Introduction

Firefighting is one of the most hazardous civilian occupations, implying variable working conditions under time urgency. Several studies [1,2, 3, 4, 5, 6] have shown that the combination of physical activity and/or exposure to external heat sources causes an increased physiological and psychological stress.

During fire emergencies, firefighters wear personal protective devices (PC), composed by a layered thermal protective clothing (flame resistant outer shell and insulating thermal liner), a heavy footwear, a helmet, and a self-contained breathing apparatus (S.C.B.A.). Despite PC+SCBA are a good barrier for the firefighters from thermal radiation, burns, injuries, smoke and noxious gases these can have negative effects on gait, metabolic and thermal efficiency, fatigue leading to a significant reduction in work capability and work duration [7, 8, 9, 10, 11, 12, 13].

Among the different parameters the maximum oxygen uptake (VO_2max) is the most frequent variable taken into account. Cardiovascular strain has been discussed in response to a real and simulated emergency and during different tasks [7, 8, 14, 15, 16]. In particular, single firefighters’ activities with PC+ SCBA showed an average VO₂ of 39.0 ml^.kg^-1.min^-1 while climbing stairs, a VO₂ of 23.4–25.7 ml^.kg^-1.min^-1 lifting and moving the hose, a VO₂ of 30.9 ml^.kg^-1.min^-1 controlling a flexible tube and a VO₂ of 36.6–44.0 ml^.kg^-1.min^-1 transporting equipment on the stairs climb [1, 17]. Perroni et al. [8], analyzing simulated emergencies of Italian firefighters, have shown that the values of VO₂ remained elevated after 30 min of rest (VO₂: 8.86 ± 2.67 ml^.kg^-1.min^-1) compared with basal values (VO₂: 4.57 ± 1.07 ml^.kg^-1.min^-1). Furthermore, it has been shown in firefighters a constant and annual physiological decrease related to age [18, 19, 20]. For these reasons, numerous studies [21, 22, 23, 24, 25] have suggested VO_2max> 33 ml^.kg^-1.min^-1, with preferably >45 ml^.kg^-1.min^-1, to successfully complete rescue activities.

Although the most accurate method to determine the aerobic power of individuals is the direct measurement of VO_2max, some aspects may limit the applicability of large-scale assessments such as a lot of time to carry out the test, sophisticated and expensive equipment, and large effort of individuals for the attainment of VO_2max. In contrast, the field trials are inexpensive, easy to administer and might be best suited to examine large number of firefighters in minimum time.

Several studies [23, 26, 27, 28, 29] have demonstrated that evaluation of VO_2max in weight-related rate (ml kg^-1 min^-1) is fundamental for job performance. Given that job performance could be influenced by body weight, by weigh of PC+SCBA and/or by further weight of supplementary fire device (i.e., operating high pressure hoses, ladders, lifting weights), the purposes of this study were; 1) analyze the utility of evaluate the maximum oxygen uptake of Italian firefighters recruits in absolute or weight-related terms by a Queen’s College Step field Test (QCST) (performed with and without wearing fire protective garments), 2) compare the effects of wearing fire protective garments and 3) analyze the differences among age groups.

Methods

Results

Means, standard deviations and statistical differences (p<0.05) of anthropometric and aerobic profile of the participant to the study are shown in Table 1.

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Table 1. Means ± standard deviations of anthropometric and aerobic data across age group.

https://doi.org/10.1371/journal.pone.0119757.t001

Wearing the PC+S.C.B.A., 26 (13%) of all firefighters failed to complete the test and have been excluded from the total data analysis (Table 2).

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Table 2. Firefighters (n°), number (n° failed) and percentage (%) of Firefighters who did not complete the Queen’s College step test with PC+ S.C.B.A.

https://doi.org/10.1371/journal.pone.0119757.t002

Comparing the average values of weight-related VO_2max of tests without (VO_2max = 48.7 ± 7.1 ml^.kg^-1.min^-1) and with PC+S.C.B.A. (VO_2max = 39.9 ± 5.4 ml^.kg^-1.min^-1) of all firefighters, we can notice a sharp decrease (22%) between the two. In contrast, the average values of absolute VO_2max evaluated without (VO_2max = 3.7 ± 0.6 L^.min^-1) and with PC+S.C.B.A. (VO_2max = 3.9 ± 0.6 L^.min^-1), we can notice an increase of 6% between test.

In particular, the higher percentage differences were observed between 26–30 yr vs 31–35 yr and between 36–40 yr vs >40 in test performed with PC+S.C.B.A. both in absolute (9.5% and 5.3%, respectively) and weight related (7.7% and 6.0%, respectively) terms. Test performed without PC+S.C.B.A. have shown the higher percentage differences between 26–30 yr vs 31–35 yr and between 36–40 yr vs >40 in absolute term (5.2% and 5.5%, respectively), whereas between 26–30 yr vs 31–35 yr and between 31–35 yr vs 36–40 in weight related values (5.9% and 5.7%, respectively).

VO_2max values showed significant differences between QCST performed with and without PC+S.C.B.A. in absolute (F_(1,169) = 42.6, p < 0.0001, ES = 0.19) and weight-related (F_(1,169) = 339.9, p < 0.0001, ES = 0.57) terms.

Analysing all subjects, the tests performed with and without PC+S.C.B.A. have shown a better correlation coefficient in L min^-1 (r = 0.67) than in ml kg^-1 min^-1 (r = 0.54). Across age groups, correlation between tests performed with and without PC+S.C.B.A. observed in absolute and weight-related values showed the same trend: <25 yr (r = 0.66 and r = 0.39, respectively), 26–30 yr (r = 0.71 and r = 0.54, respectively), 31–35 yr (r = 0.61 and r = 0.43, respectively), 36–40 yr (r = 0.60 and r = 0.25, respectively), and >40 yr (r = 0.58 and r = 0.25).

Discussion

The purpose of this preliminary study was to evaluate the maximum oxygen uptake (absolute and weight-related) of Italian firefighters recruits performed with and without wearing PC+S.C.B.A. and to analyze the differences among age groups. The main findings of this study were the statistical differences between tests performed with and without PC+S.C.B.A. in Italian firefighters recruits and a high correlation between test observed in absolute value.

It is generally recognized that VO_2max is the single best physiological indicator of muscular capacity for sustained work and that the assessment of VO_2max can determine workers’ cardiovascular health and physical capability required to carry out their duties safely and effectively. Consequently different exercise protocols are used to determine VO_2max for assessment of cardiovascular fitness, prescribed training programs and evaluate his effects on the health in an occupational setting.

Since the most precise method of measuring VO_2max is a maximal laboratory exercise test and it isn’t always possible to do, we used the QCST as it is recommended as a valid method to evaluate cardiorespiratory fitness for large numbers of population [30, 35] and the movement was similar to that used for the job performance of the firefighters (i.e. climbing stairs).

Even if we have found an increased number of firefighters who failed to complete the test with PC+S.C.B.A., they showed the lowest values compared with Perroni et al. [36, 37]. Percentage differences between the values recorded with and without PC+S.C.B.A. and observed in weight-related values, have shown a higher decrease in 31–35 yr (19%) compared with the other categories (<25 yr = 17%; 26–30 yr = 17%, 36–40 yr = 17% and >40 yr = 18%). In contrast, differences between tests observed in absolute terms have demonstrated a higher increase of values in <25 and 26–30 yr (10% and 11%, respectively) compared with 31–35 yr (3%), 36–40 yr (6%) and >40 yr (6%). Punakallio et al. [38] have found that the average annual change aerobic capacity, in male Finnish firefighters (30–44 yrs) at 3- and 13-year follow-ups, was −1.12% in absolute (L^.min⁻¹) and −1.33%. in weight-related (ml^.kg^−1.min⁻¹). A previous study by Perroni et al. [37] had hypothesized that decreased values of performance and increased percentage of failure were due to premature muscle fatigue of the lower limbs muscle, caused by an overload of the musculoskeletal system and thermoregulation, rather than by a deficit of cardiovascular system. As fatigued muscles can affect lifting techniques and can increase the risk of injury [39], this is a parameter that has to be taken into account while training programs. In this direction, Griefahn et al. [40] declare that at the same extra-load the subjects with elevated body mass (i.e. height and weight) are less affected by the load carried with lower cardiovascular strain.

In this study, the aerobic power of all firefighters measured without PC+S.C.B.A. showed lower absolute (3.7 Vs 4.6 L^.min^-1) and weight-related (48.7 Vs 58 ml^.kg^-1.min^-1) values than the values investigated by Lindberg et al. [41] on full-time firefighters (mean age: 34 yrs, range: 20–57) by submaximal treadmill running test. In addition, in the same study Lindberg et al. [41] found a correlation between work tasks and absolute and weight-related VO_2max.

Wearing PC+S.C.B.A., all ranges of age of firefighters were still within the minimum values, but below the preferable values recommended for the successful completion of a standard rescue protocol. Otherwise, absolute VO_2max data have shown values in line (2.7–4.0 L^.min^-1) with those proposed for firefighters by O’Connell et al. [17] and von Heimburg et al. [26].

Results of all our firefighters were lower than Perroni et al. [36, 37] and, analyzing the aerobic values of each age category, than Perroni et al. [37] for 31–35 yrs, 36–40 yrs and >40 yrs. The firefighters values examined in this investigation are substantially higher than the values measured by Swank et al. [42] on a cycle ergometer and by Hammer and Heath [43] on a treadmill for the same mean age.

Given that various studies [21, 44, 45, 46, 47, 48] have shown a correlation between high levels of fitness and improved job performance during actual firefighting activities and a decreased risk of injury, a periodic fitness evaluation of firefighters should be done. Although the aim of the tests is to measure specific physical capacities and evaluate the relation between different tests, a high number of experiments could be time consuming and may be physically and mentally strenuous for the firefighters. A reduction in tests number and the choice of an adequate test could be useful for the organization of specific and periodic assessment of physical fitness of firefighters. Given that 1) in this study we have found a large correlation coefficient between tests performed with and without PC+ S.C.B.A observed in L min^-1 (r = 0.67), 2) Perroni et al. [8] found the same values as in our test performed with PC+ S.C.B.A. observed in absolute rate and 3) the absolute VO_2max does not take into account the body weight of subjects and the heavy PC+S.C.B.A., we suggest that the QCST performed with PC+ S.C.B.A. observed in absolute term can be an useful tool to evaluate the cardiovascular strain of the firefighters.

Conclusion

Our results show that there are significant differences among step tests performed with and without PC+S.C.B.A., observed in absolute and weight-related rate, and that a simple procedure can be carried out for the evaluation of VO_{2 max} in large number of firefighters. A systematic and periodic assessment of fitness level of firefighters with PC+S.C.B.A. (by a correlate field test) could allow us to create specific training programs and monitor the evolution of the physical capabilities of firefighters during the job career to perform their occupational activities safely reducing the risk of injuries. Moreover, the evaluation of VO_2max observed in L^.min^-1 could facilitate a comparison between the same firefighters and different subjects during a specific period of physical conditioning.

Acknowledgments

The authors would like to express their gratitude to the Italian Firefighters Corp for the support in carrying out the experimental sessions.