The findings of this study indicate that obesity has a significant negative impact on back and core muscular endurance in firefighters, regardless of which measure of obesity is assessed. Similar trends were observed for the relationships between all markers of obesity (BMI, body fat percentage, or fat mass/fat-free body mass) assessed in this study and back and core muscular endurance.
Several factors may explain the poorer back and muscular endurance times in obese firefighters. First, the biomechanical strategies required for the muscular endurance tests used in this study are likely impacted by body mass. The Modified Biering Sorensen and the Plank tests depend on gravity's action on body mass for the entire load and the ability to support body mass against gravity for an extended period of time. Thus, individuals with high BMI values are likely at a biomechanical disadvantage during these tests given the higher body loads that need to be supported against gravity. It is unclear if the same relationship between BMI and back and core muscular endurance exists when testing strategies are used that are not dependent on gravity's effect on body mass and exercise load. The lack of significant relationships between fat-free mass and muscular endurance measures in the bivariable and multivariable regression models may indicate that overall body mass may play a larger role than muscle mass in back and core muscular endurance performance testing. Other conceivable explanations for the poorer muscular endurance times in obese firefighters include possible differences between obese and nonobese individuals in physiology and morphology of the back and core muscles, general fitness levels, perceived exertion levels on fitness tests, and familiarity with physical fitness exercise, along with a possible negative influence of other comorbidities associated with obesity.
Considering that the multivariable regression models for back and core muscular endurance accounted for only 16–19% and 29–37% of the variance, respectively, these models do not account for a large portion of the variance in back and core muscular endurance. Therefore, other factors that were not measured in this study may be involved in performance on the muscular endurance tests. Also, because the total portion of variance accounted for in the models was low, back and core muscular endurance should be directly assessed and should not be predicted from age, self-reported amount of any physical exercise per week, and measures of obesity.
The present study's findings on the relationship of body composition and back muscular endurance in firefighters appear to be similar to the relationships observed in the general population. We uncovered four previous studies that assessed relationships between back muscular endurance (as assessed by some version of the Biering Sorensen test) and body composition in the general population [6
]. In three of these four studies [6
], significant moderate negative correlations between measures of body composition (BMI and body fat percentage) and back muscular endurance were observed.
In the present study, BMI and body fat percentage were elevated in a large proportion of participants, indicating a high prevalence of overweight or obese firefighters in this study. In the current study, 29% of the firefighters were obese according to BMI, and 42% of firefighters were obese according to body fat percentage. These findings are consistent with previous studies of obesity in firefighters which reported that approximately 34% of firefighters are obese according to BMI [3
] and approximately 48% are obese according to body fat percentage [3
]. In the present study, BMI misclassified 42.9% of obese firefighters (according to body fat percentage criteria defining obese as ≥25%) as nonobese (i.e., false negative diagnosis). Such misclassifications appear to be common in male firefighters, as a previous study reported a false negative rate of 32.9% [3
]. Given the relatively high rate of mismatch between BMI and body fat percentage, assessing both BMI and body fat percentage is reasonable since each method may give unique information on the health status of firefighters.
In addition to the relationship between obesity and poor back and core muscular endurance observed in the present study, obesity has been linked with decreased job performance and increased risk of disability in firefighters. A recent study assessed fitness parameters among firefighters, including BMI, body fat percentage, and performance on a physical ability test that consisted of climbing stairs, rolling, lifting, pulling, threading a hose coupling to a hydrant, and advancing a hose, wielding a sledgehammer, and rescuing a mannequin [18
]. In this study, poor performance on the physical ability test was correlated with high BMI (r
= 0.34, P
< 0.01) and high body fat percentage (r
= 0.36, P
< 0.01). A longitudinal study of firefighters reported that obesity was a significant risk factor for job disability, and that every unit increase in BMI was associated with a 5% increase in risk of job disability [4
]. This study also reported that obese firefighters (BMI ≥ 30) had a hazard ratio of 2 for an adverse employment event, defined as injured-duty status, termination of duty, resignation, premature retirement, or death, when compared to those with a BMI <27. Whether obesity is associated with increased risk for musculoskeletal disorders, such as low back pain, in firefighters is unknown and requires further study.
The present study has noteworthy limitations that should be considered when interpreting its findings. First, this study enrolled a relatively small number of participants, which limited the number of independent variables that could be considered in multivariable regression models. The study sample was powered based on potential differences in outcomes of the randomized trial, not the regression models of this cross-sectional analysis. Also, the present study only enrolled male participants from a single fire department in the southeastern United States, and it is unclear if results can be generalized to firefighters in other settings. The cross-sectional, observational design used in this study can only report on associations observed at the time of measurement and is therefore unable to establish causation between back and core muscular endurance and body composition. Additionally, it is unclear if performance on the back and core muscular endurance tests reflects the ability to perform physical activities required of firefighters. Although physical factors such as back and core muscular endurance and obesity measures are associated with low back pain, other factors that are associated with low back pain, such as psychosocial occupational factors, were not measured in this study. Finally, the measure of self-reported physical activity used in this study, although a part of the minimum data set established by the Fire Service Joint Labor Management Wellness-Fitness Initiative, has yet to be validated.