Physical employment standards,physical training and musculoskeletal injury in physically demanding occupations

Abstract

BACKGROUND:

Physically demanding occupations such as the military, firefighting and law enforcement have adopted physical employment standards (PES). The intent of PES is to match the physical capacity of personnel with the physical demands of job tasks. Inadequate physical capacity can affect occupational task performance as well musculoskeletal injury (MSKI) risk.

OBJECTIVE:

To present contemporary evidence on the relationship(s) between PES, physical training, physical capacity and MSKI in physically demanding occupations, and provide recommendations regarding physical training for improved occupational performance and reduced MSKI risk.

METHODS:

This narrative review draws on evidence from 104 published sources.

RESULTS:

Physical training is central to the development and maintenance of occupationally-relevant physical capacity, as well as mitigating MSKI risk associated with job performance. In addition, given the prevalence of manual handling tasks, strength training needs to be emphasised in physical training regimen.

CONCLUSIONS:

PES development can inform both physical training and injury prevention strategies in physically demanding occupations. Furthermore, a physical performance continuum is essential to through-career maintenance of occupational performance and health, and the preservation of organisational capability. Finally, organisations should consider the potential to implement PES as maximal performance tests to better understand the relationship between occupational task performance and MSKI risk.

Keywords

Muscular strength cardiorespiratory endurance physical performance occupational performance musculoskeletal injury

1 Introduction

Fundamentally, the role of a physical employment standard (PES) is to match the physical capacity of the individual with physical demands of the job. Physically demanding occupations such as the military, law enforcement and firefighting have adopted PES for applicants and/or incumbents. A PES may be applied at various junctures, including prior to entry, graduation from training, annual testing, pre-deployment, and return to work. More recent efforts to remove sex-based barriers to employment (e.g. restrictions for direct combat roles in the military) and/or increase workforce diversity (e.g. sex and age) across many physically demanding occupations has resulted in increased scrutiny of many of these physicalfitness tests and standards [1, 2]. This in turn has led to an increase in PES-related projects being undertaken within physically demanding occupations, as evidenced by the open-literature publications and dedicated conferences over the last 10 years [3 –10].

The knowledge acquired from PES development has the potential to extend far beyond discrete tests and standards. Through the job task analysis that forms the foundation of a PES, extensive information is gathered on the physical demands of a role and/or occupation [11, 12], which could be used to inform decisions regarding two co-dependent areas; 1) physical training, and 2) musculoskeletal injury (MSKI) prevention. For instance, a recent PES review of 57 employment categories within the Australian Army identified 583 physically demanding tasks, of which 458 (∼79%) were classified as manual materials handling tasks [13]. Load carriage is also a common requirement for occupations such as the military, law enforcement and fire-fighting [14 –18]. Evidence has demonstrated that these loaded activities (load carriage, manual material handling) pose an injury risk to personnel, from both acute and chronic exposure [14 , 19–21]. Occupationally-relevant physical training can therefore play a role in matching the physical capacity of personnel and job task demands to preserve performance and reduce MSKI risk.

While there is a strong association between physical capacity (e.g. cardiorespiratory endurance, muscular endurance and muscular strength), and MSKI risk [22 –24], there is considerably less evidence demonstrating a link between PES performance and injury risk, particularly in military and emergency service organisations [25]. What is clear, is the significant organisational cost (e.g. medical care, medical compensation) and burden (e.g. lost duty days, decreased workforce capability, degraded employee health) associated with MSKI for physically demanding occupations [26, 27]. Increased occupationally-relevant physical capacity, which can be achieved via physical training, can reduce MSKI rates and therefore increase workforce availability. This review will link PES, physical training, and MSKI prevention with a consideration of how PES might be utilized for greater impact, as opposed to simply a minimum standard for employment. The review is divided into two sub-sections; 1) occupational demands and physical training, 2) physical capacity and injury prevention.

2 Occupational demands and physical training

Whilst it is easy to focus on the essential (or critical) tasks within physically demanding occupations, as represented by PES, it is also important to consider the broader occupational context. Job task analysis has shown that many roles within physically demanding occupations can involve sedentary or low intensity tasks punctuated with high intensity or prolonged moderate intensity activities, such as the use of force, or load carriage. The day to day physical demands of the non-emergency/non-tactical tasks, may be insufficient to elicit an adequate training stimulus for maintenance of physical capacity commensurate with the essential tasks [28, 29]. For example, following prolonged deployments (9–12 months), American and Norwegian soldiers demonstrated a decline in maximum oxygen uptake ( ${\dot{VO}}_{2 peak}$ ) post-deployment [28, 29]. In addition, cross-sectional research has demonstrated that industrial workers with a high occupational physical load had equivalent leg power and cardiorespiratory endurance when compared to peers working in occupations with low physical load, while grip strength was greater in the ‘high physical load’ group [30]. Collectively, these results show that physically demanding jobs may not necessary provide adequate stimulus to induce cardiorespiratory or musculoskeletal adaptations to maintain and/or develop the physical capacity required to successfully perform the role. Further compounding this issue is the decline in population health and fitness in western countries over recent decades. Since 1970 there has been a consistent decline in cardiorespiratory endurance of 6 to 19 year-olds across the world (27 countries, n = 25,455,527) of 0.4% per year [31]. This has a downstream effect on the recruit pool for physically demanding occupations such as the military, firefighting, and police [32 –34]. For example, Finnish conscripts demonstrated a decline in cardiorespiratory endurance, as predicted from the 12 min run, of 12% from 1979 to 2004, which equates to a ∼0.5% decline per year [35]. Physically demanding jobs will therefore likely need supplementary physical training to ensure personnel achieve and maintain the required physical capacity to safely and effectively perform essential tasks.

The benefits of improved workforce physical capacity extend beyond occupational task performance and reduced MSKI risk. Physical training can help to maintain body composition and cardiovascular health, as well as attenuate age-related declines in body composition, cardiovascular endurance, muscular strength and power [36 –45]. Physical capacity can also help personnel buffer the allostatic stress associated with occupational training and roles [46]. Specifically, cardiorespiratory endurance has been associated with attenuated hormonal and subjective stress reactivity in response to military training [47 –49]. Importantly in the physically demanding occupation context, cardiorespiratory fitness helps to moderate reactivity to acute mental stress, in the absence of physical stress [50]. Physical capacity is therefore critical in moderating occupational performance, resilience and health in physically demanding occupations.

Physical training can play a key role in addressing disparity between physical capacity of personnel and physical demands of job tasks. Evidence suggests however, that within occupational settings traditional physical training is often moderately effective, at best, in enhancing physical capacity (cardiorespiratory endurance, maximal strength) or occupational performance (e.g. box lifting, pack march) [51, 52]. For example, the rates of adaptation during basic military training have remained constant since the 1990’s, with gains in cardiovascular endurance and muscular strength often less than 10% [51 –57] (Table 1). These gains in fitness are modest when compared to laboratory and community-based interventions, where improvements in cardiovascular endurance and muscular strength in like populations (e.g. university students, and recreationally active) are typically ≥15–20% [58 –60].

Table 1
Physical and physiological performance changes following basic military training

Study Population Training duration No of PT sessions Type of PT sessions Physical capacity Performance change

Burley et al. [55] Australian Army recruits, 12 weeks 41 (3.4 sessions/week) 7 circuit, 10 endurance, 7 load carriage, 13 skill-based sessions, 4 fitness testing sessions. Muscular strength: 1 RM box lift, 1.5 m: 11% ^*

n = 173 male, 22 female Muscular endurance: Push-ups: 22% ^*

Cardiovascular endurance: MSST predicted VO_2max: 9% ^*

3.2 km, 22 kg load carriage: 8% ^*

Groeller et al. [51] Australian Army recruits, 12 weeks 3 sessions/week Circuits, swimming, running, load carriage, obstacle and ropes courses. Muscular strength: 1 RM box lift, 1.5 m: ∼9.8% ^*

n = 45 male, 6 female Muscular endurance: Push-ups: ∼21% ^*

Cardiovascular endurance: MSST predicted VO_2max: ∼9.7% ^*

3.2 km, 22 kg load carriage: ∼ - 9.2% ^*

Blacker et al. [100] British Army recruits, 12 weeks Not provided. Cardiovascular endurance: 2.4 km run: ∼ 9.0% ^*

n = 20 male, 17 female

Santtila et al. [61, 64] Finish conscripts, 8 weeks 2-3 sessions/ week, 33 hours total Endurance (running, Nordic walking, cycling, orienteering), strength training, sport. Muscular strength: Isometric leg extension: 5.2%

n = 24 males VO_2max: 13.4% ^*

Cardiovascular endurance: 3 km, 14 kg loaded run: 10.2% ^*

Dyrstad et al. [56] Norwegian Infantry recruits, 10 weeks 2 sessions/week 10 endurance sessions, 10 strength sessions Cardiovascular endurance: VO_2max: 1.3%

n = 51 males Muscular endurance: Push-ups: ∼33% ^*

Sharp et al. [65] U.S. Army recruits, n = 186 male, 168 female 8 weeks Not provided. Male Isometric leg extension: 6.7% ^*

Muscular strength: Isometric 38 cm upright pull: 4.4%

Muscular endurance: IDL1.52 m: 3.4% ^*

Cardiovascular endurance: VO_2peak: ∼3.8% ^*

Female Isometric leg extension: 1.1%

Muscular strength: Isometric 38 cm upright pull: 0%

Muscular endurance: IDL1.52 m: – 4.3%

Cardiovascular endurance: VO_2peak: ∼6.3% ^*

Williams et al. [52] British Army recruits, 11 weeks 71 (6.5 sessions/week) 19 sport, 18 circuit, 10 agility, 9 swimming, 7 endurance, 2 manual handling, 6 skill-based and testing sessions. Muscular strength: 1 RM box lift, 1.45 m: 1.7%

n = 47 male, 10 female 1 RM box lift, 1.70 m: 4.4%

Muscular endurance: Isometric 38 cm upright pull: – 0.8%

Repetitive lift, 10 kg: 7.0%

Cardiovascular endurance: Repetitive lift, 22 kg: 29.5% ^*

IDL1.45 m: 0.2%

MSST predicted VO_2max: 6.1% ^*

Loaded march, 15 kg: 3.6%

Loaded march, 25 kg: 15.7% ^*

Brock and Legg [53] British Army recruits, 7 weeks 51 (7.3 sessions/week) 17 own PT, 13 endurance, 7 sports, 4 obstacle course, 5 physical fitness testing, 3 swimming, 2 familiarisation Muscular strength: Isometric 38 cm upright pull: 3.4%

n = 73 female Muscular endurance: IDL1.52 m: 10.1% ^*

Cardiovascular endurance: MSST predicted VO_2max: 2.2% ^*

Legg and Duggan [54] British Army Artillery recruits, 8 weeks Not provided. Running, swimming, gym exercises, obstacle courses, loaded marching Muscular strength: Isometric leg extension: – 4.2% ^*

n = 62 male Muscular endurance: Isokinetic leg extension, 30° sec^–1: – 3.3%

Isokinetic leg extension, 180° sec^–1: – 4.5% ^*

Isometric leg extension endurance: – 9.6% ^*

Cardiovascular endurance: Predicted VO_2max: 3.6% ^*

^*Denotes significant (p < 0.05) change from baseline, 1 RM; one repetition-maximum, MSST; multi-stage shuttle run test, IDL; incremental dynamic lift.

Physical training in the military context, and likely other physically demanding occupations, has typically focused on the development of cardiorespiratory and muscular endurance, with training modalities such as; high-repetition circuits, distance running, and load carriage (Table 1). Proponents of traditional military physical training suggest that this model is efficacious given it has been shown to (modestly) improve cardiorespiratory endurance, muscular endurance, load carriage and repeated manual handling [51 , 61]. This training paradigm however, neglects the development of muscular strength, which is essential to the performance of manual handling tasks [13, 15]. For example, Williams et al. examined the British Army 8-week basic training course and observed no improvements in 1 repetition maximum (RM) box lift tasks, whilst cardiorespiratory endurance and repeated box lifting performance improved. A follow-up study examined the efficacy of a modified basic training program that included strength training, compared to the normal British Army basic training course [52, 62]. The modified program demonstrated greater improvements in occupationally-relevant performance (manual materials handling, load carriage) and physical capacity (cardiorespiratory endurance, muscular endurance) when compared to the normal program. Moreover, strength training alone has been shown to improve load carriage performance, in the absence of load carriage training [63]. It is suggested that rather than focusing on replicating occupational tasks such as load carriage, physical training should primarily, but not exclusively, focus on the development of underlying physical capacities (e.g. muscular strength, and cardiorespiratory endurance), that support performance in a wide array of occupational tasks. This is not likely to be achieved with physical training regimen commonly employed in physically demanding occupations, which typically lack dedicated strength training. Well-designed physical training programs however, can simultaneously improve cardiorespiratory endurance, muscular endurance and muscular strength in occupational settings.

Beyond training modality, it is essential all personnel receive a sufficient training stimulus, to at least prevent a decay in fitness (‘detraining’). Whilst this may seem intuitive, several military studies have shown heterogeneous responses in physical capacity and performance gains across a recruit cohort [55 , 64]. Specifically, recruits with the lowest fitness upon commencement of basic training showed the greatest gains in fitness [55 , 65]. Whereas ‘fitter’ recruits often demonstrated muted gains or perhaps even detrained [55 , 65]. Training programs that allow for equivalent relative intensities to ensure positive training stimuli across the entire population are essential [55]. Ability-based running groups and high-intensity intervals are examples of how this can be achieved for cardiorespiratory endurance. A recent study with Australian Army recruits also demonstrated a model for strength training with progressive and individualised loading that can be delivered to large groups (i.e. ∼100 individuals) and successfully achieve gains across the cohort [66, 67]. Looking beyond recruit training, a limited number of studies have prospectively followed recruits through basic military training and occupation specific training [51 , 57] or basic training and early career (10–12 months) [56, 68]. These studies show that physical capacity tends to plateau following basic military training (Table 2). It is acknowledged that the focus of occupation specific training is the acquisition of technical skills, as opposed to basic training, which is designed to develop foundational military skills and fitness. However, it is suggested that modernised physical training programs could yield improved physical capacity when compared to results previously reported (Table 2). This does not necessarily involve a greater allocation of time to physical training, rather improved training prescription (e.g. more individualised and progressive load, concurrent training, and high-intensity intervals) to promote a positive training stimulus.

Table 2

Physical and physiological performance changes following basic and employment category military training

Study	Population	Training duration	Physical capacity	Performance change Pre-post basic training	Post basic training to post employment-category training/10–12 months service
Groeller et al. [51]	Australian Army recruits,	12 weeks basic training, and 6–15 weeks employment category training	Muscular strength:	1 RM box lift: ∼9.8% ^*	1 RM box lift: ∼2.3%
	n = 45 male, n = 6 female		Muscular endurance:	Push-ups: ∼21% ^*	Push-ups: ∼15.7% ^*
			Cardiovascular endurance:	MSST predicted VO_2max: ∼9.7% ^*	MSST predicted VO_2max: ∼1.8%
				3.2 km, 22 kg load carriage: ∼– 9.2% ^*	3.2 km, 22 kg load carriage: ∼+4.7% ^*
Santtila et al. [57]	Finish conscripts, n = 57 males	8 weeks basic training, and 8 weeks specialised military training	Muscular strength:	Isometric leg extension: 8.1% ^*	Isometric leg extension: 1.9%
			Cardiovascular endurance:	VO_2max: 5.6% ^*	VO_2max: 0.6%
Legg and Duggan [54]	British Army Infantry recruits,	5 months	Cardiovascular endurance:	Predicted VO_2max: – 2.4% ^*
	n = 95 male,		Muscular strength:	Isometric leg extension: 5.7% ^*
	British Army Infantry Leader recruits,	11 months	Cardiovascular endurance:	Predicted VO_2max: 3.0% ^*
	n = 104 male,		Muscular strength:	Isometric leg extension: 19.9% ^*
Dyrstad et al. [56]	Norwegian Infantry recruits,	10 weeks basic training, and 10 months after enlistment	Cardiovascular endurance:	VO_2max: ∼1.3%	VO_2max: ∼– 2.5^*%
	n = 84 males		Muscular endurance:	Push-ups: ∼33% ^*	Push-ups: ∼6.3^*%
Gordon et al. [68]	South African Defence Force recruits, n = 53 males	10 weeks basic training, and 12 months after enlistment	Cardiovascular endurance:	VO_2max: ∼2.8%	VO_2max: ∼– 0.8%

^*Denotes significant change (p < 0.05), 1 RM; one repetition-maximum, MSST; multi-stage shuttle run test.

The ability to provide an individualised training stimulus is also critical to incumbents to minimise through-career declines in physical capacity and occupational performance. A cross-sectional study in U.S. Army soldiers showed that length of service and age were associated with declining body composition and cardiorespiratory endurance [43]. Firefighters and police officers have shown a similar pattern of declining physical capacity (cardiorespiratory endurance and muscular strength) with advancing age [44, 69]. It is acknowledged that age and seniority are associated, and seniority is in turn associated less physically demanding roles. Notwithstanding this the fact, for physically demanding occupations, the physical capacity of personnel is central to the operational capability of the organisation. These agencies therefore have a vested interest in maintaining the occupationally-relevant physical capacity of its workforce. Thus, organisations cannot rely on individuals maintaining the requisite (occupational) physical capacity solely under their own volition and must support the ongoing participation in physical training. Further, it is suggested that organisations within physically demanding occupations should establish a through-career physical performance continuum, that commences upon entry and continues until separation (Fig. 1) [70]. The development of a coherent, well-articulated physical performance continuum, inclusive of both physical training and PES, is critical to the optimal development and maintenance of workforce performance capacity.

Fig.1

Through-career physical performance continuum. Adapted from Billing and Drain [70].

To prepare personnel for job demands, physically demanding occupations need to consider adopting concurrent training, i.e. the simultaneous prescription of muscular strength and cardiorespiratory endurance exercise within physical training regimen [71, 72]. Current evidence is equivocal regarding the ability to concurrently optimise cardiorespiratory endurance and muscle strength [71 –75]. Factors such as training status, endurance training volume, endurance training modality (running, cycling), and recovery between sessions appear to strongly moderate muscle adaptations to a concurrent training regimen [71 –75]. For untrained and recreationally active individuals, who broadly represent recruits and incumbents in physically demanding occupations, comparable muscle adaptations (strength, hypertrophy) have been observed between concurrent training and strength training regimens [60, 72]. The volume of endurance training (frequency and duration of sessions) appears to be a particularly strong moderator of gains in muscular strength, hypertrophy and power [74]. Reductions in the volume of endurance exercise through the addition of high-intensity intervals for example, appears to minimise the interference effect of concurrent training on muscle adaptations [60]. Jones, et al., investigated a variety of resistance and endurance exercise regimens with varying ratios of each type of exercise in strength trained men. A ratio of 3:1 resistance and endurance exercise sessions per week stimulated muscular strength gains comparable with resistance exercise as a single mode of exercise. With regard to the timing of sessions, separating resistance and endurance training sessions by a minimum of six hours, but preferably 24 hours, reduces the potential interference on strength and hypertrophy gains [75].

Concurrent moderate to high load resistance exercise (% 1RM) and low volume, high intensity cardiorespiratory endurance has been shown to significantly improve physical capacity and performance of occupational tasks [60 , 78]. The majority of these investigations however have not directly implemented the experimental training regimen within an occupational environment, nor have they accounted for the addition of physical activity undertaken outside of the dedicated physical training sessions [60 , 78]. Consequently, the most effective concurrent training methodology within a physically demanding occupational setting remains unclear. A limited number of studies have implemented experimental exercise regimens within a military training environment, however differences in factors such as population, training status, training regimens and total physical activity loads (including the physical activity associated with occupational, leisure and physical training time) make comparison difficult [61 , 79]. Only one investigation has directly compared an experimental and an extant military physical training regimen, where the total number of physical training sessions and total training time were matched [66, 67]. The experimental training regimen in an Australian Army recruit cohort significantly reduced the volume of endurance type exercise (8 vs 17 sessions), whilst increasing the intensity (3 min near-maximal intervals) and increased the focus on resistance exercise. Despite a ∼50% reduction in both the number of endurance sessions and the physical activity volume within the sessions, the experimental program showed greater gains in cardiorespiratory endurance compared to the standard physical training program (12.9 vs 8.1%). The strength training to endurance training ratio was ∼2:1. The experimental concurrent training regimen also achieved superior gains in muscular strength and occupationally relevant tasks (load carriage, box lift). Importantly, this was achieved within the constraints of an occupational (recruit) training environment. The greater gains observed in the experimental group are likely attributable in part, to the reduced training volume, thus a reduced interference effect on strength development. Differences in hormone responses between the two groups provides some support for this suggestion [66]. In addition, the superior gains in the occupationally relevant tasks were achieved despite a reduced exposure to military circuits and load carriage sessions. For example, the experimental group showed greater improvement in load carriage performance when compared to the control group (11.2% vs 7.6%, p < 0.05) despite a reduction in load carriage sessions from 7 to 2 (Table 2). This highlights the importance of developing the underlying physical capacities, rather than simply replicating occupational tasks (e.g. load carriage). Physical training regimen must strike a balance between development of physical capacities and occupational task exposure. These findings are relevant to all physically demanding occupations, however the precise training variables applied within this study are not necessarily directly transferable. Each occupational setting will vary in total physical demands and allostatic stress, physical training sessions allocations, training status of the population, etc., all of which will influence both the training prescription and the response to training.

Further insight into the programming of concurrent training programs within physically demanding occupations may be gained from a study in Finnish military recruits [64] (Table 3). Two separate recruit cohorts were supplemented with either strength or endurance training on top of ‘normal training’. There were no differences between the endurance, strength or control groups in cardiorespiratory endurance or 3 km combat loaded run performance following the 8-week training course. The strength and endurance groups both increased maximal isometric leg extension despite marked differences in training regimen, whilst the control group did not change [64]. Notwithstanding the fact that more occupationally relevant performance measures could have been assessed (e.g. manual handling) [13 , 52], it is suggested that these results may be explained by an interference effect from the total physical demands of the military training courses. The volume of endurance training in the strength group may have interfered with the development of muscular strength, and therefore explain the equivalent gains when compared to the endurance group. These results highlight the importance of considering the total training volume, and the potential interference effect of high-volume endurance exercise on muscular strength development. Several studies have also documented the numerous physically demanding activities that are conducted outside of physical training lessons during military training (e.g. load carriage, field training, drill) [52 , 80–82]. All of these activities contribute to the total physical load exposure and may influence physiological adaptations and physical performance outcomes, as well as MSKI risk. In fact, two systematic reviews examining MSKI prevention in the military have recommended a reduction in physical activity volume as a priority strategy [83, 84].

Table 3

Physical and physiological performance changes following training interventions in basic military training

Study	Population	Training duration	Training intervention	Physical capacity	Performance change
Drain et al. [67]	Australian Army recruits,	12 weeks	Control group (CON):	Muscular strength:	1 RM box lift, 1.5 m: CON; 5.7%, EXP; 13.7% ^#
	Control group; n = 52 males, 15 females, experimental group; n = 59 males, 11 females.		40 PT sessions; 7 circuit, 10 endurance, 7 load carriage, 13 skill-based sessions, 3 fitness testing sessions.		Push-ups: CON; 31.6%, EXP; 46.4% ^#
			Intervention group (EXP):	Muscular endurance:	MSST predicted VO_2max: CON; 8.1%, EXP; 12.9% ^#
			40 PT sessions; 17 strength, 8 endurance, 2 load carriage, 8 skill-based sessions, 3 fitness testing, 2 swimming.	Cardiovascular endurance:	3.2 km, 22 kg load carriage: CON; 7.6%, EXP; 11.2% ^#
Santtila et al. [61, 64]	Finish conscripts, n = 24 males per group	8 weeks	Normal training (CON):	Muscular strength:	Isometric leg extension: NT; 5.2%, ST; 9.1% ^# , ET; 12.9% ^#
			33 hours, sport-related PT.
			Strength training (ST):
			44 hours sport-related PT. Additional 3 strength sessions week, 60–90 min session, whole-body program.	Cardiovascular endurance:	VO_2max: NT; 13.4% ^#, ST; 12.0% ^#, ET; 8.5% ^#
			Endurance training (ET):		3 km, 14 kg loaded run: NT; 10.2% ^#, ST; 12.4% ^#, ET; 11.6% ^#
			51 hours sport-related PT.
			Additional 3 endurance sessions week, 60–90 min session.
Williams et al. [52, 62]	British Army recruits,	11 weeks	Normal training (CON):	Muscular strength:	1 RM box lift, 1.45 m: CON; 1.7%, EXP; 12.4% ^*
			71 PT sessions; 19 sport, 18 circuit, 10 agility, 9 swimming, 7 endurance, 2 manual handling, 6 skill-based and testing		1 RM box lift, 1.70 m: CON; 4.4%, EXP’ 8.0%
	Normal training; n = 47 males, 10 females;		Modified training (EXP):		38 cm upright pull: CON; -0.8%, EXP; – 4.4%
			71 PT session; 28 strength training, 15 endurance, 8 agility, 6 manual handling, 6 sports, 4 circuit training, 4 swimming	Muscular endurance:	Repetitive lift, 10 kg: CON; 7.0%, EXP; 14.5%
	Modified training: n = 43 males, 9 females.				Repetitive lift, 22 kg: CON; 29.5%, EXP; 18.5%
					IDL1.45 m: CON; 0.2%, EXP; 14.4% ^*
					MSST predicted VO_2max: CON; 6.1%, EXP; 9.6%
				Cardiovascular endurance:	Loaded march, 15 kg: CON; 3.6%, EXP; 8.9% ^*
					Loaded march, 25 kg: CON; 15.7%, EXP; 16.7%

^*Denotes significant difference (p < 0.05) to control, ^#denotes significant difference (p < 0.05) to pre-training, 1 RM; one repetition-maximum, MSST; multi-stage shuttle run test, IDL; incremental dynamic lift.

The relationship between cardiorespiratory training volume and performance gains is not linear, nor is it always positively trending [67, 85]. For example, the impact of running mileage on performance was assessed in U.S. Navy recruits during basic training [85]. The two lowest total run mileage quartiles for the 7-week training period (11.5 to 17.5, and 18.0 to 21.5 total miles) demonstrated a faster 2.4 km run time at the end of training, when compared to the highest total run mileage quartile (25.5 to 43.5 total miles). The highest quartile of running mileage also had the highest injury rate, when compared to the other quartiles (22.4% vs 17.2%) [85]. Furthermore, Westcott et al. [86] compared two different physical training regimen in U.S. Air Force personnel who had failed the annual physical fitness assessment. The control group undertook the standard Air Force 12-week remedial physical training program that involved mostly running for approximately 60 min, 4-5 days a week. The intervention group undertook a whole-body circuit training program that involved 60 sec bouts of moderate load strength training (40–60% 1RM) alternating with 60 sec efforts on a cycle ergometer for 25 min, 3 days per week [86]. The aerobic training group (i.e. standard 12-week remedial physical training program) showed no change in 2.4 km run time, 1-minute push-ups or abdominal circumference, whereas the circuit training group showed significant improvements (p < 0.05) in all measures.

Thus for physically demanding occupations, there is evidence to support lower volume, higher intensity endurance exercise combined with resistance exercise for the simultaneous development of muscular strength and cardiorespiratory endurance [62 , 88]. Strength training will not only help to condition personnel for the many manual material handling tasks they are likely to perform but also enhance load carriage and cardiorespiratory endurance performance [63 , 90].

3 Physical capacity and injury prevention

While there is an association between physical capacity (e.g. cardiorespiratory endurance, muscular endurance and muscular strength) and MSKI risk in occupational settings [22, 23], it is difficult to attribute cause and effect, as causation is typically multifactorial [91]. Low cardiorespiratory endurance has been associated with injury risk in military personnel [24 , 92–95] and workers in manual materials handling jobs [96]. Results from a Danish 12 week basic military training course showed an overall injury rate of 28%, with an inverse relationship between “physical” fitness and overuse injury [97]. Low (estimated) cardiorespiratory endurance was also correlated with higher injury rates in male industrial workers [96]. The results demonstrated an injury incidence rate of 17% in the ‘high’ cardiorespiratory endurance group (>46 mL.kg^–1.min^–1), compared to 50% for the ‘low’ cardiorespiratory endurance group (<36 mL.kg^–1.min^–1). This must be balanced against civilian-based research demonstrating that the risk of sustaining an activity-related injury increased with higher cardiorespiratory endurance and higher total weekly physical activity [98]. Increased cardiorespiratory endurance may therefore confer a protective effect in some instances (noting cause and effect is not established) but participation in cardiorespiratory endurance activities (e.g. running) are also a risk factor for MSKI. There are numerous confounders in understanding the association between physical capacity and occupational MSKI risk, including training status, genetics, sex, occupational exposure(s), and previous injuries.

In some occupations, individuals may also be able to self-select tasks and roles to a degree, which may influence MSKI risk, whereas in other settings (e.g. operational) it may not be possible to self-select occupational exposure. Recent results from deployed U.S. Army soldiers demonstrated that load carriage (e.g. dismounted patrolling) and lifting tasks were risk factors for MSKI [19 –21]. Outside of occupational risk factors, additional risk factors included physical fitness, age and sex [21]. Unlike age and sex, physical capacity is modifiable, and increased physical capacity can decrease the relative task demands (e.g. % 1RM, % VO_2max), and in turn reduce fatigue and MSKI risk [99, 100]. This reinforces the importance of developing occupationally-relevant physical capacity (e.g. cardiorespiratory endurance, muscular strength) for both job performance and mitigating MSKI risk.

Various occupations have investigated screening options for MSKI risk, which often employ generic physical fitness tests (e.g. multi-stage shuttle run test, push-ups, sit-ups) or clinical assessments of joint strength or flexibility. For example, to help identify occupational MSKI risk in police officers, a suite of baseline measures such as range of motion and movement competency were assessed, and these officers were then monitored over a 5-year period for back injury [101]. The best predictive model, using eight mobility and fitness measures resulted in a prediction no better than a coin toss [101]. Similarly, Danish industrial workers (n = 421; 208 women, 213 men) were assessed for maximal trunk, neck, shoulder and hand strength, and then prospectively monitored over a 12-year period [102]. The results indicated that those with low muscle strength in a particular body region did not have an increased risk for future long-term sickness absence or MSKI in the same body region. Sex may be a consideration however, as the proportion of women having an episode of long-term sickness absence was 42%, compared to 32% for men, yet men demonstrated approximately double the muscle strength of women in all regions except the hands.

Further research in industrial workers examined the relationship between isometric lifting strength in the arm, torso, and leg and the incidence of reported back problems over a four-year period [103]. The results indicated that isometric strength testing was ineffective at identifying individuals at risk of back problems once age was controlled for. In addition, no significant difference was found in the incidence of back problems between sexes despite differences in isometric strength [103]. In contrast, a study examining pre-employment physical capacity screening and MSKI in workers involved in building material manufacture and supply demonstrated significant benefit [104]. Experimental subjects (n = 503) were assessed for knee, back and shoulder strength on an isokinetic dynamometer, and matched to physical capability job standards, while the control group (n = 1423) were hired into roles without screening. Non-screened applicants were 2.38 more likely to suffer a MSKI injury to the knees, back or shoulders [104].

These mixed findings are likely explained, at least in part by the fact that these screening tools provide limited insight into work tolerance or fatigue resistance. This is difficult to assess via typical test batteries such as PES, which involve discrete tasks performed to minimum standards with binary outcomes (pass/fail) as opposed to maximal performance tests with continuous variables (e.g. completion time). Unlike many countries, the Canadian Armed Forces assess both PES pass/fail, and the performance time on the four elements of the PES, where maximum effort is incentivised. From 2014 onwards, data on 8609 recruits (n = 7265 males, 1344 females) indicates that of those who passed the PES, the bottom 10% of performers on a given element were 3 times more likely to sustain an MSKI during basic training. In addition, those that failed the sandbag drag (the strength component of the PES) had a MSKI risk 6 times higher than those who passed [25]. This PES performance data demonstrates the potential utility of PES to not only predict occupational performance but also MSKI risk.

In addition to higher fidelity data of PES performance (i.e. continuous best effort performance scores rather than just pass/fail), increased collection of physiological data during task performance could better inform physical training programs, to reduce fatigue (i.e. maintain task performance) and decrease MSKI risk. Physical capacity, occupational exposure and MSKI risk are complexly linked and should not be treated as mutually exclusive. Appropriate physical training programs that focus on occupational demands and associated MSKI are therefore essential in developing and maintaining a physically resilient and available workforce.

4 Recommendations and conclusion

The development of PES helps to characterise the acute and chronic exposure to occupational tasks (e.g. manual materials handling, load carriage) which should inform physical training and MSKI prevention programs. The focus of PES and physical training should evolve beyond improving performance on select physical fitness measures and discrete occupational tasks, towards interventions for improving physical and physiological resilience to acute and chronic occupational task demands. The characteristics of these occupational tasks and evidence base for such interventions can be drawn from the JTA that underpins a PES. Future studies evaluating the effects of physical training programmes on occupational task performance (potentially assessed by PES), should measure relative strain (i.e. percentage of maximal capacity) of the participant while performing the task, in addition to the performance outcome (e.g. completion time). These data will provide more insight into an individual’s ability to cope with occupational demands. PES typically reflect a minimum binary pass/fail occupational requirement, however they can be used as a powerful tool to assess occupational performance improvements and identify MSKI risk when designed and administered to collect maximal effort as a continuous variable. Many organisations overlook the potential for PES to be implemented and used in this manner. Furthermore, physically demanding occupations should support the through-career participation (Fig. 1) in physical training to not only preserve occupationally-relevant physical performance, but also cardiovascular health. Physical training, PES and MSKI prevention strategies are co-dependent factors, and central to organisational capability within physically demanding occupations.

4.1 Summary of key recommendations

Information collected during PES development should inform physical training programs, e.g. dominant muscle groups, physical capacities, movement patterns, task duration, task frequency. MSKI data should also be used to inform physical training programs as well as evaluate program effectiveness.

Increase focus on the development on occupationally-relevant physical capacities to improve occupational performance, and reduce MSKI risk, rather than focus on task performance (e.g. load carriage).

Adopt concurrent training regimen comprised of strength training and endurance training (including high-intensity intervals).

Consider total physical activity exposure (e.g. occupational training, tasks and leisure time activity) when developing physical training programs.

Consider implementing PES as maximal performance tests, rather than dichotomous pass/fail tests, to better understand the relationship between occupationally-relevant physical capacity and MSKI risk.

Develop a physical performance continuum to manage the through-career performance capacity and MSKI risk of personnel. This should include longitudinal MSKI surveillance.

Conflict of interest

None to report.

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