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Validity of submaximal aerobic capacity and strength tests in firefighters

Validity of submaximal aerobic capacity and strength tests in firefighters Occupational Medicine, 2024, 74, 161–166 https://doi.org/10.1093/occmed/kqae004 Advance access publication 21 February 2024 Validity of submaximal aerobic capacity and strength tests in firefighters 1 2, 2 W. Hart , D. Taylor and D. C. Bishop North Yorkshire Fire and Rescue Service, Transport and Logistics Hub, Thirsk, UK, School of Sports and Exercise Science, University of Lincoln, Lincoln, UK . Correspondence to: D. Taylor, School of Sports and Exercise Science, University of Lincoln, Lincoln LN6 7TS, UK . Tel: +44 (0)1522 886845; e-mail: [email protected] Background: Typically, the fitness of UK firefighters is assessed via submaximal estimate methods due to the low demands on time, money, expertise and equipment. However, the firefighter-specific validity of such testing in relation to maximum aerobic capacity ( ) and par- VO 2 max ticularly muscular strength is not well established. Aims: To examine the validity of submaximal methods to estimate and maximal strength in operational firefighters. VO 2 max Methods: Twenty-two full-time operational firefighters (3 female) completed same-day submaximal (Chester Step Test; CST) and maximal (treadmill) assessments of , with a sub-sample of 10 firefighters (1 female) also completing submaximal and maximal back-squat (i.e. VO 2 max one repetition maximum; 1RM) assessments. All participants then completed the Firefighter Simulation Test (FFST) within 2–4 days. –1 –1 ˙ ˙ Results: CST underestimated actual VO by 1.4 ml·kg ·min (~3%), although values were positively correlated (r = 0.61, VO 2 max 2 max P < 0.01) and not significantly different. Estimated values negatively correlated with FFST performance (r = –0.42). Predicted 1RM VO 2 max underestimated actual 1RM by ~2%, although these values were significantly correlated (r = 0.99, P < 0.001) and did not significantly differ. The strongest predictive model of FFST performance included age, body mass index, and direct maximal measures of 1RM and VO . 2 max Conclusions: Submaximal back-squat testing offers good validity in estimating maximum firefighter strength without exposure to the fatigue associated with maximal methods. The CST provides a reasonably valid and cost-effective estimate which translates to firefighting task VO 2 max performance, although the error observed means it should be used cautiously when making operational decisions related to VO bench- 2 max marks. IN TR OD UC TION significantly differ from values obtained via direct maximal tread - mill testing [7] and there remains limited evidence regarding the Firefighters are expected to possess sufficient levels of cardio - validity of the CST in firefighting specifically. As such, there is a respiratory fitness and muscular strength to successfully meet need for research that helps inform and support decisions made the demands of the occupation [1–4]. It is, therefore, standard using the CST within the service (i.e. when profiling the fitness practice for firefighters to complete annual cardiorespiratory of active personnel). fitness and strength tests, with a failure to meet service-specific Maximum multiple (e.g. five repetition max) or single repe- standards deeming them unfit to perform operational duties. tition max (1RM) tests are commonly used to assess the mus- These are used alongside the recently developed Firefighting cular strength of firefighters, with good levels of validity reported Simulation Test (FFST) to confirm that operational fire and across various upper-body 1RM tests in comparison with job- rescue staff meet minimum job-specific functional requirements specific tasks [11,12]. Whilst lower-body muscular strength [5,6]. Given the implications of test outcomes to individual fire- may also contribute to firefighter fitness and task performance gh fi ters and the level of coverage provided by each service, the [13,14], methods that have been validated specifically for fire- need for practitioners to have reliable and valid data to assess gh fi ters are lacking. Furthermore, maximal strength testing and develop firefighter cardiorespiratory fitness and muscular may be time consuming and lead to fatigue that may place fire- strength is essential [6]. gh fi ters at greater risk during subsequent operational duties. A common method of testing cardiorespiratory fitness in UK Submaximal methods may help establish minimum operational firefighters is the submaximal Chester Step test (CST), which standards of muscular strength in an efficient manner whilst offers good test–retest reliability in estimating maximal aerobic also mitigating the risks associated with maximal test protocols capacity (VO ) [7,8] and is considered an accessible method 2 max [15–17]. The integration of laser-optic device technology with of assessing VO within occupational settings such as fire- 2 max submaximal barbell lifting has emerged as a relatively simple and gh fi ting [9,10]. However, VO estimates from the CST may 2 max © The Author(s) 2024. Published by Oxford University Press on behalf of the Society of Occupational Medicine. This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. 162 • OCCUPATIONAL MEDICINE (NYFRS) volunteered to participate in the study, with phys- K e y le a r nin g p oin ts ical characteristics for males, females and the overall participant group presented in Table 1. All participants completed written, What is already known about this subject: informed consent and health screening before starting the study, • Firefighting requires sufficient levels of cardiorespiratory which was approved by the institutional ethics review com- fitness and muscular strength to meet the demands of the mittee (application number: 2020-3432) in accordance with the job, with a failure to meet minimum standards in these Declaration of Helsinki. All participants had previous experi- components leading to withdrawal from operational ser- ence of the physical tests required by (conducted in) this study vice. as part of the annual fitness assessment and/or physical training • Annual fitness testing of firefighters is commonly per - schedule within NYFRS, including lower-body strength training. formed via submaximal estimate methods due to The first visit required all participants to first complete the relatively low demands on time, money, expertise and CST followed by a maximal incremental treadmill test, each sep- equipment. arated by a rest period of at least 30 minutes. The CST protocol • The firefighter-specific validity of testing in relation to followed that of Sykes [20], with heart rate (Polar H10, Polar, aerobic capacity and particularly muscular strength is not Finland) recorded at the end of each 2-minute stage. Estimated well established, which may lead to erroneous operational was established from heart rate using the CST com- VO 2 max decisions being made. puterized prediction software [20,21]. Maximal incremental testing was performed on a motorized treadmill (Gym Gear –1 What this study adds: T98, UK), with a 3-minute warm-up completed at 5.6 km·h , –1 • The Chester Step Test provides a reasonably valid and followed by a 0.6 km·h increase in speed and a 1% increase cost-effective VO estimate in firefighters, although in gradient every 2 minutes thereae ft r. These increments con- 2 max –1 the associated error means it should be used cautiously tinued until a speed of 11.2 km·h was achieved, ae ft r which when making operational decisions. the gradient was increased by 2% per minute until volitional • A novel submaximal back-squat protocol can provide exhaustion. Breath-by-breath oxygen uptake was recorded via a a valid estimate of lower-body maximal strength in metabolic cart (Cortex Metalyzer 3B, Germany) with heart rate firefighters. recorded using an integrated chest transmitter belt (Polar H10, Polar, Finland). All cardiorespiratory data were time averaged What impact this may have on practice or policy: as per the recommendations of Robergs et al. [22], whereby VO • If the Chester Step Test is utilized during firefighter re- 2 max and maximum heart rate (HR ) were calculated as the max cruitment and/or annual fitness testing then borderline highest 30 seconds average value recorded. –1 –1 A sub-sample of 10 participants (one female) also completed results (i.e. within ±12.16 ml·kg ·min of the operational submaximal and maximal lower-limb strength testing during benchmark for VO ) should be confirmed on a case- 2 max by-case basis via maximal incremental testing and direct their first visit, having had a 1-hour recovery period following gas analysis. treadmill testing. The physical characteristics of males, females and the overall sub-group are presented in Table 2. Strength • Estimating maximal strength via submaximal protocols and laser-optic device technology is a sufficiently valid, simple and cost-effective method to be considered as part Table 1. Mean ± SD physical characteristics for males, females and of the standard approach to fitness profiling across the fire the overall participant group and rescue service. • The sex- and age-specific relevance of operational bench- Male (n = 19) Female (n = 3) All (n = 22) marks for both maximum aerobic capacity and strength Age (years) 34.1 ± 7.9 30.7 ± 7.6 33.6 ± 7.8 should continue to be critically explored via firefighter- Mass (kg) 90.3 ± 11.8 66.3 ± 2.1 87.0 ± 13.5 specific research studies, particularly in females. Height (cm) 182.4 ± 5.4 171.0 ± 5.3 180.9 ± 6.6 Body mass 27.1 ± 3.1 22.7 ± 1.2 26.5 ± 3.3 cost-effective way to provide valid and reliable estimates of max - index (kg/m ) imal strength [18,19]. However, this approach has not yet been explored in relation to physically demanding occupations which Significantly different from men (P < 0.05). rely on profiling of muscular strength, such as the UK Fire and Rescue Service. Table 2. Mean ± SD physical characteristics for males, females and Given these points, this study examined the validity of the overall sub-group submaximal methods to estimate and lower-body 1RM VO 2 max Male Female All (n = 10) in operational firefighters, relative to direct maximal methods (n = 9) (n = 1) and in relation to FFST performance. Age (years) 32.7 ± 6.1 39 33.3 ± 5.8 Mass (kg) 93.0 ± 12.4 68 90.5 ± 14.1 M E THODS Height (cm) 180.9 ± 5.5 169 179.7 ± 6.4 A total of 22 fully trained full-time operational firefighters (three Body mass index (kg/m ) 28.4 ± 3.0 23.8 27.9 ± 3.2 female) from the North Yorkshire Fire and Rescue Service W. HART ET AL.: AEROBIC CAPACITY AND STRENGTH TESTING IN FIREFIGHTERS • 163 testing adopted a standardized incremental back-squat protocol associations between estimated and directly measured values [18] with an instructor-led mobilization warm-up using a for VO and 1RM and between FFST performance and the 2 max 20-kg Olympic barbell (Gym Gear Ltd, UK). A FLEX (Kinetic estimated and directly measured values for both and VO 2 max Performance Technologies, Australia) laser-optic device was 1RM. e fitt d to the right-hand side of the barbell to calculate the move- Stepwise multiple regression tests were used to establish pre- ment displacement of the barbell during each lift. A series of dictors of performance in the FFST test. The modelling strategy progressive loads were then completed based on the estimated focussed on separate analyses of those parameters associated with 1RM value provided by the FLEX device via the FLEX Stronger submaximal testing or those associated with maximal testing. app (Kinetic, Australia; firmware version: A714) with stand - Durbin–Watson values <1 or >3 were considered as a violation ardized encouragement and rest between lifts [23]. Following of statistical assumptions [27]. The prediction model(s) with the submaximal protocol prescribed by the FLEX Stronger app the highest proportion of explained variance (R ) and the lowest (i.e. 5+ sets, 1–3 reps, between 50–85% of estimated 1RM, with standard error of the estimate (SEE) were then selected. Non- 2-minute recovery between), an estimated 1RM value was calcu- standardized beta correlation coefficients were used to construct lated by the software and recorded. The submaximal 1RM esti - prediction equations for FFST completion time. mate was not revealed to participants until they had completed Given the potential effect of biological sex on each variable the subsequent maximal strength testing protocol. This required and the relatively small number of females within the study the completion of progressively heavier single lifts until failure, sample, all analyses were performed with and without female with each lift interspersed with a 2-minute recovery (and with participant data. If the removal of female data substantially two ae tt mpts given to lift a failed weight). As such, 1RM was changed the outcome of any analysis, then this was reported defined as the maximum load (kg) successfully lie ft d for a full separately from the outcomes for whole group data. For all repetition [24]. The first visit concluded with an FFST demon- analyses, statistical significance was set at an alpha value of stration to re-familiarize participants with the test. P < 0.05. The second visit required participants to complete the FFST to the best of their ability on a flat 25 m course, as per the stand - R E S U LT S ardized approach of Siddall et al. [25]. The FFST requires the VO , HR and FFST performance data for males, females completion of job-specific tasks (i.e. equipment carry, casualty 2 max max evacuation, hose run) across a timed 1025 m shuttle-run cir - and the overall participant group are summarized in Table 3. cuit, in the shortest possible overall time. Participants wore There was a significant moderate positive correlation (r = 0.61, P < 0.01) between estimated and directly measured full-firefighting PPE (i.e. tunic, salopeett s, boots, gloves and VO 2 max values, which were not significantly different from one another. helmet), and standardized verbal feedback was provided [6]. Heart rate was recorded throughout (Polar H10, Polar, Finland), Bland–Altman analysis established the mean bias by which the with HR calculated as the highest 30-second average value re- CST underestimated directly measured VO was 1.4 ml·kg 2 max peak 1 –1 ·min (~3%) with a 95% confidence interval for LoA of ±12.2 corded during the test [6,22]. –1 –1 ml·kg ·min (Figure 1). Data analysis was completed using SPSS for Windows (Version 28), with means and standard deviations calculated 1RM data for males, females and the overall participant for all variables and normal distribution confirmed by Shapiro– group are summarized in Table 4. A strong positive correlation (r = 0.995, P < 0.01) was observed between estimated and dir- Wilk tests. Differences between estimated and directly meas - ectly measured back-squat 1RM, with no significant difference ured VO and 1RM values were assessed via paired t-tests, 2 max with Bland–Altman plots [26] used to assess within-subject between these values. Bland–Altman analysis revealed the mean variation in estimated versus directly measured values, with bias by which the submaximal method underestimated dir- ectly measured 1RM was 2.1 kg (~2%), with a 95% confidence 95% limits of agreement (LoA) calculated. Pearson’s Product– interval for LoA of ±8.7 kg (Figure 2). Moment correlation coefficients established the strength of Table 3. Mean ± SD FFST performance, VO and HR values for males, females and the overall participant group 2 max max Male (n = 19) Female (n = 3) All (n = 22) FFST time (s) 585.6 ± 42.1 632.3 ± 47.6 592.0 ± 44.8 –1 * FFST HR (beats·min ) 181 ± 6 186 ± 2 182 ± 6 peak FFST HR (%HR ) 96 ± 2 96 ± 3 97 ± 2 peak max –1 –1 ˙ 48.0 ± 5.6 50.1 ± 2.7 48.3 ± 5.3 VO (ml·kg ·min ) 2 max –1 –1 ˙ 46.7 ± 8.3 47.9 ± 3.4 46.9 ± 7.8 VO (CST) (ml·kg ·min ) 2 max –1 4.3 ± 0.6 3.3 ± 0.3 4.2 ± 0.6 VO (L·min ) 2 max –1 HR (beats·min ) 189 ± 8 194 ± 6 190 ± 8 max –1 HR (Tanaka) (beats·min ) 184 ± 6 187 ± 5 184 ± 5 max Significantly different from men (P < 0.05). 164 • OCCUPATIONAL MEDICINE FFST performance of all participants was 592 ± 45 seconds, When based on only submaximal data, the strongest predic- –1 with an HR of 182 ± 6 beats·min (97 ± 2 % HR ) ob- tion model for FFST performance included age, BMI, estimated peak max served. Both estimated and directly measured values for ˙ and estimated 1RM, irrespective of female data being VO VO 2 max 2 max demonstrated a weak negative correlation with FFST perform- included (R = 0.592, R = 0.350, SEE = 56.4 seconds) or re- ance (r = –0.467 and –0.379, respectively). The removal of fe- moved (R = 0.805, R = 0.648, SEE = 36.5 seconds) from the male data substantially increased the strength of relationship analysis. between directly measured and FFST performance VO 2 max (r = –0.561). For all sub-group data, weak negative correlations were observed for estimated and directly measured 1RM values DIS CU S S ION versus FFST performance (r = –0.388 and –0.320, respectively). This study has shown that the CST underestimates VO 2 max The strongest overall prediction models for FFST perform- –1 –1 by 1.4 ml·kg ·min (~3%) in full-time operational firefighters ance included age, body mass index (BMI), and direct maximal compared to direct maximal treadmill assessment, with a –1 measures of 1RM and (L·min ) irrespective of female VO 2 max margin of error (i.e. 95% confidence interval for LoA) of ±12.16 data being included (R = 0.764, R = 0.583, SEE = 45.2 sec- –1 –1 ml·kg ·min . Greater mean differences have been reported onds) or removed (R = 0.891, R = 0.587, SEE = 27. 9 seconds) across a number of studies examining submaximal VO es- 2 max from the analysis. As such, the following prediction equations timate methods in physically demanding occupations, including of FFST performance time were produced for male-only and firefighting, with mean error values of between 2.9 and 7.0 mixed-sex data: –1 –1 ml·kg ·min being accepted for tests to be deemed suitably FFST completion time (seconds) valid for use [28–31]. More specifically, Dolezal et al. [32] re - (male only data)= 537.101 (2.096 × Age) ˙ ported greater mean error (~11%) in VO estimates derived 2 max (0.380 × 1RM)(62.841 × VO )+(16.033 × BMI) for firefighters from submaximal treadmill testing than the cur - 2max rent study, supporting the relative validity of the CST. Further FFST completion time (seconds) to these points, we found a significant positive correlation and (mixed - sex data)= 748.432 (2.666 × Age) no statistically significant differences between estimated and dir - (0.407 × 1RM)(90.867 × VO )+(14.189 × BMI) ˙ ectly measured values. There was also a significant nega - 2max VO 2 max tive correlation between estimated VO values derived from 2 max When based on only main group data (i.e. without 1RM the CST and FFST performance time. values), the strongest prediction model for FFST performance –1 These findings suggest that the CST has sufficient validity to included age, BMI and directly measured VO (L·min ) ir- 2 max provide a relatively low-cost estimate of which trans- 2 VO 2 max respective of female data being included (R = 0.657, R = 0.432, lates well to role-specific task performance of firefighters, but SEE = 36.5 seconds) or removed (R = 0.682, R = 0.465, that practitioners within the fire service should use and in- SEE = 35.9 seconds) from the analysis. terpret the CST with a degree of caution, particularly when making operational decisions on borderline cases. In cases where individuals fall within the LoA, it is advised that a more accurate follow-up test is used to clarify results, such as max- imal breath-by-breath testing [32]. However, given that a rela- tively large proportion of firefighters may fall into these LoA VO for estimated , the need to deploy more sophisticated 2 max tests across a substantial number of firefighters is likely to out - weigh any savings that the initial use of the CST currently offers. There may, therefore, be a need for firefighter-specific prediction equations to be developed to translate CST VO 2 max data more effectively (i.e. estimate with greater ac- VO 2 max curacy). As highlighted by Dolezal et al. [32], this may require a critical evaluation of age-predicted HR calculations used max Figure 1. Bland–Altman plot of predicted (CST) versus directly as part of the CST [33], given the inherent error of using gen- measured VO . Solid (black) line indicates mean bias (–1.4 2max –1 –1 eralized equations in populations differing from the original ml·kg ·min ) and dashed lines indicate 95% limits of agreement –1 –1 –1 –1 (upper 10.8 ml·kg ·min , lower –13.5 ml·kg ·min ). research context. Table 4. Mean ± SD estimated and directly measured 1RM values for males, females and the overall sub-group Male (n = 9) Female (n = 1) All (n = 10) Squat 1RM (kg) 141.2 ± 35.3 92.5 136.3 ± 34.8 Squat 1RM (FLEX) (kg) 139.1 ± 32.5 90.6 134.2 ± 32.5 Squat 1RM (1RM/body mass) 1.5 ± 0.3 1.4 1.5 ± 0.3 Squat 1RM (FLEX) (1RM/body mass) 1.5 ± 0.3 1.3 1.5 ± 0.3 One repetition maximum (1RM), submaximal 1RM estimate (FLEX). W. HART ET AL.: AEROBIC CAPACITY AND STRENGTH TESTING IN FIREFIGHTERS • 165 and therefore has the potential to be easily adopted across fire and rescue services. Whilst the sample size of the current study is equivalent or greater than previous similar research (e.g. [38]), this equated to ~8% of the total pool of full-time firefighters operating in the NYFRS and so the findings would have been enhanced by a larger, more representative, sample. More specifically, it would appear that the current study group was younger and more aer- obically fit than the typical (i.e. service mean) profile of NYFRS firefighter and also relative to previous firefighter studies (i.e. –1 –1 >40 years and <48 ml·kg ·min ) [5,6]. It is, therefore, im- portant for future research to establish how older (i.e. middle- Figure 2. Bland–Altman plot of estimated versus directly measured aged) firefighters with lower fitness levels respond to each of 1RM. The solid (black) line indicates mean bias (–2.1 kg) and the tests examined in the current study. Whilst the proportion dashed lines indicate 95% limits of agreement (upper 6.6 kg, lower of females in the study (~13%) was relatively higher than that –10.7 kg). seen currently within the NYFRS (~8%), and falls within the range used in previous research to establish the minimum fitness standards of firefighters (i.e. 7–19%) [5,6], the inclusion of more This study has also demonstrated that estimates of back-squat females in future research work (and from a broader range of fire 1RM derived using the FLEX laser-optic device are significantly and rescue services) would enhance our understanding of fitness correlated to, and not significantly different from, directly meas - testing and development in physically demanding occupations ured back-squat 1RM values. Whilst this novel testing method such as firefighting. appears to underestimate 1RM by ~2% compared to direct max- A further limitation of this study was the relatively narrow imal assessment, with a margin of error (i.e. 95% confidence range of submaximal test protocols examined. In terms of aer- interval for LoA) of ±8.7 kg (~6%), this level of accuracy would obic capacity estimation, the current study did not consider the appear to compare favourably to the CST and aforementioned relative validity of the Chester Treadmill/Walk Test, for example. studies of submaximal estimate protocols. W hilst there VO 2 max Similarly, strength testing was limited to only the back-squat is limited evidence from physically demanding occupation set- movement, so there would be value in establishing the relative tings, such agreement would appear to sit favourably with that validity of submaximal 1RM estimates across a range of upper reported by studies examining submaximal velocity-based and lower body lifts that may be relevant to the demands of fire- estimation of back-squat 1RM [34]. Furthermore, both esti- gh fi ting (e.g. Overhead press, deadli) [ ft 4,12,14]. Finally, whilst mated and directly measured back-squat 1RM were included the reliability of the laser-optic device technology and associated in the strongest models for predicting FFST performance from maximal strength estimates used in the current study have been submaximal and direct maximal methods, respectively. This is reported previously [18], there is a lack of firefighter-specific in keeping with the work of Michaelides et al. [35], which es- evidence relating to this. Given that relative strength levels may tablished that strength measures are able to explain a significant influence the reliability of estimated 1RM derived using this proportion of variation in firefighter ability test performance. method [24], future studies should examine its population- The current findings, therefore, add further support to the im- specific reliability for both male and female firefighters. portance of strength and more specifically to the testing and training of lower-body strength in operational firefighters FUNDIN G [4,13,14]. Whilst practitioners should adopt a degree of cau- tion interpreting estimated 1RM values, this novel approach None declared. to submaximal strength testing may offer sufficient validity to profile firefighter fitness effectively, without exposure to levels of A C KN OWL ED G M EN T S fatigue associated with direct maximal testing. A key strength of this study is that it is the first, to our know - a Th nk you to North Yorkshire Fire and Rescue Service for pro - ledge, to directly examine the validity of the CST in a group of viding the operational members of staff to carry out this study operational UK firefighters which allows the findings to be gen- and facilities and equipment provided by the service Physiology eralized to this population. Whilst CST reliability in firefighters department. has been examined [7,36], it is important for the population- specific validity of any submaximal estimate test to be established C OM PE TIN G IN TER E S T S [37]. Given the widespread use of this test within the fire service, and firefighter research, this study offers important population- The authors declare that the research was conducted in the ab - specific insight into the use and interpretation of CST results sence of any commercial or financial relationships that could be obtained from firefighters. Another key strength is that this study construed as a potential conflict of interest. North Yorkshire Fire has established a novel lower-body submaximal strength test that and Rescue Service had no internal influence on the research, has good levels of firefighter-specific validity. The submaximal and the research was conducted outside of normal testing rou- FLEX strength test is relativity cheap, portable and accessible, tines and away from operational duty time. 166 • OCCUPATIONAL MEDICINE 20. Sykes K . Capacity assessment in the workplace: a new step test. Occup R EFER EN C E S Health (Lond) 1995;47:20–22. 1. Gledhill N, Jamnik VK. 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Validity of submaximal aerobic capacity and strength tests in firefighters

Hart, W; Taylor, D; Bishop, D C
Occupational Medicine , Volume 74 (2): 6 – Feb 21, 2024
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Occupational Medicine, 2024, 74, 161–166 https://doi.org/10.1093/occmed/kqae004 Advance access publication 21 February 2024 Validity of submaximal aerobic capacity and strength tests in firefighters 1 2, 2 W. Hart , D. Taylor and D. C. Bishop North Yorkshire Fire and Rescue Service, Transport and Logistics Hub, Thirsk, UK, School of Sports and Exercise Science, University of Lincoln, Lincoln, UK . Correspondence to: D. Taylor, School of Sports and Exercise Science, University of Lincoln, Lincoln LN6 7TS, UK . Tel: +44 (0)1522 886845; e-mail: [email protected] Background: Typically, the fitness of UK firefighters is assessed via submaximal estimate methods due to the low demands on time, money, expertise and equipment. However, the firefighter-specific validity of such testing in relation to maximum aerobic capacity ( ) and par- VO 2 max ticularly muscular strength is not well established. Aims: To examine the validity of submaximal methods to estimate and maximal strength in operational firefighters. VO 2 max Methods: Twenty-two full-time operational firefighters (3 female) completed same-day submaximal (Chester Step Test; CST) and maximal (treadmill) assessments of , with a sub-sample of 10 firefighters (1 female) also completing submaximal and maximal back-squat (i.e. VO 2 max one repetition maximum; 1RM) assessments. All participants then completed the Firefighter Simulation Test (FFST) within 2–4 days. –1 –1 ˙ ˙ Results: CST underestimated actual VO by 1.4 ml·kg ·min (~3%), although values were positively correlated (r = 0.61, VO 2 max 2 max P < 0.01) and not significantly different. Estimated values negatively correlated with FFST performance (r = –0.42). Predicted 1RM VO 2 max underestimated actual 1RM by ~2%, although these values were significantly correlated (r = 0.99, P < 0.001) and did not significantly differ. The strongest predictive model of FFST performance included age, body mass index, and direct maximal measures of 1RM and VO . 2 max Conclusions: Submaximal back-squat testing offers good validity in estimating maximum firefighter strength without exposure to the fatigue associated with maximal methods. The CST provides a reasonably valid and cost-effective estimate which translates to firefighting task VO 2 max performance, although the error observed means it should be used cautiously when making operational decisions related to VO bench- 2 max marks. IN TR OD UC TION significantly differ from values obtained via direct maximal tread - mill testing [7] and there remains limited evidence regarding the Firefighters are expected to possess sufficient levels of cardio - validity of the CST in firefighting specifically. As such, there is a respiratory fitness and muscular strength to successfully meet need for research that helps inform and support decisions made the demands of the occupation [1–4]. It is, therefore, standard using the CST within the service (i.e. when profiling the fitness practice for firefighters to complete annual cardiorespiratory of active personnel). fitness and strength tests, with a failure to meet service-specific Maximum multiple (e.g. five repetition max) or single repe- standards deeming them unfit to perform operational duties. tition max (1RM) tests are commonly used to assess the mus- These are used alongside the recently developed Firefighting cular strength of firefighters, with good levels of validity reported Simulation Test (FFST) to confirm that operational fire and across various upper-body 1RM tests in comparison with job- rescue staff meet minimum job-specific functional requirements specific tasks [11,12]. Whilst lower-body muscular strength [5,6]. Given the implications of test outcomes to individual fire- may also contribute to firefighter fitness and task performance gh fi ters and the level of coverage provided by each service, the [13,14], methods that have been validated specifically for fire- need for practitioners to have reliable and valid data to assess gh fi ters are lacking. Furthermore, maximal strength testing and develop firefighter cardiorespiratory fitness and muscular may be time consuming and lead to fatigue that may place fire- strength is essential [6]. gh fi ters at greater risk during subsequent operational duties. A common method of testing cardiorespiratory fitness in UK Submaximal methods may help establish minimum operational firefighters is the submaximal Chester Step test (CST), which standards of muscular strength in an efficient manner whilst offers good test–retest reliability in estimating maximal aerobic also mitigating the risks associated with maximal test protocols capacity (VO ) [7,8] and is considered an accessible method 2 max [15–17]. The integration of laser-optic device technology with of assessing VO within occupational settings such as fire- 2 max submaximal barbell lifting has emerged as a relatively simple and gh fi ting [9,10]. However, VO estimates from the CST may 2 max © The Author(s) 2024. Published by Oxford University Press on behalf of the Society of Occupational Medicine. This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. 162 • OCCUPATIONAL MEDICINE (NYFRS) volunteered to participate in the study, with phys- K e y le a r nin g p oin ts ical characteristics for males, females and the overall participant group presented in Table 1. All participants completed written, What is already known about this subject: informed consent and health screening before starting the study, • Firefighting requires sufficient levels of cardiorespiratory which was approved by the institutional ethics review com- fitness and muscular strength to meet the demands of the mittee (application number: 2020-3432) in accordance with the job, with a failure to meet minimum standards in these Declaration of Helsinki. All participants had previous experi- components leading to withdrawal from operational ser- ence of the physical tests required by (conducted in) this study vice. as part of the annual fitness assessment and/or physical training • Annual fitness testing of firefighters is commonly per - schedule within NYFRS, including lower-body strength training. formed via submaximal estimate methods due to The first visit required all participants to first complete the relatively low demands on time, money, expertise and CST followed by a maximal incremental treadmill test, each sep- equipment. arated by a rest period of at least 30 minutes. The CST protocol • The firefighter-specific validity of testing in relation to followed that of Sykes [20], with heart rate (Polar H10, Polar, aerobic capacity and particularly muscular strength is not Finland) recorded at the end of each 2-minute stage. Estimated well established, which may lead to erroneous operational was established from heart rate using the CST com- VO 2 max decisions being made. puterized prediction software [20,21]. Maximal incremental testing was performed on a motorized treadmill (Gym Gear –1 What this study adds: T98, UK), with a 3-minute warm-up completed at 5.6 km·h , –1 • The Chester Step Test provides a reasonably valid and followed by a 0.6 km·h increase in speed and a 1% increase cost-effective VO estimate in firefighters, although in gradient every 2 minutes thereae ft r. These increments con- 2 max –1 the associated error means it should be used cautiously tinued until a speed of 11.2 km·h was achieved, ae ft r which when making operational decisions. the gradient was increased by 2% per minute until volitional • A novel submaximal back-squat protocol can provide exhaustion. Breath-by-breath oxygen uptake was recorded via a a valid estimate of lower-body maximal strength in metabolic cart (Cortex Metalyzer 3B, Germany) with heart rate firefighters. recorded using an integrated chest transmitter belt (Polar H10, Polar, Finland). All cardiorespiratory data were time averaged What impact this may have on practice or policy: as per the recommendations of Robergs et al. [22], whereby VO • If the Chester Step Test is utilized during firefighter re- 2 max and maximum heart rate (HR ) were calculated as the max cruitment and/or annual fitness testing then borderline highest 30 seconds average value recorded. –1 –1 A sub-sample of 10 participants (one female) also completed results (i.e. within ±12.16 ml·kg ·min of the operational submaximal and maximal lower-limb strength testing during benchmark for VO ) should be confirmed on a case- 2 max by-case basis via maximal incremental testing and direct their first visit, having had a 1-hour recovery period following gas analysis. treadmill testing. The physical characteristics of males, females and the overall sub-group are presented in Table 2. Strength • Estimating maximal strength via submaximal protocols and laser-optic device technology is a sufficiently valid, simple and cost-effective method to be considered as part Table 1. Mean ± SD physical characteristics for males, females and of the standard approach to fitness profiling across the fire the overall participant group and rescue service. • The sex- and age-specific relevance of operational bench- Male (n = 19) Female (n = 3) All (n = 22) marks for both maximum aerobic capacity and strength Age (years) 34.1 ± 7.9 30.7 ± 7.6 33.6 ± 7.8 should continue to be critically explored via firefighter- Mass (kg) 90.3 ± 11.8 66.3 ± 2.1 87.0 ± 13.5 specific research studies, particularly in females. Height (cm) 182.4 ± 5.4 171.0 ± 5.3 180.9 ± 6.6 Body mass 27.1 ± 3.1 22.7 ± 1.2 26.5 ± 3.3 cost-effective way to provide valid and reliable estimates of max - index (kg/m ) imal strength [18,19]. However, this approach has not yet been explored in relation to physically demanding occupations which Significantly different from men (P < 0.05). rely on profiling of muscular strength, such as the UK Fire and Rescue Service. Table 2. Mean ± SD physical characteristics for males, females and Given these points, this study examined the validity of the overall sub-group submaximal methods to estimate and lower-body 1RM VO 2 max Male Female All (n = 10) in operational firefighters, relative to direct maximal methods (n = 9) (n = 1) and in relation to FFST performance. Age (years) 32.7 ± 6.1 39 33.3 ± 5.8 Mass (kg) 93.0 ± 12.4 68 90.5 ± 14.1 M E THODS Height (cm) 180.9 ± 5.5 169 179.7 ± 6.4 A total of 22 fully trained full-time operational firefighters (three Body mass index (kg/m ) 28.4 ± 3.0 23.8 27.9 ± 3.2 female) from the North Yorkshire Fire and Rescue Service W. HART ET AL.: AEROBIC CAPACITY AND STRENGTH TESTING IN FIREFIGHTERS • 163 testing adopted a standardized incremental back-squat protocol associations between estimated and directly measured values [18] with an instructor-led mobilization warm-up using a for VO and 1RM and between FFST performance and the 2 max 20-kg Olympic barbell (Gym Gear Ltd, UK). A FLEX (Kinetic estimated and directly measured values for both and VO 2 max Performance Technologies, Australia) laser-optic device was 1RM. e fitt d to the right-hand side of the barbell to calculate the move- Stepwise multiple regression tests were used to establish pre- ment displacement of the barbell during each lift. A series of dictors of performance in the FFST test. The modelling strategy progressive loads were then completed based on the estimated focussed on separate analyses of those parameters associated with 1RM value provided by the FLEX device via the FLEX Stronger submaximal testing or those associated with maximal testing. app (Kinetic, Australia; firmware version: A714) with stand - Durbin–Watson values <1 or >3 were considered as a violation ardized encouragement and rest between lifts [23]. Following of statistical assumptions [27]. The prediction model(s) with the submaximal protocol prescribed by the FLEX Stronger app the highest proportion of explained variance (R ) and the lowest (i.e. 5+ sets, 1–3 reps, between 50–85% of estimated 1RM, with standard error of the estimate (SEE) were then selected. Non- 2-minute recovery between), an estimated 1RM value was calcu- standardized beta correlation coefficients were used to construct lated by the software and recorded. The submaximal 1RM esti - prediction equations for FFST completion time. mate was not revealed to participants until they had completed Given the potential effect of biological sex on each variable the subsequent maximal strength testing protocol. This required and the relatively small number of females within the study the completion of progressively heavier single lifts until failure, sample, all analyses were performed with and without female with each lift interspersed with a 2-minute recovery (and with participant data. If the removal of female data substantially two ae tt mpts given to lift a failed weight). As such, 1RM was changed the outcome of any analysis, then this was reported defined as the maximum load (kg) successfully lie ft d for a full separately from the outcomes for whole group data. For all repetition [24]. The first visit concluded with an FFST demon- analyses, statistical significance was set at an alpha value of stration to re-familiarize participants with the test. P < 0.05. The second visit required participants to complete the FFST to the best of their ability on a flat 25 m course, as per the stand - R E S U LT S ardized approach of Siddall et al. [25]. The FFST requires the VO , HR and FFST performance data for males, females completion of job-specific tasks (i.e. equipment carry, casualty 2 max max evacuation, hose run) across a timed 1025 m shuttle-run cir - and the overall participant group are summarized in Table 3. cuit, in the shortest possible overall time. Participants wore There was a significant moderate positive correlation (r = 0.61, P < 0.01) between estimated and directly measured full-firefighting PPE (i.e. tunic, salopeett s, boots, gloves and VO 2 max values, which were not significantly different from one another. helmet), and standardized verbal feedback was provided [6]. Heart rate was recorded throughout (Polar H10, Polar, Finland), Bland–Altman analysis established the mean bias by which the with HR calculated as the highest 30-second average value re- CST underestimated directly measured VO was 1.4 ml·kg 2 max peak 1 –1 ·min (~3%) with a 95% confidence interval for LoA of ±12.2 corded during the test [6,22]. –1 –1 ml·kg ·min (Figure 1). Data analysis was completed using SPSS for Windows (Version 28), with means and standard deviations calculated 1RM data for males, females and the overall participant for all variables and normal distribution confirmed by Shapiro– group are summarized in Table 4. A strong positive correlation (r = 0.995, P < 0.01) was observed between estimated and dir- Wilk tests. Differences between estimated and directly meas - ectly measured back-squat 1RM, with no significant difference ured VO and 1RM values were assessed via paired t-tests, 2 max with Bland–Altman plots [26] used to assess within-subject between these values. Bland–Altman analysis revealed the mean variation in estimated versus directly measured values, with bias by which the submaximal method underestimated dir- ectly measured 1RM was 2.1 kg (~2%), with a 95% confidence 95% limits of agreement (LoA) calculated. Pearson’s Product– interval for LoA of ±8.7 kg (Figure 2). Moment correlation coefficients established the strength of Table 3. Mean ± SD FFST performance, VO and HR values for males, females and the overall participant group 2 max max Male (n = 19) Female (n = 3) All (n = 22) FFST time (s) 585.6 ± 42.1 632.3 ± 47.6 592.0 ± 44.8 –1 * FFST HR (beats·min ) 181 ± 6 186 ± 2 182 ± 6 peak FFST HR (%HR ) 96 ± 2 96 ± 3 97 ± 2 peak max –1 –1 ˙ 48.0 ± 5.6 50.1 ± 2.7 48.3 ± 5.3 VO (ml·kg ·min ) 2 max –1 –1 ˙ 46.7 ± 8.3 47.9 ± 3.4 46.9 ± 7.8 VO (CST) (ml·kg ·min ) 2 max –1 4.3 ± 0.6 3.3 ± 0.3 4.2 ± 0.6 VO (L·min ) 2 max –1 HR (beats·min ) 189 ± 8 194 ± 6 190 ± 8 max –1 HR (Tanaka) (beats·min ) 184 ± 6 187 ± 5 184 ± 5 max Significantly different from men (P < 0.05). 164 • OCCUPATIONAL MEDICINE FFST performance of all participants was 592 ± 45 seconds, When based on only submaximal data, the strongest predic- –1 with an HR of 182 ± 6 beats·min (97 ± 2 % HR ) ob- tion model for FFST performance included age, BMI, estimated peak max served. Both estimated and directly measured values for ˙ and estimated 1RM, irrespective of female data being VO VO 2 max 2 max demonstrated a weak negative correlation with FFST perform- included (R = 0.592, R = 0.350, SEE = 56.4 seconds) or re- ance (r = –0.467 and –0.379, respectively). The removal of fe- moved (R = 0.805, R = 0.648, SEE = 36.5 seconds) from the male data substantially increased the strength of relationship analysis. between directly measured and FFST performance VO 2 max (r = –0.561). For all sub-group data, weak negative correlations were observed for estimated and directly measured 1RM values DIS CU S S ION versus FFST performance (r = –0.388 and –0.320, respectively). This study has shown that the CST underestimates VO 2 max The strongest overall prediction models for FFST perform- –1 –1 by 1.4 ml·kg ·min (~3%) in full-time operational firefighters ance included age, body mass index (BMI), and direct maximal compared to direct maximal treadmill assessment, with a –1 measures of 1RM and (L·min ) irrespective of female VO 2 max margin of error (i.e. 95% confidence interval for LoA) of ±12.16 data being included (R = 0.764, R = 0.583, SEE = 45.2 sec- –1 –1 ml·kg ·min . Greater mean differences have been reported onds) or removed (R = 0.891, R = 0.587, SEE = 27. 9 seconds) across a number of studies examining submaximal VO es- 2 max from the analysis. As such, the following prediction equations timate methods in physically demanding occupations, including of FFST performance time were produced for male-only and firefighting, with mean error values of between 2.9 and 7.0 mixed-sex data: –1 –1 ml·kg ·min being accepted for tests to be deemed suitably FFST completion time (seconds) valid for use [28–31]. More specifically, Dolezal et al. [32] re - (male only data)= 537.101 (2.096 × Age) ˙ ported greater mean error (~11%) in VO estimates derived 2 max (0.380 × 1RM)(62.841 × VO )+(16.033 × BMI) for firefighters from submaximal treadmill testing than the cur - 2max rent study, supporting the relative validity of the CST. Further FFST completion time (seconds) to these points, we found a significant positive correlation and (mixed - sex data)= 748.432 (2.666 × Age) no statistically significant differences between estimated and dir - (0.407 × 1RM)(90.867 × VO )+(14.189 × BMI) ˙ ectly measured values. There was also a significant nega - 2max VO 2 max tive correlation between estimated VO values derived from 2 max When based on only main group data (i.e. without 1RM the CST and FFST performance time. values), the strongest prediction model for FFST performance –1 These findings suggest that the CST has sufficient validity to included age, BMI and directly measured VO (L·min ) ir- 2 max provide a relatively low-cost estimate of which trans- 2 VO 2 max respective of female data being included (R = 0.657, R = 0.432, lates well to role-specific task performance of firefighters, but SEE = 36.5 seconds) or removed (R = 0.682, R = 0.465, that practitioners within the fire service should use and in- SEE = 35.9 seconds) from the analysis. terpret the CST with a degree of caution, particularly when making operational decisions on borderline cases. In cases where individuals fall within the LoA, it is advised that a more accurate follow-up test is used to clarify results, such as max- imal breath-by-breath testing [32]. However, given that a rela- tively large proportion of firefighters may fall into these LoA VO for estimated , the need to deploy more sophisticated 2 max tests across a substantial number of firefighters is likely to out - weigh any savings that the initial use of the CST currently offers. There may, therefore, be a need for firefighter-specific prediction equations to be developed to translate CST VO 2 max data more effectively (i.e. estimate with greater ac- VO 2 max curacy). As highlighted by Dolezal et al. [32], this may require a critical evaluation of age-predicted HR calculations used max Figure 1. Bland–Altman plot of predicted (CST) versus directly as part of the CST [33], given the inherent error of using gen- measured VO . Solid (black) line indicates mean bias (–1.4 2max –1 –1 eralized equations in populations differing from the original ml·kg ·min ) and dashed lines indicate 95% limits of agreement –1 –1 –1 –1 (upper 10.8 ml·kg ·min , lower –13.5 ml·kg ·min ). research context. Table 4. Mean ± SD estimated and directly measured 1RM values for males, females and the overall sub-group Male (n = 9) Female (n = 1) All (n = 10) Squat 1RM (kg) 141.2 ± 35.3 92.5 136.3 ± 34.8 Squat 1RM (FLEX) (kg) 139.1 ± 32.5 90.6 134.2 ± 32.5 Squat 1RM (1RM/body mass) 1.5 ± 0.3 1.4 1.5 ± 0.3 Squat 1RM (FLEX) (1RM/body mass) 1.5 ± 0.3 1.3 1.5 ± 0.3 One repetition maximum (1RM), submaximal 1RM estimate (FLEX). W. HART ET AL.: AEROBIC CAPACITY AND STRENGTH TESTING IN FIREFIGHTERS • 165 and therefore has the potential to be easily adopted across fire and rescue services. Whilst the sample size of the current study is equivalent or greater than previous similar research (e.g. [38]), this equated to ~8% of the total pool of full-time firefighters operating in the NYFRS and so the findings would have been enhanced by a larger, more representative, sample. More specifically, it would appear that the current study group was younger and more aer- obically fit than the typical (i.e. service mean) profile of NYFRS firefighter and also relative to previous firefighter studies (i.e. –1 –1 >40 years and <48 ml·kg ·min ) [5,6]. It is, therefore, im- portant for future research to establish how older (i.e. middle- Figure 2. Bland–Altman plot of estimated versus directly measured aged) firefighters with lower fitness levels respond to each of 1RM. The solid (black) line indicates mean bias (–2.1 kg) and the tests examined in the current study. Whilst the proportion dashed lines indicate 95% limits of agreement (upper 6.6 kg, lower of females in the study (~13%) was relatively higher than that –10.7 kg). seen currently within the NYFRS (~8%), and falls within the range used in previous research to establish the minimum fitness standards of firefighters (i.e. 7–19%) [5,6], the inclusion of more This study has also demonstrated that estimates of back-squat females in future research work (and from a broader range of fire 1RM derived using the FLEX laser-optic device are significantly and rescue services) would enhance our understanding of fitness correlated to, and not significantly different from, directly meas - testing and development in physically demanding occupations ured back-squat 1RM values. Whilst this novel testing method such as firefighting. appears to underestimate 1RM by ~2% compared to direct max- A further limitation of this study was the relatively narrow imal assessment, with a margin of error (i.e. 95% confidence range of submaximal test protocols examined. In terms of aer- interval for LoA) of ±8.7 kg (~6%), this level of accuracy would obic capacity estimation, the current study did not consider the appear to compare favourably to the CST and aforementioned relative validity of the Chester Treadmill/Walk Test, for example. studies of submaximal estimate protocols. W hilst there VO 2 max Similarly, strength testing was limited to only the back-squat is limited evidence from physically demanding occupation set- movement, so there would be value in establishing the relative tings, such agreement would appear to sit favourably with that validity of submaximal 1RM estimates across a range of upper reported by studies examining submaximal velocity-based and lower body lifts that may be relevant to the demands of fire- estimation of back-squat 1RM [34]. Furthermore, both esti- gh fi ting (e.g. Overhead press, deadli) [ ft 4,12,14]. Finally, whilst mated and directly measured back-squat 1RM were included the reliability of the laser-optic device technology and associated in the strongest models for predicting FFST performance from maximal strength estimates used in the current study have been submaximal and direct maximal methods, respectively. This is reported previously [18], there is a lack of firefighter-specific in keeping with the work of Michaelides et al. [35], which es- evidence relating to this. Given that relative strength levels may tablished that strength measures are able to explain a significant influence the reliability of estimated 1RM derived using this proportion of variation in firefighter ability test performance. method [24], future studies should examine its population- The current findings, therefore, add further support to the im- specific reliability for both male and female firefighters. portance of strength and more specifically to the testing and training of lower-body strength in operational firefighters FUNDIN G [4,13,14]. Whilst practitioners should adopt a degree of cau- tion interpreting estimated 1RM values, this novel approach None declared. to submaximal strength testing may offer sufficient validity to profile firefighter fitness effectively, without exposure to levels of A C KN OWL ED G M EN T S fatigue associated with direct maximal testing. A key strength of this study is that it is the first, to our know - a Th nk you to North Yorkshire Fire and Rescue Service for pro - ledge, to directly examine the validity of the CST in a group of viding the operational members of staff to carry out this study operational UK firefighters which allows the findings to be gen- and facilities and equipment provided by the service Physiology eralized to this population. Whilst CST reliability in firefighters department. has been examined [7,36], it is important for the population- specific validity of any submaximal estimate test to be established C OM PE TIN G IN TER E S T S [37]. Given the widespread use of this test within the fire service, and firefighter research, this study offers important population- The authors declare that the research was conducted in the ab - specific insight into the use and interpretation of CST results sence of any commercial or financial relationships that could be obtained from firefighters. Another key strength is that this study construed as a potential conflict of interest. North Yorkshire Fire has established a novel lower-body submaximal strength test that and Rescue Service had no internal influence on the research, has good levels of firefighter-specific validity. The submaximal and the research was conducted outside of normal testing rou- FLEX strength test is relativity cheap, portable and accessible, tines and away from operational duty time. 166 • OCCUPATIONAL MEDICINE 20. Sykes K . Capacity assessment in the workplace: a new step test. Occup R EFER EN C E S Health (Lond) 1995;47:20–22. 1. Gledhill N, Jamnik VK. 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Occupational Medicine – Oxford University Press

Published: Feb 21, 2024

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