Why are we doing this Boss? Justification and implications of aerobic fitness testing in the military

Abstract

BACKGROUND:

Physical fitness is a key tenet of military organisations worldwide. Specifically, many consider aerobic fitness (AF) an essential physical attribute for ensuring optimal military performance and readiness. However, the intricate relationship between AF and various facets of military job performance necessitates comprehensive review to ascertain the appropriateness and effectiveness of its assessment.

OBJECTIVE:

This narrative review aims to describe the relationship between AF and factors influencing individual military performance and readiness, and explores the implications of the enforcement of in-service, generic AF test standards in military populations.

METHODS:

Databases (PubMed and Google Scholar) were searched for all relevant published peer-reviewed literature as at August 2023.

RESULTS:

Inconsistent associations were found between AF and outcomes influencing individual military performance (physical capabilities, cognitive capabilities, presenteeism and productivity, resilience, and technical/tactical capabilities) and readiness (mental health and wellbeing and physical health). Consequently, the level of AF needed for acceptable or optimal military performance remains uncertain.

CONCLUSIONS:

AF is a cornerstone of health and performance, yet linking generic AF test standards to job performance is complex, with multiple factors interacting to influence outcomes. From existing literatures, there does not appear to be a specific level of AF at, and/or above, which acceptable military performance is achieved. As such, the enforcement of and emphasis on in-service, pass/fail, generic AF test standards in military populations is questionable and requires thoughtful re-evaluation. Role/task-specific AF should be assessed through evidence-based PES and the use of generic AF tests limited to the monitoring and benchmarking of health-related fitness.

Keywords

Military personnel law enforcement cardiorespiratory fitness fitness testing work performance military health

1 Introduction

Significant emphasis is placed on physical fitness in the military, with high levels thought to enhance military performance [1]. The demands of the various military occupational specialities (MOS) are diverse and although many military roles may bear similarities to jobs in the civilian world, the work environment is often significantly different. This reflects the range of unique stressors that military personnel are exposed to during training and in combat, including physical (e.g. sleep deprivation, undernutrition, strenuous exercise), environmental (e.g. noise, climatic extremes, altitude, pollutants) and psychological (e.g. anxiety, demanding cognitive tasks, long work hours) stressors [2]. Such stressors have been associated with negative outcomes, including reductions in physical and cognitive performance, fatigue, illness and injury [2]. High levels of physical fitness may enhance soldiers’ resilience to these stressors and enable them to accomplish required tasks while remaining healthy and uninjured [1].

Historically, generic physical fitness tests (e.g. maximal run, press-ups to failure and sit-ups to failure) have been used to assess the job-related fitness of military personnel [3]. However, more recently, the in-service enforcement of standards based on such tests has been criticised because of insufficient evidence validating their association with military performance [4]. Occupational, or job-related, fitness assessments (Physical Employment Standards, PES) are an alternative type of physical capability test, increasingly used in military organisations, whereby performance on standardised job-task simulations (e.g. carrying an object a specific distance) is assessed [4].

In comparison to generic fitness assessments, PES assessments are generally considered to provide a more valid and accurate indication of a soldier’s ability to perform essential physically demanding job tasks. That said, generic physical fitness tests are often used alongside, and in some cases instead of, PES assessments. Generic physical fitness tests have recently been included as a health-related fitness assessment of serving personnel but in some military organisations, they are used as a specific job requirement [3]. While the type of generic fitness test(s) used varies between military organisations, aerobic fitness (AF) is the most commonly assessed attribute [3]. AF can be defined as the ability to perform dynamic exercise involving large muscle groups at a moderate to high intensity for prolonged periods [1]. It is widely recognised that the most accurate assessment of AF is laboratory determination of maximal oxygen uptake, ${\dot{VO}}_{2 max}$ (i.e. the maximal (_max) rate ( $\dot{V}$ ) of the body to use oxygen (O₂) during peak or maximal exercise per unit time) [5]. However, such testing is often not feasible in the military context [5]. As such, field tests including timed running performance (e.g. 1.5 mile/2.4 km run) and the Multi Stage Fitness Test (MSFT) are more commonly used [4]. The resulting test score may then be used to predict ${\dot{VO}}_{2 max}$ , via the application of one of many conversion equations; however, prediction error and over- and under-estimation of ${\dot{VO}}_{2 max}$ is reported [5].

As at March 2023, nine of the ten nations who participated in a NATO Task Group (Human Factors and Medicine (HFM) Research Task Group (RTG) 269) on Combat Integration: Implications for Physical Employment Standards, enforced a generic AF requirement for in-service personnel, as shown in Table 1 [3]. Canadian Armed Forces personnel complete an AF test, but for health-related fitness monitoring purposes; a performance standard is not enforced. Given the widespread use of generic AF tests by military organisations worldwide, a review of the evidence for generic AF testing and standards and their association with occupational performance and readiness is warranted. The purpose of this review is twofold; first, to examine the relationship between AF and individual military performance and readiness, and second, to discuss the implications of this for the enforcement of generic, in-service AF test standards in the military.

Table 1
Aerobic fitness tests and standards administered by international military organisations for incumbent personnel [3], as at March 2023

Military Organisation Aerobic Fitness Test Aerobic Fitness Test Standard

United States Army 2 mile run Indexed by age and sex

United States Marine Corps 3 mile run Indexed by age and sex

Royal Navy 2.4 km run, MSFT or 1.6 km Rockport Walk Test¹ Indexed by age and sex

Royal Air Force MSFT Indexed by age and sex

Australian Army 2.4 km run Indexed by age and sex

Royal Australian Air Force 2.4 km run or 5 km walk Indexed by age and sex

Royal Australian Navy 2.4 km run, 5 km walk, MSFT or 500 m swim Indexed by age and sex

New Zealand Army 2.4 km run Indexed by age and sex

Royal New Zealand Navy MSFT Indexed by age and sex

Norwegian Armed Forces (Forsvaret) 3 km run Indexed by age, sex and trade

German Armed Forces (Bundeswehr) 1 km run ≤6 min 30 sec

French Armed Forces 12-minute run or MSFT Indexed by age and sex

Danish Armed Forces 12-minute run or MSFT Indexed by age

Netherlands Armed Forces 12-minute run Indexed by age and sex

Military Organisation	Aerobic Fitness Test	Aerobic Fitness Test Standard
United States Army	2 mile run	Indexed by age and sex
United States Marine Corps	3 mile run	Indexed by age and sex
Royal Navy	2.4 km run, MSFT or 1.6 km Rockport Walk Test¹	Indexed by age and sex
Royal Air Force	MSFT	Indexed by age and sex
Australian Army	2.4 km run	Indexed by age and sex
Royal Australian Air Force	2.4 km run or 5 km walk	Indexed by age and sex
Royal Australian Navy	2.4 km run, 5 km walk, MSFT or 500 m swim	Indexed by age and sex
New Zealand Army	2.4 km run	Indexed by age and sex
Royal New Zealand Navy	MSFT	Indexed by age and sex
Norwegian Armed Forces (Forsvaret)	3 km run	Indexed by age, sex and trade
German Armed Forces (Bundeswehr)	1 km run	≤6 min 30 sec
French Armed Forces	12-minute run or MSFT	Indexed by age and sex
Danish Armed Forces	12-minute run or MSFT	Indexed by age
Netherlands Armed Forces	12-minute run	Indexed by age and sex

¹In process of being replaced by a role-related PES with an aerobic component.

2 Methods

This narrative review was conducted to provide a general and critical overview of previously published research on AF in the military. A literature search was carried out in PubMed and Google Scholar using a range of search terms, as listed in Table 2. All articles that included a search term from List 1 and List 2 were taken into consideration. However, articles were only included in this review if they met the following inclusion criteria: (i) the research was peer-reviewed; (ii) the research was published in English from 1990 to August 2023; and iii) the research design was observational, experimental, or quasi-experimental, or a literature review. Sixty-three studies assessing the relationship between AF and military performance and/or readiness outcomes were included. An additional sixteen studies were included to provide context on AF testing or performance and readiness outcomes; these studies only included search terms from either List 1 or List 2 in Table 2. Finally, the North Atlantic Treaty Organization (NATO) report of HFM RTG 269 was included, as this is considered a seminal document in the field of fitness testing in military organisations.

Table 2
Search list terms for the literature review

Search List 1 Search List 2

“Aerobic Fitness”; “Aerobic Capacity”; “Cardiorespiratory Endurance”; “Multistage Fitness Test”; “Run”; “Maximal oxygen uptake ( ${\dot{VO}}_{2 max}$ )” “Load carriage”; “Obstacle course”; “Mobility course” “Recovery”; “Heat tolerance”; “Cognition”; “Presenteeism”; “Productivity”; “Absenteeism”; “Resilience”; “Marksmanship”; “Mental health”; “Wellbeing”; “Illness”; “Injury”; “Disease”; “Mortality”; “Attrition”

Search List 1	Search List 2
“Aerobic Fitness”; “Aerobic Capacity”; “Cardiorespiratory Endurance”; “Multistage Fitness Test”; “Run”; “Maximal oxygen uptake ( ${\dot{VO}}_{2 max}$ )”	“Load carriage”; “Obstacle course”; “Mobility course” “Recovery”; “Heat tolerance”; “Cognition”; “Presenteeism”; “Productivity”; “Absenteeism”; “Resilience”; “Marksmanship”; “Mental health”; “Wellbeing”; “Illness”; “Injury”; “Disease”; “Mortality”; “Attrition”

3 The relationship between AF and individual military performance

The job-related demands of the various MOS are diverse, both within and between domains (maritime, land, and air). Individual military performance is considered to be a complex and multifaceted construct arising from several factors, including physical capabilities, cognitive function, work presenteeism and productivity, resilience, and technical/tactical capabilities [6], as summarised in Fig. 1. However, irrespective of the specific demands of military roles, many military organisations adhere to the principle of ‘Universality of Service’, or ‘Soldier First’, which holds that members must be able to perform common defence and security duties, not just the duties of their military occupation speciality. As such, although many military personnel may spend long periods engaged in sedentary desk-based roles, they may be required to move, at relatively short notice, to a field role with very different demands. Regular PES assessments help ensure readiness to perform such roles but where generic AF tests and standards are also used, there is a need to ensure that these are evidence-based and justified. This section of the review will examine the relationship between AF and factors influencing successful individual military performance.

Fig. 1

Key factors influencing individual military performance. (Note: It is acknowledged that there are other capabilities, extending beyond those shown in the figure).

3.1 Physical capabilities

3.1.1 Load carriage

Load carriage refers to military personnel carrying equipment and supplies on their body [7]. A relationship between higher AF and reduced cardiovascular strain has been found during fixed duration load carriage in male Royal Marine recruits [8] and healthy volunteers [9]. Additionally, several studies have demonstrated an association between higher AF levels and faster load carriage march time to completion [7, 10]. Collectively, these findings suggest that military personnel with higher AF levels can work at a lower relative intensity, which in turn allows for greater task performance.

As previously mentioned, ${\dot{VO}}_{2 max}$ , assessed by field or laboratory tests, is a common measurement used to establish AF. Both predicted and laboratory-determined ${\dot{VO}}_{2 max}$ can be expressed as an absolute value (L·min^–1), or as a value that is relative to body mass (ml·kg^–1·min^–1) [11]. In circumstances where body mass must be supported, relative ${\dot{VO}}_{2 max}$ is generally of greater relevance, but in an operational setting, military personnel rarely walk or run without carrying external load, adding to the complexity of this relationship and in part explaining why the observed relationship between AF and load carriage appears to be influenced by the method by which AF is expressed. Studies have reported an association between absolute ${\dot{VO}}_{2 max}$ and load carriage performance and/or strain measures, but these associations were not observed for relative ${\dot{VO}}_{2 max}$ [7 , 12], likely owing to the considerable mass of the encumbered soldier that is not capable of metabolising oxygen.

The mass of the load itself appears to influence the relationship between ${\dot{VO}}_{2 max}$ and the metabolic demands (% ${\dot{VO}}_{2 max}$ ) of load carriage. Lyons et al. [9] examined this relationship in healthy male volunteers during a 60 min march with 0 kg, 20 kg and 40 kg loads. The metabolic demands of the 0 kg and 20 kg load carriage tasks were associated with relative ${\dot{VO}}_{2 max}$ [9]. Similarly, an association between relative ${\dot{VO}}_{2 max}$ and physiological strain was recently demonstrated by Oeschger et al. [13] with a higher relative ${\dot{VO}}_{2 max}$ (≥58.8 ml·kg^–1·min^–1) associated with lower mean heart rates and peak body core temperatures during a 34 km loaded (∼23 kg) march in Swiss Armed Forces soldiers. In contrast, with a heavier load, specifically 40 kg, absolute ${\dot{VO}}_{2 max}$ was more strongly correlated with metabolic demands than relative ${\dot{VO}}_{2 max}$ [13]. These findings suggest that a high absolute ${\dot{VO}}_{2}$ reserve (i.e. the difference between maximum and resting ${\dot{VO}}_{2}$ ) is essential for heavy load carriage performance, as humans become less efficient when carrying increasingly heavier loads due to the sharp increase in energy expenditure that occurs per kg of load [13].

Higher body mass and higher muscle mass also appear to be beneficial for load carriage performance. For example, when carrying the same fixed mass, at an equivalent velocity, the fixed load represents a lower percentage of body mass and a lower metabolic cost for an individual with a higher body mass, compared to an individual with a lower body mass [12]. Heavier individuals have also been shown to perform better in a load carriage task (specifically, longer time to exhaustion when running at 9.5 km.h^–1 with an 18 kg load) compared with lighter individuals [12]. Therefore, describing an individual’s ${\dot{VO}}_{2 max}$ relative to body mass is likely to underestimate a heavier individual’s ability to perform load carriage [12].

It is difficult to directly compare previous study findings as the methods of assessing the relationship between AF and load carriage vary considerably in terms of the tests used to assess AF, the mass of the load carried, the distance or duration of the march, and as previously mentioned, the method of expressing AF. Further adding to the complexity, load carriage performance is known to be influenced by numerous factors, including personal characteristics (e.g. sex, age, injury profile, load carriage experience), task characteristics (e.g. load mass, equipment design, movement speed, work to rest ratio), and environmental conditions (e.g. terrain, climate) [14].

In summary, it is apparent that a relationship exists between higher AF and enhanced load carriage performance. However, due to the methodological heterogeneity across studies and a multitude of factors known to influence load carriage, definitive conclusions cannot be drawn as to what level of AF is required for acceptable or optimal load carriage performance.

3.1.2 Operationally-relevant mobility

To gain a more global understanding of the impact of AF on soldier physical capabilities, several studies have sought to simulate various physical operational demands using obstacle/mobility courses involving sprinting, casualty recovery, climbing, crawling, lifting and carrying [15 –19]. These activities require varying degrees of agility, mobility, speed, and strength; however, when performed back-to-back (as in an obstacle course), the aerobic demand increases and the aerobic system plays a significant role in influencing fatigue, recovery, and reserve capacity for subsequent tasks [1]. Indeed, an association between higher AF and faster obstacle course time to completion has been reported in soldiers [18 –20] and healthy volunteers [15, 16]. However, in addition to AF, other factors were also significantly associated with better performance, including anthropometric measures [15, 17], jump height [16, 18], core stability [19] and muscular strength [18, 19]. Furthermore, of the studies reviewed, there appears to be only one study that has attempted to specify a required level of AF based on obstacle course performance. Orantes-Gonzalez et al. [17] found that male Spanish Army soldiers with a predicted ${\dot{VO}}_{2 max}$ ranging from 55–65 ml·kg^–1·min^–1 had the fastest time to completion on an obstacle course, versus those with a lower relative ${\dot{VO}}_{2 max}$ (35–55 ml·kg^–1·min^–1), leading researchers to suggest a ${\dot{VO}}_{2 max}$ greater than 55 ml·kg^–1·min^–1 should be used as a criteria for enrolment in the Spanish Army. However, it is not clear what level of AF is required for ‘acceptable’ (as opposed to faster) performance. Obstacle course time to completion, as an indicator of physical competence, may also lack ecological validity in the military, as faster is not always better or necessary; rather, it is often more important to perform a task at the required minimum pace, for a prolonged period of time.

Research suggests that soldiers most commonly perform submaximal intensity work (i.e. 50% ${\dot{VO}}_{2 max}$ ), often over extended durations [20]. Military personnel with lower AF levels may still be able to adequately perform real-world physical military tasks to the required standards. Alternatively, those with higher levels of AF may not be able to perform the required tasks to the required standard. Indeed this has been observed in initial trainees, with 23.5% of US Army recruits misclassified (6.7% false positives - accepting a recruit who may not be successful in training and 16.8% false negatives - declining entry to a recruit who may be successful in training) when researchers used the US Army’s Occupational Physical Assessment Test (OPAT) cut-off scores to predict recruits’ success in training [21]. These findings suggest that generic AF test performance does not necessarily predict success in training or successful task performance [21].

In summary, where speed of completion of consecutive soldiering tasks is important, higher AF is likely beneficial, however, the specific level of AF associated with acceptable operationally relevant mobility (or other) task performance is yet to be determined.

3.1.3 Recovery

In addition to having the competence to perform physically demanding military tasks, the ability to overcome mental and physical fatigue and recover quickly after physical stress is important for soldiers. All recovery is aerobic and as such, higher levels of AF generally enhance recovery from exercise through various physiological mechanisms (e.g. heart rate recovery, lactate removal and phosphocreatine regeneration) [22]. Nevertheless, only a handful of studies have specifically examined AF and recovery in military populations. Hoffman et al. [23] assessed infantry soldiers’ ability to recover from three 140-m sprints (separated by 2 minutes of passive rest), with a fatigue index score calculated by dividing the mean time of the three sprints by the fastest time. A reduction in the rate of fatigue was found in soldiers with AF test scores (2 km run times) equal to or faster than the population mean [23]. However, researchers found no further benefit in the recovery rate from high-intensity exercise with high levels of AF ( ${\dot{VO}}_{2 max}$ : 52 ml/kg/min), compared to moderate levels of AF ( ${\dot{VO}}_{2 max}$ : 44 ml/kg/min), [23], suggesting that only moderate (rather than high) AF levels may be required for enhanced recovery.

Recovery has also been examined in military personnel undergoing a 2-week Survival, Evasion, Resistance and Escape (SERE) training course [24]. Fitness test scores (timed run, press-ups and sit-ups) for 20 US Navy sailors and Marines were used to evaluate overall fitness. To assess recovery from arduous activity, adrenergic responses (i.e. the body’s response to stress measured via norepinephrine and neurotransmitter neuropeptide Y) were then evaluated during survival training [24]. At the end of survival training, a quicker return of adrenergic responses to baseline levels was found in those with a higher overall fitness score, suggesting that physical fitness may have a protective effect in recovery following a period of high stress military training [24]. Conversely, in Swiss Armed Forces soldiers, no relationship between AF and recovery was found. Soldiers were divided into three fitness groups (high ${\dot{VO}}_{2 peak}$ : 61.7±2.2 ml·kg^–1·min^–1; middle ${\dot{VO}}_{2 peak}$ : 55.1±2.1 ml·kg^–1·min^–1; and low ${\dot{VO}}_{2 peak}$ : 44.9±4.8 ml·kg^–1·min^–1) and recovery ability was examined during the third of four 19–24 min recovery breaks provided during a 34 km loaded march [13]. Heart rate and core temperature data were collected during the last minute before the break and at the tenth minute of the recovery break, and the difference between these values was documented as recovery ability [13]. Higher relative ${\dot{VO}}_{2 peak}$ was correlated with less likelihood of dropout from the march [13]. However, recovery ability did not differ between the three fitness groups [13]. This finding may be explained by the fairly high AF levels across all groups [13] and supports previous findings that only moderate AF levels are needed for enhanced recovery [23]. In summary, given the limited literature and conflicting evidence, further research is needed to better elucidate the relationship between AF and recovery in militarypopulations.

3.1.4 Heat tolerance

Military personnel often deploy to environments (heat, cold, altitude) that differ to those in which they typically live and train in. Duties in hot environments present a unique challenge as carrying heavy loads and wearing personal protective equipment exacerbates heat production and further restricts heat loss [25]. Military studies investigating heat stress have tended to use the standardised heat tolerance test developed by the Israeli Defence Force, which requires participants to walk at 5 km.h^–1 2% gradient, in 40°C and 40% relative humidity. Participants are deemed ‘heat intolerant’ if they meet the criteria of core body temperature >38.5°C, heart rate >150 bpm, or when either fail to plateau [26, 27]. In studies examining both military personnel and the general population, AF was predictive of - and associated with - heat intolerance, whereby relative ${\dot{VO}}_{2 max}$ was significantly lower in heat intolerant participants [26, 27]. Similarly, in healthy adult volunteers, lower physiological strain during heat stressful exercise (35–40°C and 30% relative humidity) has been found in participants with higher AF levels ( ${\dot{VO}}_{2 peak}$ >60 ml·kg^–1·min^–1) [28] ( ${\dot{VO}}_{2 max}$ = 64±4 ml·kg^–1·min^–1) [29] versus those with lower AF levels ( ${\dot{VO}}_{2 peak}$ <50 ml·kg^–1·min^–1) [28] ( ${\dot{VO}}_{2 max}$ = 45±3 ml·kg^–1·min^–1) [29]. Other factors have also been shown to influence heat tolerance during heat-stressful exercise in military populations, including prior heat illness, age, sex and body composition [25].

In summary, despite research demonstrating a strong relationship between higher AF and improved thermoregulatory function and exercise tolerance in the heat, the specific ${\dot{VO}}_{2 max}$ required to elicit these benefits is currently unknown.

3.2 Cognitive capabilities

In addition to physical competence, cognition is critical for successful military performance. As summarised in Table 3, a number of studies have sought to determine the influence of AF on cognitive capabilities, with researchers tending to use a battery of tests to assess a broad range of cognitive functions in various physiological states. Different military populations have been investigated, specifically Royal Norwegian Navy sailors [30], Irish Defence Personnel [31], Spanish young adult healthy volunteers [32], US Air Force personnel [33], US service members [34], and Spanish active-duty military personnel [35]. Inconsistent findings are apparent in these studies, with both positive- and null- associations found between various cognitive functions and AF (Table 3). For example, mixed results were reported by Beckner et al. [34] examining the relationship between AF and cognition in 54 soldiers during a 5-day multi-factorial stress scenario. In this study, soldiers were divided into three fitness groups (high ${\dot{VO}}_{2 peak}$ : >51.53 ml·kg^–1·min^–1, n = 18; moderate ${\dot{VO}}_{2 peak}$ : >44.87 – ≤51.53 ml·kg^–1·min^–1, n = 18; and low ${\dot{VO}}_{2 peak}$ : ≤44.87 ml·kg^–1·min^–1, n = 18) and cognitive function was assessed via a 10-part cognitive test battery. A significant association between AF and vigilant attention was demonstrated; Psychomotor Vigilance Test (PVT) accuracy remained stable in the high AF group during simulated military operational stress, whereas a significant decline occurred in both low- and moderate- AF groups [34]. However, as shown in Table 3, nine other cognitive functions were not influenced by AF [34].

Table 3
Summary of studies examining associations between cognitive functions and aerobic fitness

Cognitive measures Association with Aerobic Fitness

Positive Negative Null

Reaction time [32, 33] [30, 35]

Executive functioning [31] [31, 33]

(e.g. Stroop test, Grooved pegboard test)

Processing efficiency [32, 33] [34]

Episodic memory [33]

Short-term memory [31, 33]

Working memory [30, 31] [33, 34]

Fluid intelligence [33] [33]

Scanning, and visual tracking [31]

Vigilant attention [32, 34]

(e.g. Psychomotor Vigilance Task)

Sensorimotor speed [34]

Spatial orientation [34]

Spatial learning [34]

Abstraction and concept formation [34]

Complex scanning, memory, and visual tracking [34]

Emotional identification through facial expressions [34]

Abstract reasoning [34]

Risk decision making [34]

Attention [31]

(e.g. Colour Trails Test, Trail making test)

Cognitive measures	Association with Aerobic Fitness
Reaction time	[32, 33]	[30, 35]
Executive functioning	[31]	[31, 33]
(e.g. Stroop test, Grooved pegboard test)
Processing efficiency	[32, 33]	[34]
Episodic memory	[33]
Short-term memory		[31, 33]
Working memory	[30, 31]	[33, 34]
Fluid intelligence	[33]	[33]
Scanning, and visual tracking	[31]
Vigilant attention	[32, 34]
(e.g. Psychomotor Vigilance Task)
Sensorimotor speed		[34]
Spatial orientation		[34]
Spatial learning		[34]
Abstraction and concept formation		[34]
Complex scanning, memory, and visual tracking		[34]
Emotional identification through facial expressions		[34]
Abstract reasoning		[34]
Risk decision making		[34]
Attention	[31]
(e.g. Colour Trails Test, Trail making test)

In summary, findings in military populations are equivocal, with a relationship between AF and cognitive performance only demonstrated for some domains of cognition. Methodological differences (e.g. test populations, tests used) likely explain these conflicting results. Nevertheless, from the studies reviewed, there is no evidence to suggest that AF has any detrimental effect on cognitive performance. Therefore, improving AF should be promoted as a potential method for enhancing cognition. However, the use of generic AF test standards as an indicator of acceptable cognitive function is not justified, as there is no defined level of AF above which enhanced/optimised cognitive function occurs.

3.3 Presenteeism and productivity

With increasing rank and service, it is common for military personnel to become more desk-bound, with a greater emphasis on cognitive rather than physical tasks [36]. In such situations, presenteeism (i.e. being at work but not fully functioning due to illness, injury, or other conditions [37]) and productivity (i.e. the rate of output per unit of input [38]) become important factors influencing individual military performance. No studies have examined the impact of AF on these factors in military personnel. General population studies have reported associations between higher levels of AF and an increase in the quantity of office-type work performed [39], a reduction in the amount of extra effort required to do the job [39], and reduced absenteeism [40]. Therefore, AF should be encouraged as a potential method for improving presenteeism and productivity; however, given the lack of military studies, more research is needed.

3.4 Resilience

Resilience, defined by the Technical Corporation Program (TTCP) as “the capacity of the individual, team and organisation to recover quickly, resist, and possibly thrive in the face of direct/indirect stressors and adverse situations in garrison, training, and operational environments” is a complex construct, reflecting not only an individual’s physiology and psychology, but also the influence of factors such as sex, environment, and training [6]. Many of the physical and cognitive capabilities discussed previously have (or are) components of resilience, but several military studies have focussed specifically on the influence of AF on psychological resilience and the role of mental processes and behaviour in protecting an individual from the potential negative effect of stressors.

Luria at al. reported an association between higher baseline fitness scores (timed 2 km run, sit-ups and push-ups) and lower levels of perceived stress, before and during a physically and cognitively demanding two-day selection activity, in Israeli Defence Force recruits [41]. In this study, AF comprised 70% of the overall fitness score [41]. Similarly, passing of the Army Physical Fitness Test (APFT), involving a 2-mile run, push-ups and sit-ups, was associated with higher resilience and confidence in managing reactions to stress in US Army recruits during a 10-week basic combat training course [42]. Furthermore, a relationship between higher predicted absolute ${\dot{VO}}_{2 max}$ and lower levels of perceived stress has been demonstrated in male Swiss Army recruits during 11 weeks of basic military training [43].

In contrast, Beckner et al. [34] found no association between AF and resilience during a 5-day simulated military operational protocol. This conflicting finding may be explained by the soldier population and context; Active Duty, Reserve, National Guard and Reserve Officer Training Corps soldiers [34], compared to recruits in training [41 –43]. Generic AF on entry may predict subsequent resilience and stress management during selection courses and training, however, it remains unclear how altered AF levels subsequently influence long-term resilience of serving personnel.

3.5 Technical/tactical capabilities

For effective military performance, technical capabilities, e.g. how to use the weapon or drive the vehicle, must be complemented by tactical capabilities, e.g. when to use the weapon, where to drive the vehicle. Several of the physical and cognitive capabilities discussed previously contribute to technical/tactical performance, but marksmanship (i.e. the application of the fundamental skills of firing a weapon with precision and accuracy) is an essential technical/tactical capability that has received specific attention in the literature. Equivocal evidence has been presented on the relationship between AF and marksmanship in military and law enforcement personnel. A positive association [44] and no association [45] were found for static shooting accuracy, while AF was not associated with dynamic and positive identification shooting scenarios [44]. A limitation of these studies is that they examined a shooting task(s) in isolation [44, 45]; in real-world military operations, other activities and/or tasks are typically performed prior to shooting. Nevertheless, no association between AF and static shooting accuracy following exercise (upper body fatiguing exercise) has been demonstrated in US Army soldiers [46]. Therefore, the relationship between AF and marksmanship is currently unclear, with no defined AF level below or above which marksmanship will decline or be enhanced.

4 The relationship between generic aerobic fitness and other aspects of military readiness

Although the previous section has highlighted inconsistent relationships between AF and factors affecting individual military performance, generic AF has been shown to provide additional benefits that may in turn enhance military readiness. Individual military readiness is defined as “a soldier’s ability to deploy on a mission or perform their usual military duties” [47] and is influenced by mental and physical health, which in turn affect morbidity, mortality and attrition.

4.1 Mental health and wellbeing

Psychological stress and mental ill-health (e.g. depression and anxiety) are prevalent in military organisations and can negatively impact military readiness [48]. In US Army soldiers, higher overall APFT scores have been linked to stronger psychological skills (e.g. stress-reactions, self-confidence, commitment, fear control) [49]. While the relative contribution of AF cannot be determined from this study (APFT score reflects a combination of AF and strength) [49], a relationship between lower levels of AF and higher psychological stress and/or distress has been demonstrated in military personnel in Taiwan [50] and Finland [51]. Higher overall fitness and AF levels have also been associated with lower risk of depression [52] and mood disturbance [53] during basic military combat training. Furthermore, an inverse association between AF levels and trait anxiety has been reported in male US military personnel undergoing advanced instruction to become special warfare officers [54].

It should, however, be recognised that fitness testing in and of itself may negatively impact mental health and wellbeing [55]. In some military organisations, there are a range of negative consequences associated with fitness test failure, including remedial physical training, not attaining a promotion, inability to attend development programs, and ultimately the inability to remain on active duty [55]. As such, fitness testing may lead to mental distress [55] and should therefore only be included where justified. That aside, there is general consensus that higher levels of AF are beneficial for mental health and wellbeing, although the actual level of AF required to realise these benefits has not yet been determined.

4.2 Physical health

4.2.1 Acute illness and injury

Illness and injury are a major concern in the military, with both negatively impacting occupational readiness and task performance. A relationship between low AF and increased illness-related absenteeism has been demonstrated in male Finnish soldiers [56]. Upper respiratory tract symptom (URTS) (e.g. sore throat, runny nose, cough) episodes are a common form of acute illness experienced by military personnel, and during basic combat training, higher AF levels on entry have been linked to lower incidence [57] and shorter duration [58] of URTS episodes in male recruits.

Several studies have also examined the role of AF in exertional heat illness (EHI), which presents as a serious occupational hazard and common cause of hospitalisation in military personnel [59]. EHI comprises a broad spectrum of syndromes ranging from exertional heat cramps, heat exhaustion, and heat injury, to life-threatening heat stroke [60]. Lower levels of AF have been associated with increased risk for EHI in military personnel [60, 61].

The relationship between AF and injury, particularly musculoskeletal injury (MSKI), has received considerable attention in the military literature. MSKI causes more morbidity among soldiers than any other health condition [62]. An association between low levels of AF and increased risk for MSKI has been demonstrated in recruit [63 –65] and non-recruit [66, 67] military populations. However, conflicting findings have also been reported, for example, no relationship between AF and injury was reported in Swiss military personnel [68], whereas, a positive association between AF and injury was demonstrated in US military personnel [69]. Physical fitness testing itself has also been shown to cause injury, with an injury rate of 7.6% reported in 1,532 US soldiers; however, the majority of injuries were attributed to the sit-up event (51% ) compared with the run (32% ) and press-up (11% ) events [70], with a low proportion resulting in time-loss [70].

Despite conflicting findings, high levels of AF generally appear to protect against injury; indeed, a meta-analysis reported a relative risk of 2.34 (95% CI 2.02–2.70) for run times, whereby slower run times significantly increased risk for injury [71]. Furthermore, a recent systematic review concluded that strong scientific evidence exists for low physical fitness as a modifiable risk factor for MSKI [62].

In summary, higher levels of generic AF generally enhance physical health, although the specific level of AF needed to reduce illness and injury risk is yet to be determined, if indeed such a level exists. It is important to note that illness and injury risk are influenced by numerous factors, including lifestyle, genetics, medical status, physiology, biomechanics, and training load [62], therefore, reliance on a specific, generic AF test score to indicate adequate physical health for successful job performance is likely not appropriate.

4.2.2 Chronic disease and mortality

Chronic, non-communicable diseases represent a significant challenge in the military. Crump et al. [72 –74] conducted several large longitudinal cohort studies to assess the relationship between AF and non-communicable disease risk in Swedish male military conscripts. Over the 15–43 year study periods, low relative ${\dot{VO}}_{2 max}$ at age 18 was associated with increased risk for type 2 diabetes [72], hypertension [73] and ischemic heart disease [74]. High levels of AF have also been associated with a reduction in cardiovascular risk factors (e.g. cholesterol, blood pressure, smoking and obesity) in male and female US basic law enforcement trainees [75] and in male but not female Chinese Army personnel [76].

Similar findings have been reported in the general population, with high levels of AF being associated with lower risk of CVD, morbidity and mortality. Importantly, in the general population, researchers have sought to determine the level of AF needed to protect against CVD, for example, Mandsager et al. [77] stratified subjects by AF into five performance groups and observed an inverse association between AF and long-term mortality (Fig. 2). Furthermore, there was no upper limit of benefit of increased AF on mortality risk [77]. Although specific AF levels associated with disease and mortality have not yet been determined in military populations, general population datasets provide useful AF benchmarks.

Fig. 2

Risk (hazard ratio [HR]) of all-cause mortality for AF performance groups [77].

5 From standards to benchmarking: Shifting the aerobic fitness testing paradigm

5.1 Generic aerobic fitness standards as a military requirement

Despite some studies demonstrating an association between higher levels of AF and factors influencing military performance, this is not a consistent finding. An absolute value, above which benefits are conferred or acceptable performance is achieved, has not been clearly determined. Indeed, different levels of AF might be required for different aspects of performance, and/or for different individuals. Moreover, the military operating environment is undergoing transformation, marked by the integration of new technologies such as artificial intelligence and robotics. This shift is accompanied by a change in emphasis within human capabilities, transitioning from traditional attributes, like load carriage and physical strength, to prioritising decision dominance and mental endurance [78]. The findings of this review would suggest that there is unlikely to be one level of generic AF that is associated with ‘acceptable’ performance in all aspects of all military roles. Neither is there likely to be one level of AF that is associated with ‘optimal’ individual military performance. Such fitness levels are difficult to determine when ‘acceptable’ and ‘optimal’ job performance are difficult to quantify, and when previous studies have used a variety of methods to measure and express AF.

Unjustified emphasis on any aspect of fitness has the potential to lose (or miss the opportunity to recruit) highly effective personnel and encourages serving personnel to spend a disproportionate amount of time and effort on an aspect of their readiness that may not be the most beneficial. As described by Reilly et al. [79], in nations where PES and physical fitness tests must be linked to a bona fide job requirement, a clear link between the task and the required level of AF must be demonstrated to avoid legal challenge and unnecessary barriers to retention [79]. Where organisations also adopt the Soldier First principle, or Universality of Service, requirements on personnel at all levels of the organisation should reflect actual and reasonable expectations, not nice to haves.

5.2 Generic aerobic fitness tests and benchmarks as a military enabler

Although this review has highlighted that the evidence for in-service, generic AF standards per se is equivocal, the review has also demonstrated the actual and potential benefits of increased AF for the performance and health of military personnel, and this should be innovatively and proactively encouraged. In-service, generic AF testing could be used as one component of a health-related fitness assessment, separate to but alongside a role-related PES, to provide a more complete fitness profile of military personnel [3]. However, rather than using pass/fail standards, evidence-based age and sex benchmarking of generic AF test scores could be used. Adjusted benchmarks based on age and sex are justified in such a context, whereas standards for occupational/job-related fitness assessments (i.e. PES) should be the same for everyone, as job-related requirements remain the same. This approach would enable military personnel to obtain a better understanding of their own fitness level, provide motivation for continuous improvement (or maintenance), allow progress to be tracked, and provide meaningful metrics to the chain of command to guide and prioritise program delivery [3]. Other aspects of the health-related fitness evaluation might include body composition, muscular strength, muscular endurance, and flexibility, all of which are known to have a relationship with good health [80]. The goal of such an evaluation would be to help promote continuous improvement, rather than be pass or fail, as per the Canadian Armed Forces FORCE Fitness Profile. In addition to fitness, it should be recognised that other important factors also influence individual military performance and readiness, including physical activity levels, diet, smoking, alcohol consumption, stress and anxiety, personality traits, and sleep. Therefore, to optimise military performance and readiness, emphasis should not be placed on fitness alone; rather, a holistic approach should be adopted, incorporating nutrition, lifestyle, and behavioural strategies.

6 Conclusion

This review explored the relationship between generic AF and various aspects of military performance and readiness. While some associations were identified, there is no specific threshold of generic AF that confers improved military performance or readiness. Until further research can establish how much fitness is enough, the enforcement of standalone, in-service, pass/fail, generic AF test standards in military organisations lacks justification. That said, this review has also demonstrated that increased AF benefits key outcomes influencing individual military performance and readiness, and thus, task-specific, in-service assessments of AF (in the form of evidence-based PES) may be warranted for specific sub-populations, and the monitoring of health-related fitness (via generic AF testing) would be beneficial for all, with benchmarks rather than pass-fail standards for the latter.

While research suggests no upper limit to the benefits of increased AF, it is vital to recognise that AF is just one element affecting performance, with lifestyle and behavioural factors also playing significant roles. Emphasis needs to shift from “how do we test” to “how do we best support military, law enforcement and emergency services personnel to enhance and maintain their physical fitness throughout their careers?” Organisations should promote a culture that encourages positive lifestyle behaviours, alongside assessing and supporting fitness, through consultation and guidance rather than strict testing and remedial actions. The literature suggests that enforcement of AF test standards, without evidence to support a bona-fide job requirement, opens military organisations up to legal challenge and has the potential to limit the recruitment and retention of effective personnel. As highlighted in the final report of NATO HFM RTG 269, military members are incentivised to achieve fitness and activity goals when they understand the importance of being fit and healthy, when they know that PES and physical fitness tests are fair and justified, and when a balance of carrot and stick (bonus and malus) approaches is used. To continually advance military performance and readiness, careful consideration of the use and application of generic, in-service AF tests and standards, across the range of military contexts, is required.

Disclaimer

The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or reflecting the views of the New Zealand Defence Force.

Ethical approval

Not applicable.

Informed consent

Not applicable.

Conflict of interest

Not applicable.

Footnotes

Acknowledgments

Not applicable.

Funding

Not applicable.

References

Friedl

Knapik

Häkkinen

Baumgartner

Groeller

Taylor

NAS

, et al. Perspectives on aerobic and strength influences on military physical readiness: Report of an international military physiology roundtable. J Strength Cond Res. 2015;29(Suppl 11):S10–23.

Karl

Hatch

Arcidiacono

Pearce

Pantoja-Feliciano

Doherty

, et al. Effects of psychological, environmental and physical stressors on the gut microbiota. Front Microbiol. 2018;9:2013.

Reilly

Drain

Blacker

Sharp

Hauret

. HFM: Combat integration: Implications for physical employment standards. NATO STO-TR-HFM-269; 2019.

Hauschild

DeGroot

Hall

Grier

Deaver

Hauret

, et al. Fitness tests and occupational tasks of military interest: A systematic review of correlations. Occup Environ Med. 2017;74(2):144–53.

Cooper

Baker

Tong

Roberts

Hanford

. The repeatability and criterion related validity of the 20m multistage fitness test as a predictor of maximal oxygen uptake in active young men. Br J Sports Med. 2005;39(4):e19.

Nindl

Billing

Drain

Beckner

Greeves

Groeller

, et al. Perspectives on resilience for military readiness and preparedness: Report of an international military physiology roundtable. J Sci Med Sport. 2018;21(11):1116–24.

Rayson

Holliman

Belyavin

. Development of physical selection procedures for the British Army. Phase 2: Relationship between physical performance tests and criterion tasks. Ergonomics. 2000;43(1):73–105.

Fallowfield

Blacker

Willems

MET

Davey

Layden

. Neuromuscular and cardiovascular responses of Royal Marine recruits to load carriage in the field. Appl Ergon. 2012;43(6):1131–7.

Lyons

Allsopp

Bilzon

. Influences of body composition upon the relative metabolic and cardiovascular demands of load-carriage. Occup Med (Chic Ill). 2005;55(5):380–4.

10.

Williams

Rayson

. Can simple anthropometric and physical performance tests track training-induced changes in load-carriage ability?. Mil Med. 2006;171(8):742–8.

11.

Maciejczyk

Więcek

Szymura

Szyguła

Wiecha

Cempla

. The influence of increased body fat or lean body mass on aerobic performance. PLoS One. 2014;9(4):e95797.

12.

Bilzon

JLJ

Allsopp

Tipton

. Assessment of physical fitness for occupations encompassing load-carriage tasks. Occup Med (Chic Ill). 2001;51(5):357–61.

13.

Oeschger

Roos

Wyss

Buller

Veenstra

Gilgen-Ammann

. Influence of Soldiers’ Cardiorespiratory Fitness on Physiological Responses and Dropouts During a Loaded Long-distance March. Mil Med. 2023;188(7-8):e1903–9.

14.

Taylor

Peoples

Petersen

. Load carriage, human performance, and employment standards. Appl Physiol Nutr Metab. 2016;41(6):S131–47.

15.

Huang

H-C

Nagai

Lovalekar

Connaboy

Nindl

. Physical fitness predictors of a warrior task simulation test. J Strength Cond Res. 2018;32(9):2562–8.

16.

Harman

Gutekunst

Frykman

Sharp

Nindl

Alemany

, et al. Prediction of simulated battlefield physical performance from field-expedient tests. Mil Med. 2008;173(1):36–41.

17.

Orantes-Gonzalez

Heredia-Jimenez

Escabias

. Body mass index and aerobic capacity: The key variables for good performance in soldiers. Eur J Sport Sci. 2022;22:(1);1467–74.

18.

Pihlainen

Santtila

Häkkinen

Kyröläinen

. Associations of physical fitness and body composition characteristics with simulated military task performance. J Strength Cond Res. 2018;32(4):1089–98.

19.

Stocker

Leo

. Predicting military specific performance from common fitness tests. J Phys Educ Sport. 2020;20(5):2454–9.

20.

Pihlainen

Santtila

Häkkinen

Lindholm

Kyröläinen

. Cardiorespiratory responses induced by various military field tasks. Mil Med. 2014;179(2):218–24.

21.

Foulis

Canino

Cohen

Gebhardt

Redmond

Sharp

. US Army physical demands study: Accuracy of occupational physical assessment test classifications for combat arms soldiers. Work. 2019;63(4):571–9.

22.

Coote

. Recovery of heart rate following intense dynamic exercise. Exp Physiol. 2010;95(3):431–40.

23.

Hoffman

. The relationship between aerobic fitness and recovery from high-intensity exercise in infantry soldiers. Mil Med. 1997;162(7):484–8.

24.

Szivak

Lee

Saenz

Flanagan

Focht

Volek

, et al. Adrenal stress and physical performance during military survival training. Aerosp Med Hum Perform. 2018;89(2):99–107.

25.

Ashworth

Cotter

Kilding

. Methods for improving thermal tolerance in military personnel prior to deployment. Mil Med Res. 2020;7(1):1–17.

26.

Lisman

Kazman

O’Connor

Heled

Deuster

. Heat tolerance testing: Association between heat intolerance and anthropometric and fitness measurements. Mil Med. 2014;179(11):1339–46.

27.

Alele

Malau-Aduli

AEO

Crowe

. Individual anthropometric, aerobic capacity and demographic characteristics as predictors of heat intolerance in military populations. Medicina (B Aires). 2021;57(2):173.

28.

Selkirk

McLellan

. Influence of aerobic fitness and body fatness on tolerance to uncompensable heat stress. J Appl Physiol. 2001;91(5):2055–63.

29.

Ogden

Fallowfield

Child

Davison

Fleming

Delves

, et al. Influence of aerobic fitness on gastrointestinal barrier integrity and microbial translocation following a fixed-intensity military exertional heat stress test. Eur J Appl Physiol. 2020;120(10):2325–37.

30.

Hansen

Johnsen

Sollers

Stenvik

Thayer

. Heart rate variability and its relation to prefrontal cognitive function: The effects of training and detraining. Eur J Appl Physiol. 2004;93(3):263–72.

31.

Hickey

Donne

O’Brien

. Effects of an eight week military training program on aerobic indices and psychomotor function. BMJ Mil Heal. 2012;158(1):41–6.

32.

Luque-Casado

Perakakis

Hillman

Kao

S-C

Llorens

Guerra

, et al. Differences in sustained attention capacity as a function of aerobic fitness. Med Sci Sport Exerc. 2016;48(5):887–95.

33.

Zwilling

Strang

Anderson

Jurcsisn

Johnson

Das

, et al. Enhanced physical and cognitive performance in active duty Airmen: Evidence from a randomized multimodal physical fitness and nutritional intervention. Sci Rep. 2020;10(1):1–13.

34.

Beckner

Conkright

Eagle

Martin

Sinnott

LaGoy

, et al. Impact of simulated military operational stress on executive function relative to trait resilience, aerobic fitness, and neuroendocrine biomarkers. Physiol Behav. 2021;236:113413.

35.

Janicijevic

Miras-Moreno

Castilla

Vera

Redondo

Jiménez

, et al. Association of military-specific reaction time performance with physical fitness and visual skills. PeerJ. 2022;10:e14007.

36.

Schulze

Lindner

Goethel

Müller

Kundt

Stoll

, et al. Evaluation of the physical activity of German soldiers depending on rank, term of enlistment, and task area. Mil Med. 2015;180(5):518–23.

37.

Lohaus

Habermann

. Presenteeism: A review and research directions. Hum Resour Manag Rev. 2019;29(1):43–58.

38.

Low

Gramlich

Engram

. Self-paced exercise program for office workers: Impact on productivity and health outcomes. Aaohn J. 2007;55(3):99–105.

39.

Pronk

Martinson

Kessler

Beck

Simon

Wang

. The association between work performance and physical activity, cardiorespiratory fitness, and obesity. J Occup Environ Med. 2004;46(1):19–25.

40.

Tucker

Aldana

Friedman

. Cardiovascular fitness and absenteeism in 8,301 employed adults. Am J Heal Promot. 1990;5(2):140–5.

41.

Luria

Torjman

. Resources and coping with stressful events. J Organ Behav Int J Ind Occup Organ Psychol Behav. 2009;30(6):685–707.

42.

Williams

Brown

Bray

Anderson Goodell

Rae Olmsted

Adler

. Unit cohesion, resilience, and mental health of soldiers in basic combat training. Mil Psychol. 2016;28(4):241–50.

43.

Tuch

Teubel

La Marca

Roos

Annen

Wyss

. Physical fitness level affects perception of chronic stress in military trainees. Stress Heal. 2017;33(5):490–7.

44.

Muirhead

Orr

Schram

Kornhauser

Holmes

Dawes

. The relationship between fitness and marksmanship in Police Officers. Safety. 2019;5(3):54.

45.

Kayihan

Ersöz

Özkan

Koz

. Relationship between efficiency of pistol shooting and selected physical-physiological parameters of police. Polic an Int J Police Strateg Manag. 2013;36(4):819–32.

46.

Evans

Scoville

Ito

Mello

. Upper body fatiguing exercise and shooting performance. Mil Med. 2003;168(6):451–6.

47.

Larson

Adams

Ritter

Linton

Williams

Saadoun

, et al. Associations of early treatments for low-back pain with military readiness outcomes. J Altern Complement Med. 2018;24(7):666–76.

48.

Smith

Zamorski

Smith

Riddle

LeardMann

Wells

, et al. The physical and mental health of a large military cohort: Baseline functional health status of the Millennium Cohort. BMC Public Health. 2007;7(1):1–13.

49.

Hammermeister

Pickering

McGraw

Ohlson

. Relationship between psychological skill profiles and soldier physical fitness performance. Mil Psychol. 2010;22(4):399–411.

50.

Lin

K-H

Chen

Y-J

Yang

S-N

Liu

M-W

Kao

C-C

Nagamine

, et al. Association of psychological stress with physical fitness in a military cohort: The CHIEF study. Mil Med. 2020;185(7-8):e1240–6.

51.

Appelqvist-Schmidlechner

Vaara

Mäkinen

Vasankari

Kyröläinen

. Relationships between leisure time physical activity, physical fitness and mental health among young adult males. Eur Psychiatry. 2017;41(S1):S179–S179.

52.

Crowley

Wilkinson

Wigfall

Reynolds

Muraca

Glover

, et al. Physical fitness and depressive symptoms during army basic combat training. Med Sci Sports Exerc. 2015;47(1):151.

53.

Ahn

Kim

Jeong

. Physical fitness level and mood state changes in basic military training. Int J Environ Res Public Health. 2020;17(23):9115.

54.

Taylor

Markham

Reis

Padilla

Potterat

Drummond

SPA

, et al. Physical fitness influences stress reactions to extreme military training. Mil Med. 2008;173(8):738–42.

55.

Boykin

Rice

. Physical readiness is more than physical fitness: Relationships between army physical fitness test scores and self-reports of physical and psychological fitness. Int Conf Appl Hum Factors Ergon, Springer; 2019, pp. 176–83.

56.

Kyröläinen

Häkkinen

Kautiainen

Santtila

Pihlainen

Häkkinen

. Physical fitness, BMI and sickness absence in male military personnel. Occup Med (Chic Ill). 2008;58(4):251–6.

57.

Dimitriou

Lockey

Castell

. Is baseline aerobic fitness associated with illness and attrition rate in military training?. BMJ Mil Heal. 2017;163(1):39–47.

58.

Mazidi

Katsiki

Mikhailidis

Sattar

Banach

. Lower carbohydrate diets and all-cause and cause-specific mortality: A population-based cohort study and pooling of prospective studies. Eur Heart J. 2019;40(34):2870–9.

59.

Hakre

Gardner

Kark

Wenger

. Predictors of hospitalization in male Marine Corps recruits with exertional heat illness. Mil Med. 2004;169(3):169–75.

60.

Bedno

Urban

Boivin

Cowan

. Fitness, obesity and risk of heat illness among army trainees. Occup Med (Chic Ill). 2014;64(6):461–7.

61.

Gardner

Kark

Karnei

Sanborn

Gastaldo

Burr

, et al. Risk factors predicting exertional heat illness in male Marine Corps recruits. Med Sci Sports Exerc. 1996;28(8):939–44.

62.

Sammito

Hadzic

Karakolis

Kelly

Proctor

Stepens

, et al. Risk factors for musculoskeletal injuries in the military: A qualitative systematic review of the literature from the past two decades and a new prioritizing injury model. Mil Med Res. 2021;8(1):1–40.

63.

Robinson

Siddall

Bilzon

Thompson

Greeves

Izard

, et al. Low fitness, low body mass and prior injury predict injury risk during military recruit training: A prospective cohort study in the British Army. BMJ Open Sport Exerc Med. 2016;2(1):e000100.

64.

Knapik

Darakjy

Hauret

Canada

Scott

Rieger

, et al. Increasing the physical fitness of low-fit recruits before basic combat training: An evaluation of fitness, injuries, and training outcomes. Mil Med. 2006;171(1):45–54.

65.

Müller-Schilling

Gundlach

Böckelmann

Sammito

. Physical fitness as a risk factor for injuries and excessive stress symptoms during basic military training. Int Arch Occup Environ Health. 2019;92(6):837–41.

66.

Rappole

Grier

Anderson

Hauschild

Jones

. Associations of age, aerobic fitness, and body mass index with injury in an operational Army brigade. J Sci Med Sport. 2017;20(Suppl 4):S45–50.

67.

Teyhen

Shaffer

Butler

Goffar

Kiesel

Rhon

, et al. What risk factors are associated with musculoskeletal injury in US Army Rangers? A prospective prognostic study. Clin Orthop Relat Res. 2015;473(9):2948–58.

68.

Wyss

Von Vigier

Frey

Mäder

. The Swiss Army physical fitness test battery predicts risk of overuse injuries among recruits. J Sports Med Phys Fitness. 2012;52(5):513–21.

69.

Giovannetti

Bemben

Cramer

. Relationship between estimated aerobic fitness and injury rates among active duty at an Air Force base based upon two separate measures of estimated cardiovascular fitness. Mil Med. 2012;177(1):36–40.

70.

Evans

Reynolds

Creedon

Murphy

. Incidence of acute injury related to fitness testing of US Army personnel. Mil Med. 2005;170(12):1005–11.

71.

Tomes

Sawyer

Orr

Schram

. Ability of fitness testing to predict injury risk during initial tactical training: A systematic review and meta-analysis. Inj Prev. 2020;26(1):67–81.

72.

Crump

Sundquist

Winkleby

Sieh

Sundquist

. Physical fitness among Swedish military conscripts and long-term risk for type 2 diabetes mellitus: A cohort study. Ann Intern Med. 2016;164(9):577–84.

73.

Crump

Sundquist

Winkleby

Sundquist

. Interactive effects of physical fitness and body mass index on the risk of hypertension. JAMA Intern Med. 2016;176(2):210–6.

74.

Crump

Sundquist

Winkleby

Sundquist

. Interactive effects of obesity and physical fitness on risk of ischemic heart disease. Int J Obes. 2017;41(2):255–61.

75.

McMurray

Ainsworth

Harrell

Griggs

Williams

. Is physical activity or aerobic power more influential on reducing cardiovascular disease risk factors?. Med Sci Sports Exerc. 1998;30(10):1521–9.

76.

Huang

C-P

Chen

W-L

. Relevance of physical fitness and cardiovascular disease risk. Circ J. 2021;85(5):623–30.

77.

Mandsager

Harb

Cremer

Phelan

Nissen

Jaber

. Association of cardiorespiratory fitness with long-term mortality among adults undergoing exercise treadmill testing. JAMA Netw Open. 2018;1(6):e183605–e183605.

78.

Dyches

Friedl

Greeves

Keller

McClung

McGurk

, et al. Physiology of health and performance: Enabling success of women in combat arms roles. Mil Med. 2023;188(Suppl 4):19–31.

79.

Reilly

Gebhardt

Billing

Greeves

Sharp

. Development and implementation of evidence-based physical employment standards: Key challenges in the military context. J Strength Cond Res. 2015;29:S28–33.

80.

Gagnon

Spivock

Reilly

Mattie

Stockbrugger

. The FORCE fitness profile - adding a measure of health-related fitness to the Canadian Armed Forces operational fitness evaluation. J Strength Cond Res. 2015;29:S192–8.

Cognitive measures	Association with Aerobic Fitness
	Positive	Negative	Null
Reaction time	[32, 33]		[30, 35]
Executive functioning	[31]		[31, 33]
(e.g. Stroop test, Grooved pegboard test)
Processing efficiency	[32, 33]		[34]
Episodic memory	[33]
Short-term memory			[31, 33]
Working memory	[30, 31]		[33, 34]
Fluid intelligence	[33]		[33]
Scanning, and visual tracking	[31]
Vigilant attention	[32, 34]
(e.g. Psychomotor Vigilance Task)
Sensorimotor speed			[34]
Spatial orientation			[34]
Spatial learning			[34]
Abstraction and concept formation			[34]
Complex scanning, memory, and visual tracking			[34]
Emotional identification through facial expressions			[34]
Abstract reasoning			[34]
Risk decision making			[34]
Attention	[31]
(e.g. Colour Trails Test, Trail making test)