Effects of Cardiovascular Disease Risk Factors, Musculoskeletal Health, and Physical Fitness on Occupational Performance in Firefighters—A Systematic Review and Meta-Analysis

Background: Firefighting is a strenuous occupation, which necessitates that firefighters stay in good physical condition and maintain adequate cardiovascular and musculoskeletal fitness to perform their duties with minimal health and safety risks. The aim of this review is to determine the effects of cardiovascular disease risk factors, musculoskeletal health, and physical fitness on the occupational performance of firefighters. Methods: PubMed/Medline, SCOPUS, Web of Science, EBSCOHost, and ScienceDirect were searched without time-restriction. The appraisal tool for cross-sectional studies and the Critical Appraisal Skills Programme toolkit were used to conduct the methodological assessment. Data were analyzed using Review Manager 5.3, and MedCalc® statistical software. Results: Age had a moderate effect on occupational performance (Z = 5.15, p < 0.001), whereas gender had a large effect size on occupational performance (Z = 4.24, p < 0.001). A significant moderate negative correlation was found between cardiorespiratory fitness and occupational performance (R = −0.584, p < 0.001). Significant low negative correlations were found between upper body endurance (R = −0.344, p < 0.001), abdominal endurance (R = −0.308, p < 0.001), grip strength (R = −0.421, p < 0.001), upper body strength (R = −0.318, p < 0.001), and lower body strength (R = −0.216, p = 0.020) and occupational performance. Conclusions: Aged firefighters with poor body composition and lower levels of physical fitness performed worse on all occupational performance tasks.


Introduction
Firefighting is a hazardous occupation that places high physiological and psychological stressors on firefighters, thereby, posing significant risks to their health and wellbeing [1][2][3]. In addition, the environmental stressors include extreme temperatures, and hazardous chemicals and fumes [3][4][5][6][7][8]. The extreme environmental conditions necessitate that firefighters wear heavy, insulated personal protective equipment (PPE), which often includes self-contained breathing apparatus (SCBA) that places tremendous strain on their cardiovascular system [6,8,9]. Moreover, firefighters are required to perform strenuous work duties, such as emergency rescues, first aid and resuscitation, and emergency extrication from vehicles, all while working irregular hours [1,8,10,11]. These types of strenuous and irregular working conditions place significant strain on the musculoskeletal and cardiovascular systems of firefighters, increasing the risk of serious injuries and sudden cardiac events, while on duty [1,[12][13][14].
Existing research indicates that many firefighters have multiple cardiovascular disease (CVD) risks factors or poor overall cardiovascular health [3,[15][16][17][18][19], poor musculoskeletal health [20][21][22][23] and inadequate physical fitness [24][25][26][27], which significantly and negatively affect their occupational performance [20,21,[28][29][30][31]. Extensive scientific literature indicates that among emergency services professionals, firefighters have one of the highest percentages of mortality (45%) due to sudden cardiac death (SCD), with the majority related to underlying CVD risk factors [1,10]. These deleterious consequences are likely, at least partially, attributed to inadequate physical fitness, which invariably results in overexertion and increased cardiovascular strain [7,8,32], particularly when wearing full protective gear. Under these conditions, studies have shown the induction of maximum physiological responses, and often with adverse health outcomes [9,32,33]. In addition, firefighters have been reported to have the highest incidence of musculoskeletal injuries among all emergency services personnel [1], which is likely attributable to a combination of the weight of the PPE [32], the high prevalence of obesity [34][35][36], the necessity for sudden changes in posture and gait on rescue [35,36] and the high musculoskeletal demand of their professional duties [37][38][39]. The combination of extraordinary musculoskeletal health demands, deteriorating cardiovascular health and inadequate physical fitness in many firefighters, may lead to significant morbidity and mortality in this population [40,41]. In addition, the progressively deteriorating cardiovascular and musculoskeletal health with increasing age, and the overall poor physical fitness significantly and negatively affect firefighters' occupational performance [15,17,20,21,[28][29][30]. Consequently, firefighters who are unable to perform their duties with sufficient competency and rigour are at risk of underperformance while on-duty [30,42], thereby, placing their lives as well as those of other civilians at increased risk, and increasing the potential loss of property and infrastructure. Firefighters who are not fit for active duty may be at increased risk of sustaining cardiovascular events and musculoskeletal injuries [27,30,43,44].
Measuring firefighters job performance while on active duty is an inherently difficult and costly task, due to the physical nature of their occupation and the high likelihood of equipment becoming lost or damaged [45]. This is particularly true for fire departments in developing countries or those fire departments that cannot afford to equip firefighters with this equipment [46,47]. Therefore, to assess firefighters' work ability, fire and rescue departments use occupational simulation protocols to determine if firefighters are able to perform their duties with sufficient rigor [6,28,33]. Previous research has indicated that occupational simulation protocols are the closest representation of the stressors of firefighting [48]. Globally, an alarming number of firefighters are at increased cardiovascular disease (CVD) risk, while suffering from multiple musculoskeletal disorders and operating under suboptimal levels of physical fitness. This negatively effects their occupational performance and limits their ability to cope with the on-duty demands [2,6,31,39,49,50]. However, there have been no previous systematic reviews investigating the effects of CVD risk factors, musculoskeletal health, and physical fitness on the occupational performance of firefighters, which motivated the need for the present study.
The relative lack of systematic reviews on this current topic was somewhat surprising, given the nature of the occupation. Providing more information on the effect that cardiovascular disease risk factors, musculoskeletal health and physical fitness have on occupational performance may provide valuable evidence in informing policy makers and fire departments. For more information on the aim and objectives of this review, please refer to the published protocol: Ras et al. [51]. Briefly, the aim of this systematic review and meta-analysis was to determine the effects of CVD risk factors, musculoskeletal health and physical fitness on the occupational performance of firefighters. The objectives of the review were (i) to investigate the effects of cardiovascular health on the occupational performance of firefighters; (ii) to investigate the effects of musculoskeletal health on the occupational performance of firefighters; (iii) to investigate the effects of physical fitness on the occupational performance of firefighters, (iv) and, to investigate the relationship between cardiovascular health, musculoskeletal health and physical fitness on the occupational performance of firefighters.

Materials and Methods
The guidelines for Meta-analysis of Observational Studies in Epidemiology (MOOSE) and Quality of Reporting of Meta-analysis (QUOROM) guided our methods when conducting this review [52,53]. When considering studies for this review, the PRISMA guidelines for systematic reviews was followed, and the outcomes for each step was described in a flow-diagram [54] (Figure 1).

Selection of Studies
All studies, as full-text articles, that met the inclusion criteria were selected for screening. Every attempt was made to contact the authors for full-text articles or missing data. Thereafter, the full-text articles were assessed independently by two reviewers using the Rayyan ® intelligent systematic review (RIS) tool [55]. When screening the studies, three categories were used, namely, included, excluded and unsure. Any uncertainties regard-

Summary of Methods
The study design of choice is a quantitative systematic review, where participants included adult, full-time, part-time and volunteer firefighters between the ages of 18 to 65 years. The exposures assessed included cardiovascular health, musculoskeletal health, and physical fitness in relation to the occupational performance of firefighters. The inclusion criteria were as follows: (i) studies that recruit full-time adult firefighters, with no limitations to publication year; (ii) studies investigating the effects of cardiovascular health, musculoskeletal health and/or physical fitness on the occupational performance of firefighters; (iii) studies available in full-text. Exclusion criteria included: (i) studies focusing on other outcome measures as the main exposures or outcomes; (ii) systematic reviews or other types of reviews; (iii) articles that are non-English. The protocol for this study has been published and more information on the methods involved in the current manuscript may be found at: Ras et al. [51].

Search Strategy for Identification of Studies
A detailed literature search was conducted by the two primary reviewers (JR and RN), tasked with independently identifying studies, extracting the data, verifying the data collected and grading the quality of the results. JR was the principal investigator tasked with data analysis, narratively synthesising the data and writing up of the systematic review. A third reviewer (LL) was tasked with adjudicating and resolving any disagreement between the two independent reviewers.

Electronic Literature Search
The following journal databases were searched: PubMed/Medline, SCOPUS, Web of Science, EBSCOHost and ScienceDirect with no limitation to publication year. Keywords and medical subject heading (MeSH) terms were used in various arrangements depending on the specific database. A combination of the appropriate terms (search string) was used to ensure the inclusion of the relevant components of the participants, exposure, comparison, and outcome (PECO). The details of the search strategy can be found in Supplementary S1.

Additional Searches for Grey Literature
The search strategy was completed by searching the following databases for grey literature: Google, Google Scholar and Networked Digital Library of Theses and Dissertation.

Selection of Studies
All studies, as full-text articles, that met the inclusion criteria were selected for screening. Every attempt was made to contact the authors for full-text articles or missing data. Thereafter, the full-text articles were assessed independently by two reviewers using the Rayyan ® intelligent systematic review (RIS) tool [55]. When screening the studies, three categories were used, namely, included, excluded and unsure. Any uncertainties regarding study inclusion were discussed between the two reviewers. In the event of disagreement, a discussion was held with the third reviewer, and resolved by the latter.

Data Extraction and Data Management
A researcher-generated data extraction form was used (Supplementary S2 and Supplementary S3). The information extracted was the general study details, such as authors, date of study publication, study title, study design and country of study, the exposure assessed, and the outcome measures. Study characteristics were collected, such as sampling method and sample size, and details of the participants. In addition, the details of exposure and the outcome variables were extracted, i.e., the study must have reported on at least one of the exposure variables in relation to firefighter occupational performance.

Critical Appraisal of Included Studies
The appraisal tool for cross-sectional studies (AXIS checklist) (Table 1) [56] and The Critical Appraisal Skills Programme (CASP) toolkit (Middle Way, Oxford, UK) ( Table 2) (https://casp-uk.net/casp-tools-checklists/ (accessed on 1 March 2021)) were used to conduct the methodological assessment of each study included. The CASP toolkit (Middle Way, Oxford, UK) was previously used in systematic reviews on firefighters and tactical personnel to assess study methodologies, and allows for fair and equitable assessment of a variety of study types. The AXIS toolkit was shown to be a reliable and valid tool for assessing the quality of cross-sectional studies [56].

Classification of Age and Obesity and Physical Fitness for Meta-Analysis
Age was classified as male firefighters over the age of 45 years, and obesity was classified as a BMI of 30 kg·m 2 or higher or a bodyfat percentage (BF%) over 25%. For cardiorespiratory fitness, only studies that included either absolute (mL·kg·min) or relative (L·min) . VO2max were used. These estimates included both from direct gas analysis and those estimated with maximal or submaximal . VO2max. For upper body and abdominal muscular endurance, the push up and sit ups endurance tests were preferred. For upper body strength grip strength and the bench press were used as the preferred measures, and for lower body only studies including the leg press or squat were included. For flexibility, the sit and reach test was preferred. These physical fitness tests were favored due to their frequency of use across multiple studies when used to indicate overall health-related physical fitness in firefighters.

Assessment of Overall Effect Size
The outcome measure (occupational performance) was analysed as a continuous variable. The mean difference (MD) and standardized mean difference (SMD), with 95% confidence intervals (CI), of estimation was used to estimate the effect, using the inverse variance method of meta-analysis, between cardiovascular health and occupational performance and physical fitness and occupation performance in firefighters [81]. For the correlation analysis, MedCalc ® statistical software Ltd., Ostend, Belgium (version 20.104), was used to perform the correlation meta-analysis. Investigators grouped the "R" values according to cardiovascular disease risk factors, physical fitness components and overall performance, and combined them into a single representative effect estimate [82]. Metaanalysis techniques were applied using the number of studies, original "R" values and sample sizes to generate the pooled "R" values between each cardiovascular health component, fitness component and job task component [82]. Where one study, or insufficient studies were present, a meta-analysis on the pooled "R" was not calculated [82]. The original "R" values were converted to a common test metric using the Fisher's "R" to "Z" transformation [82]: The Fisher's Z values from the original studies were combined using random effect models for all analysis' performed [82]. The following was used to indicate the strength of correlation, 0.00 to 0.30 (−0.00 to −0.30) for negligible correlation; 0.30 to 0.50 (−0.30 to −0.50) for low correlation; 0.50 to 0.70 (−0.50 to −0.70) for moderate correlation; 0.70 to 0.90 (−0.70 to −0.90) for high correlation and 0.90 to 1.00 (−0.90 to −1.00) for very high correlation [82].
Selection process likely to select subjects that were representative of the target population?
Measures undertaken to address and categorize non-responders?
Were the risk factor and outcome variables measured appropriate to the aims of the study?
Were the risk factor and outcome variables measured correctly using instruments/measurements that had been trialed, piloted or published previously? Clear which tests were used to determine statistical significance and/or precision estimates?
Were the methods sufficiently described to enable them to be repeated?

Results
Were the basic data adequately described?
Does the response rate raise concerns about non-response bias?
If appropriate, was information about non-responders described?

N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A
Were the results internally consistent?
Were the results presented for all the analyses described in the methods?

Discussion
Were the authors' discussions and conclusions justified by the results?
Were the limitations of the study discussed? × Other  TOTAL SCORE  15  15  15  15  15  15  15  16  16  16  16  17  17  17  17  17  17  17  17  17  17  17  18  19  19  19  19 Note: NA-indicated questions that were not applicable for grading of the overall study quality; -indicates yes; ×-indicates no. Note: NA-indicated questions that were not applicable for grading of the overall study quality; -indicates yes.

Assessment of Heterogeneity
Heterogeneity was evaluated using the Chi-square test, I 2 test and Cohen's Q test [83]. The following was used to explain I 2 statistics: (1) 0% to 30%: may not be important; (2) 31% to 60%: may indicate moderate heterogeneity; (3) 61% to 80%: may indicate substantial heterogeneity; (4) 81% to 100%: considerable heterogeneity. Regardless of whether homogeneity or heterogeneity were present between studies, a random-effects model was preferred in order to maintain consistency in the interpretation of results [83]. To assess the risk of bias between studies, the Egger's test and Begg's test were performed.

Subgroup Analysis and Investigation of Heterogeneity
When heterogeneity was present, a subgroup analysis was performed to explore the sources of heterogeneity [81,83]. Where applicable, subgroup analysis included the following: weight of personal protective equipment (PPE), the sex of the firefighters (male and females), the number of tasks performed and if tasks were sequentially or discretely conducted, for full-time firefighters. For the weight of PPE, studies that had a combined weight of PPE above 22 kgs. The number of tasks performed included studies where firefighters performed five or more tasks during the occupational simulation protocols. Sequential tasks included studies that included tasks that were performed sequentially, i.e., followed a specific order, whereas discrete tasks included studies that had no specific order. An additional subgroup analysis was included for cardiorespiratory fitness, which included studies that estimated cardiorespiratory fitness directly by using gas analysis. Although all exposures were measured using a standard physical ability test or simulated work-related tasks, the methods used could be different, which required comparing and converting certain measurements to produce similar findings for comparison.

Study Selection
Initially, the electronic database searches yielded 8084 publications, with an addition of 10 studies found through reference list searching ( Figure 1). After removal of duplicates, 3363 studies remained and were screened using title and abstract information. Of these, 3300 studies were excluded for not meeting the inclusion criteria, leaving 63 studies that were designated for full-text screening. A total of 33 studies were excluded after screening the full text, and 30 studies were eligible to proceed to data extraction. After data extraction, 25 studies were included for the final meta-analysis.

Assessment of the Strengths and Weaknesses of Studies
The strengths and weaknesses of the studies were assessed and the most frequent weakness of the studies, according to the AXIS checklist, were: (a) the sample size was not justified (13/27); (b) the sample frame not taken from an appropriate sample base (22/27); (c) the sample selection not likely to select participants that represented the target population (21/27) ( Table 1). These weaknesses were largely due to the nature of the study types and the relatively small sample sizes. The quality of the included studies was acceptable. A score of 15 point was given a score of "moderate", 16 to 17 point given a score of "good" and scores between 18−19 given a score of "high" quality ( Table 1). Scores of lower quality (<15) were excluded from this review. Then strengths and weaknesses of cohort studies were assessed according to the CASP checklist for cohort studies, and all studies were of high quality, with N/A given to two questions, namely: (a) What are the results of this study? and (b) How precise are the results? (Table 2).

Study Characteristics
The included studies encompassed 27 cross-sectional studies and three cohort studies conducted between the period of 1987 and 2022, and included 2585 firefighters. Studies were conducted in different global regions and encompassed multiple variations of occupational simulated tasks. A summary of the included studies is presented in Table 3.

Cardiovascular Disease Risk Factors, Musculoskeletal Health and Occupational Performance
The results indicated that only two cardiovascular disease risk factors were consistently studied according to occupational performance in firefighters, and included age and obesity ( Table 3). The studies reported that older firefighters' completion times and performance on each individual task was significantly lower compared to younger firefighters [39,58,74,77]. When firefighters were aged (over 45 years in males), overall performance was significantly reduced. Obesity was reported to significantly reduce overall occupational performance and performance on each individual task [30,57,60,65,73,74,77,79,80]. Resting diastolic blood pressure and diastolic blood pressure at completion of the simulation event was significantly related to occupational performance in firefighters [72,84]. The results indicated that, although males tended to be at higher risk for cardiovascular disease, they also performed significantly better overall and in each occupational task compared to female firefighters [39,58,64,72]. Only one study was found that investigated the relationship between musculoskeletal health and occupational performance in firefighters.
Although not statistically significant, the study found that firefighters who reported having moderate-to-severe muscle and joint problems took approximately 10 s longer to complete the five flights of stairs while carrying a 22 kg high-rise pack than firefighters not reporting those problems [29].
3.6. The Effect of Aging, Obesity, Heart Rate and Gender on Occupational Performance in Firefighters Figure 2 shows the effects of age and obesity on occupational performance in firefighters. Due to the different methods used to determine firefighters' performance on occupational performance tasks, the standardized mean difference (SMD) was used to determine overall effect size. Age had a moderate significant pooled random effect on occupational performance [SMD = 0.66, 95%CI (0.41, 0.91), Z = 5.15, p < 0.001] [39,58,74,77]. The level of heterogeneity was low (I 2 = 4%) and there was no evidence of publication bias (Egger test p = 0.397). For obesity, there was a large random effect size, that was not statistically significant [SMD = 1.89, 95%CI (−2.25, 6.03), Z = 0.90, p = 0.37; I 2 = 93%] [57,79] (Figure 3).  Figure 2 shows the effects of age and obesity on occupational performance in firefighters. Due to the different methods used to determine firefighters' performance on occupational performance tasks, the standardized mean difference (SMD) was used to determine overall effect size. Age had a moderate significant pooled random effect on occupational performance [SMD = 0.66, 95%CI (0.41, 0.91), Z = 5.15, p < 0.001] [39,58,74,77]. The level of heterogeneity was low (I 2 = 4%) and there was no evidence of publication bias (Egger test p = 0.397). For obesity, there was a large random effect size, that was not statistically significant [SMD = 1.89, 95%CI (−2.25, 6.03), Z = 0.90, p = 0.37; I 2 = 93%] [57,79] ( Figure 3).

Correlation between Obesity, Aging and Resting Heart Rate on Occupational Performance
In Table 4, there was a low positive correlation between BF% and occupational time (R = 0.316, p < 0.001) [30,42,57,60,65,73,74,77,80]. There was moderate heterogeneity between studies (I 2 = 54.51%). In subgroups analyses, the correlation between BF% and completion time increased for all subgroups and was highest for the subgroups of males only (R = 0.413, p < 0.001) and full-time firefighters only (R = 0.388, p < 0.001). In addition, these subgroups had the least heterogeneity present (I 2 ≤ 16.4%). There was a modest positive correlation between age and occupational performance (R = 0.224, p < 0.001) [30,57,65,66,69,74,77,80]. There was moderate-to-substantial heterogeneity present between studies (I 2 = 74.1%). The correlation coefficient increased in studies that included either only male firefighters (R = 0.282, p < 0.001) or full-time firefighters (R = 0.323, p < 0.001) for the association between age and occupational performance. In addition, heterogeneity significantly decreased to 32.5% in male only studies and 0% in full-time firefighters' studies. There was a low positive correlation between heart rate and occupational performance in firefighters (R = 0.387, p < 0.001), with no evidence of heterogeneity [57,73].
For age, there was a low positive correlation between age and stair climb times (R = 0.345, p < 0.001; I 2 = 62.3%) [30,69,76,77]. After subgroup analysis, 0.0% heterogeneity was present when studies that analysed full-time male firefighters only were included. In addition, the correlation was strongest between age and stair climb times when studies that included only full-time male firefighters were analysed (R = 0.434, p < 0.001) [30,69,77].

Summary of Evidence
The results of this systematic review and meta-analysis indicated that the effect of cardiovascular risk status and musculoskeletal health status on occupational performance are understudied, and large gaps exist in the literature. Only two cardiovascular disease risk factors were frequently studied, namely age and obesity, and both had a significant effect on occupational performance. The results indicated that as firefighters aged and accumulated more adipose tissue, their completion times increased, which was consistent for all tasks investigated. In addition, we found a significant effect of physical fitness on occupational performance with cardiorespiratory fitness, muscular endurance, and upper body strength, all related to all individual tasks performance. These results are consistent with two systematic reviews, one on firefighters and the other on military personnel, that also found that aerobic capacity, muscular endurance and muscular strength are related to completion times in emergency occupations [87,88]. In addition, the current study results indicated that the weight of PPE worn significantly influenced the performance of all tasks. Moreover, the weight of PPE was related to overall occupational performance and individual task performance according to age, obesity and all physical fitness measures. This may be due to the weight of the equipment placing an extra burden on firefighters' abilities to perform their tasks efficiently, especially when compounded with excessive adipose accumulation and older age. The weight of PPE may become particularly important when conducting occupational performance tasks, as using full PPE may represent the truest simulation of the burden firefighters face physiologically while on active duty. These results are supported by a systematic review that indicated that the weight of PPE and SCBA gear elicit a significant physiological response in firefighters [32].
Globally, firefighting is regarded as one of the most physically demanding occupations that require high levels of physical fitness in order for them to perform their jobs effectively [88]. Moreover, firefighters are expected to remain in peak physical conditioning, especially as they age, to ensure they do not become a liability as they remain in the fire services [88,89]. The results of the current review supported this standpoint, as less physically fit firefighters that had increased fat mass were the most likely to perform poorly on the occupational performance tasks. Firefighting induces significant physiological responses [9,33] and, therefore, fitter firefighters perform significantly better than unfit firefighters, even as they age.

The Effect of Age, Obesity, Blood Pressure, Heart Rate and Gender on Occupational Performance
The results indicated that age had a significant moderate effect on occupational performance in the current study. In addition, a significant correlation existed between aging and overall occupational performance, particularly among full-time career firefighters. Ageing is considered a CAD risk factor, particularly in men 45 years and older and woman 55 years and older, due to the progressive reduction in arterial elasticity, increased inflammatory responses and reduction in key growth factors responsible for maintenance of arterial health [90][91][92][93]. Moreover, diastolic blood pressure was shown to significantly affect occupational performance in firefighter, however, the literature on this is limited, and more research should be conducted to allow for meta-analysis. Previous research indicated that blood pressure significantly affected work capacity in athletes [94] and job performance in emergency personnel [95] alike, which supports the results of the current study. Regular physical activity maintains cardiovascular health, however, firefighters generally become less physically active as they age [96][97][98][99], particularly in firefighters in the City of Cape Town Fire and Rescue Service [15,22]. Firefighters that are older, especially those aged 45 years or older, should engage in regular physical activity to maintain their work performance to acceptable standards [96,97,99,100]. There was a significant positive correlation between age and stair climb performance. Older firefighters performed significantly worse compared to younger firefighters and showed the strongest correlation when occupational performance simulation protocols included five or more sequential tasks. Age did not correlate with hose drag, victim rescue and forcible entry performance. The results suggest that muscular endurance and strength are of greater significance in performing the hose drag, victim rescue and forcible entry tasks successfully. Aging had a much larger effect on cardiorespiratory fitness as opposed to muscular endurance and strength, which may explain why aged firefighters performed worse on the stair climb [37,38,96,[99][100][101]. The present results indicated that cardiorespiratory fitness was the most significant factor in optimal performance in firefighters, and that older firefighters with lower cardiorespiratory fitness had the lowest overall occupational performance, particularly those that are obese [38,99,[102][103][104]. A study by Von Heimburg [71] reported that firefighters that performed best on the hose drag had a better dragging technique and higher cardiorespiratory fitness, but no significant difference between age was present. The years of experience as a firefighter may, somewhat, reduce the effect of age on task performance, especially those tasks where economical and explosive technique, rather than absolute power, may prove to be most beneficial, such as hose drag, victim rescue and forcible entry.
Obesity had a significant large moderate effect on occupational performance in firefighters, indicating that non-obese firefighters performed significantly better on the occupational performance tasks. This was further strengthened by the correlation analysis which indicated that as firefighters' age increased, overall simulation performance significantly decreased, and in particular, the stair climb, and victim drag events, especially when the weight of PPE was controlled for. Obesity increases the amount of non-functional excess weight that firefighters are required to overcome while performing their duties, reducing their overall performance on simulated tasks [99,[102][103][104][105]. Although research has indicated that increased body mass, to a point, may benefit certain strength or upper body stamina related tasks, overall task performance was not benefited, particularly related to the stair climb task [71,79]. Obese firefighters, generally, have a much lower cardiorespiratory fitness level, which may account for the reduced occupational performance seen in this group [26,27,103,106]. To maintain high work performance, firefighters should maintain a healthy weight throughout their careers, especially those firefighters involved in smoke diving and emergency rescues [3,6,8,107]. Although there were no studies investigating other CVD risk factors, obesity has been associated with increased risk status. Reducing obesity may not only improve overall occupational performance, but may also reduce all-cause mortality related to CVD in firefighters [2,3,6,16,108]. Increased adiposity reduced the overall performance times in stair climb, hose drag, victim rescue, forcible entry, and equipment hoist times in firefighters. Firefighters that were obese, performed significantly worse on each task. Most firefighting tasks were negatively affected by increased fat mass in firefighters, which is consistent with previous research indicating that obesity reduces performance [87].
Resting heart rate had a significant positive correlation with completion times, indicating that a higher resting heart rate resulted in worse performance on the occupational performance tasks. Resting heart rate (RHR) is closely linked to cardiorespiratory health and cardiorespiratory fitness. Higher RHRs have been linked to cardiovascular disease and poor cardiorespiratory fitness and increased cardiovascular risk [27,106,109]. Nazari et al. [33] reported that high heart rates and near maximum heart rates are reached during occupational performance tasks.
The current results indicated that gender had a significant effect on completion times in firefighters, with males performing significantly better than female firefighters. This is consistent with previous results that indicated males were stronger and fitter than their female counterparts and performed the occupational tasks faster. This may be due to many tasks being strength and endurance based, favouring male firefighters [31,86]. This is most likely due to males being taller, more muscular, and stronger than female firefighters, which has been shown to be a significant predictor of performance times [79]. Female firefighters may need to engage in more frequent off-duty strength training to maintain the minimum levels of strength needed to perform firefighting tasks optimally.

The Effect of Physical Fitness on Occupational Performance
The results indicated that a moderate negative correlation existed between cardiorespiratory fitness and completion times. Fitter firefighters performed significantly better on the occupational performance tasks compared to less fit firefighters. Studies suggest that firefighting require a minimum . VO2max of 42 mL·kg·min and, unsurprisingly, firefighters with higher cardiorespiratory fitness levels performed significantly better. This is supported by Hauschild et al. [88], where the review indicated that emergency personnel that had higher cardiorespiratory fitness performed better in the simulated tasks. Although all physical fitness parameters, except flexibility, was significantly correlated to occupational performance, cardiorespiratory fitness had the highest correlation with overall performance. Maintenance of cardiorespiratory fitness may be the most important aspect in the maintenance of optimal work performance in firefighters. This is especially true when firefighters that find themselves in emergency situations and are required to work at moderate-to-vigorous levels of intensity for prolonged periods of time. Cardiorespiratory fitness was significantly and negatively correlated to stair climb and hose drag times, especially when subgroup analysis was performed on studies including heavier equipment weights (>22 kgs). The stair climb and hose drag tasks require firefighters to perform locomotive move either climbing a flight of stairs or dragging a hose, which require the use of large muscle groups that require large amounts of oxygen. Fitter firefighters are able to utilize the available oxygen more efficiently, performing better on these locomotive tasks. Heavier equipment increased the cardiorespiratory load of each firefighting task, and require a higher fitness level for adequate completion [32].
Upper body (push-up) and abdominal (sit-ups) endurance had a significant negative correlation with overall completion times, particularly when firefighters performed five or more tasks and when equipment weighed more than 22 kgs. Many of the tasks' firefighters are required to perform involve forceful repetitive upper body exertive movements. Higher levels of upper body muscular endurance allow firefighters to sustain a particular amount of force over a number of repetitions [61,62,66,77]. Such as the door breach, which require firefighters to sustain maximal force during each hit to move the tyre or sled the desired distance [57,66,110]. Significant negative correlations were present between upper body and abdominal endurance and stair climb, hose drag, victim rescue, and forcible entry performance and, in particular, when subgroup analysis was performed on studies with equipment weighing more than 22 kgs and five or more tasks. Higher levels of upper body and abdominal stamina positively affected performance in stair climb, hose drag, victim rescue and forcible entry tasks. For all tasks, firefighters are required to wear their full protective equipment and SCBA gear which places significant strain on the upper body muscular [79,111,112]. Higher levels of upper body endurance will reduce the muscular strain of wearing PPE and SCBA gear while performing the occupational tasks. As indicated by Marcel-Millet [9], there are significant physiological differences between firefighters that wore PPE and SCBA gear, compared to those without. Focussing on improving firefighter stamina may prove to be particularly important to maintain high levels of occupational performance.
Grip strength, upper body strength and lower body strength were all significantly and negatively correlated with overall simulation performance in firefighters, particularly in males, where five or more tasks were performed while wearing equipment weighing more than 22 kgs. In general, stronger firefighters completed the simulation protocols significantly quicker than weaker firefighters. As mentioned previously, stronger firefighters are capable of producing higher levels of force with each movement, as most studies indicated significant relationships existed between muscle strength and endurance in firefighters [42,57,61,62]. In addition, higher levels of strength reduce the effort required to perform each task, allowing them to sustain the minimum required level of force for longer. This allows firefighters to move the tyre or sled further with each swing of the sledgehammer, or hoisting equipment further with each pull. More specifically, grip strength correlated negatively to hose drag, victim rescue, forcible entry and equipment hoist times, upper and lower body strength was negatively correlated to hose drag and victim rescue times, only, and in particular, when heavier equipment was used. Surprisingly, lower body strength was not correlated with better performance in the stair climb task. Grip strength appeared to be the most significant strength measure to maintain overall occupational performance in firefighters. This may be due to firefighting requiring firefighters to constantly grip and hold objects in place while producing high levels of force, such as sledgehammers, axes, jaws of life and fire hoses [42,61,62,77]. Higher levels of upper and lower body strength may allow firefighters to carry and drag the hose and victim with less effort [61,62,65,87]. There were insufficient studies available to analyse the effect of upper and lower body strength on forcible entry or equipment hoist times.
Flexibility was the only physical fitness parameter that was not significantly correlated to overall occupational performance, however, was negatively correlated to stair climb times in firefighters. More flexible firefighters may be able to have longer strides while climbing the stairs, as the hamstring is able to stretch further with less discomfort, improving the stair climb performance. A systematic review reported that hamstring flexibility was a key factor sprinting, jumping and agility [113]. Although the present study did not find a significant correlation between flexibility and other firefighter tasks, maintenance of flexibility may assist in maintaining high levels of occupational performance in firefighters [57,66,73]. Importantly, higher flexibility has been shown to reduce the incidence of injury in firefighters [43,44,114].

Limitations of the Study
The large number of cross-sectional studies are a limitation of the current study. Heterogeneity was introduced due to differences in weight of equipment and age ranges of the firefighters across different studies. However, this was at least partially mitigated through subgroup analysis. A limited number of studies were conducted on the relationship between cardiovascular and musculoskeletal health and occupational performance, which negatively impacted the meta-analysis on these variables. The older studies included in this systematic review, may have influenced the results, as advancements in PPE, work environments and intervention techniques, have in different physical demands, compared with previous years. Limitations in the quality of evidence are described below.

Applicability of Evidence
The results indicated that non-obese, younger male firefighters that have a high cardiorespiratory fitness level, and those that have high levels of muscular endurance and strength have the most favorable overall occupational performance. Cardiorespiratory fitness, along with upper body and abdominal endurance should be prioritized in exercise training programmes. Moreover, tasks that were performed sequentially where the weight of PPE worn was over 22 kgs significantly and negatively affected overall performance times. Taller, heavier male firefighters may have the most favorable performance outcomes when performing occupational specific duties. An inherent limitation of the evidence is that all firefighters recruited to participate in the studies are apparently healthy and injury free. More studies need to be conducted on firefighters with cardiovascular disease risk factors and underlying musculoskeletal health issues. Regular aerobic training, along with strength training may prove to be particularly beneficial for older firefighters who are smaller in stature and have a lower body mass, and in particular, female firefighters.

Quality of Evidence
Critical appraisal of the included studies for the majority of studies were acceptable; however, few studies appraised were low in overall quality. Three studies scored 15 points in the AXIS appraisal too, but was largely due to the small sample sizes of the studies. Due to the difficulty of performing occupational simulation tasks, most studies included small number of firefighters to participate. High heterogeneity was present between approximately half of the analysis, and possibly due to the difference is sample sizes, which may have influenced the means, standard deviations, as well the correlation strength between the included variables. Although high heterogeneity was present, the studies provided valuable information on factors affecting occupational performance.

Gaps in the Literature
The effects of cardiovascular disease risk factors and musculoskeletal health on firefighters' occupational performance are understudied. Particularly, studies related to the effect of cardiovascular risk factors, such as hypertension, diabetes, dyslipidaemia, and cigarette smoking on occupational performance, and the effect of musculoskeletal health issues on occupation performance. More research should be conducted on cardiovascular risks and musculoskeletal health related to occupational performance in firefighters.

Implications for Future Research
More research should be conducted investigating the effect of cardiovascular disease risk factors, and overall risk status on occupational performance. The effect of musculoskeletal health on work performance is also understudied.

Conclusions
Age and obesity significantly affected occupational performance in firefighters, increasing task completion times across all events. Physical fitness is integral to occupational performance in firefighters, with cardiorespiratory fitness, muscular endurance and upper body strength having the most significant effect on total completion times and all individual tasks. The weight of PPE is an essential consideration, as this significantly impacts completion times, highlighted by the heterogeneity caused between studies, particularly when five or more tasks were performed sequentially while firefighters wore PPE weighing over 22 kg. Moreover, younger, stronger, and heavier male firefighters performed significantly better than older, lighter and weaker firefighters, which emphasizes the importance of maintaining a suitable body composition, and appropriate levels of muscular endurance and strength as firefighters age. Firefighting departments should adopt regular physical activity, focused on maintaining cardiorespiratory fitness, muscular endurance, and upper body strength, to maintain firefighters' physical fitness and dietary recommendations, to reduce the likelihood of overweight and obesity in firefighters, which is particularly important as they age.

Patents Protocol Registration
Details of the protocol for this systematic review were registered on PROSPERO (CRD42021258898) and can be accessed at: https://www.crd.york.ac.uk/prospero/display_ record.php?RecordID=258898 (accessed on 21 January 2022).