in agreement with Poston et al. (28), who found that obese firefighters (career and volunteers) were much more likely to have substandard fitness than nonobese counterparts were. The present study, limited to career firefighters, found that only 20% were meeting the Centers for Disease Control and Prevention goals for weekly aerobic exercise, and over a third had substandard fitness as assessed by objective testing. Therefore, our study has significant implications for the US fire service given the prevalence of obesity and substandard fitness prevalence. In particular, firefighters should be engaging in more frequent aerobic PA. In addition, given that excess BMI has a major limiting effect on the benefits of PA in terms of CRF and CVD risk, weight loss needs to be promoted among obese firefighters.
In conclusion, we found significant associations between higher levels of PA across all three PA dimensions with increasing CRF. However, it seems that increasing exercise frequency during the week has the most beneficial effects on specific CVD risk factors and may be the most important single dimension in designing fitness interventions for firefighters. Further investigations taking into consideration total energy expenditure would be desirable but likely difficult from a practical standpoint. Ongoing longitudinal studies that demonstrate various PA dimensions as independent predictors of CVD events and other health outcomes among firefighters would provide the most definitive evidence.
This investigation was supported by the Federal Emergency Management Agency Assistance to Firefighters Grant program's awards EMW-2006-FP-01493 (S.N. Kales) and EMW-2009-FP-00835 (S.N. Kales).
The authors do not have professional relationships with companies or manufacturers who will benefit from the results of the present study.
The authors would like to thank all of the participating firefighters and Fire Departments; the staff and clinical leadership of the clinics who examined the firefighters; Ms. Brianne Tuley, Dr. Lilly Ramphal, and the late Dr. William Patterson for their contributions to the underlying longitudinal study.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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Keywords:©2011The American College of Sports Medicine
EXERCISE; METS; OCCUPATIONAL; METABOLIC; EPIDEMIOLOGY