Effect of Exercise Training Programs on Physical Fitness Domains in Military Personnel: A Systematic Review and Meta-Analysis

ABSTRACT

Introduction

Physical training is important to prepare soldiers for the intense occupational demands in the military. However, current physical training may not address all fitness domains crucial for optimizing physical readiness and reducing musculoskeletal injury. The effects of nontraditional military physical training on fitness domains have been inconsistently reported, which limits the design of the ideal training program for performance optimization and injury prevention in the military. The aim of this systematic review was to identify the effects of exercise training on various fitness domains (i.e., aerobic fitness, flexibility, muscular endurance, muscular power, muscular strength, and occupationally specific physical performance) that contribute to occupational performance and musculoskeletal injury risk in military personnel.

Methods

An extensive literature search was conducted in January 2021 and was subsequently updated in July 2021 and December 2021. Included studies consisted of comparative groups of healthy military personnel performing traditional and nontraditional military physical training with at least one assessment representative of a fitness domain. Study appraisal was conducted using the PEDro scale. Meta-analysis was conducted via forest plots, standard mean difference (SMD, effect size), and intertrial heterogeneity (I²).

Results

From a total of 7,350 records, 15 studies were identified as eligible for inclusion in this review, with a total of 1,613 participants. The average study quality via the PEDro score was good (5.3/10; range 4/10 to 6/10). Nontraditional military physical training resulted in greater posttraining values for muscular endurance (SMD = 0.46; P = .004; I² = 68%), power (SMD = 1.57; P < .0001; I² = 90%), strength via repetition maximum testing (SMD = 1.95; P < .00001; I² = 91%), and occupationally specific physical performance (SMD = 0.54; P = .007; I² = 66%) compared to the traditional group. There was no significant difference for aerobic fitness (SMD = −0.31; P = .23; I² = 86%), flexibility (SMD = 0.58; P = .16; I² = 76%), and muscular strength via maximal voluntary contraction (SMD = 0.18; P = .28; I² = 66%) between training groups.

Conclusions

The current systematic review identified that nontraditional military physical training had a greater posttraining effect on muscular endurance, power, strength measured via repetition maximum, and occupationally specific physical performance compared to traditional military physical training. Overall, these findings suggest that nontraditional military physical training may be beneficial in optimizing occupational performance while potentially reducing musculoskeletal injury risk.

INTRODUCTION

The arduous nature and intense occupational demands of the military are well known.^1,2 Soldiers must undertake many physically demanding tasks, from carrying heavy loads over long distances and uneven terrain to sprinting across the battlefield to seek cover and negotiate obstacles.^1,2 During deployment, these physical activities can be conducted over many hours or days, placing great physical strain on soldiers.³ Soldiers may also face additional stressors, including sleep deprivation, calorie restriction, and environmental extremes during training and on deployment.¹ These occupational demands therefore require unique training with high levels of strength, endurance, speed, power, and agility crucial for soldiers to be physically and mentally prepared to effectively operate.^2,3

Currently, the primary goal of military physical training is to improve or maintain the physical performance of soldiers in preparation for the battlefield.^2,4,5 Historically, this focus has led to a traditional physical training program heavily geared toward the development of aerobic fitness and muscular endurance.^1–4,6 While little evidence exists on the benefits of traditional training on combat readiness,⁷ improved performance during annual physical fitness tests, which may or may not reflect real-world mission requirements, results in the continuation of this training regimen.^2,4 However, traditional military physical training may not address other fitness domains, such as flexibility, muscular power, and muscular strength, aspects crucial for optimizing physical readiness and reducing musculoskeletal injury (MSKI) risk.^2,3,8 Preventable MSKI accounts for nearly 60% of soldiers’ “limited duty days in the U.S.” Army⁹ and 40% of clinical presentations in the Australian Defence Force,¹⁰ costing billions of dollars and compromising the readiness and occupational performance of military personnel.¹¹ Therefore, military physical training that targets a range of fitness domains may be beneficial for both optimal performance and reduced MSKI risk.^1,12–14

As indicated above, traditional military physical training typically consists of exercises to improve aerobic fitness and muscular endurance that are adapted to a specific military group and have been used for a substantial amount of time over the last 20-30 years.^2,4 In contrast, nontraditional military physical training involves nontraditional activities or practices (e.g., advanced or structured resistance training programs) that some military groups have chosen to adopt in recent years.^1,6,15 Previous studies have examined the effects of traditional and nontraditional military physical training on various fitness domains in an effort to determine the optimal training regime for the military.^1,12,13,16 However, the effects of these training programs on fitness domains in military personnel have been inconsistent. For example, some studies have reported greater improvements in muscular strength with resistance training compared to traditional military physical training,^17–19 while other studies have found poorer muscular strength.²⁰ Similar inconsistencies have also been observed with the addition of power exercises to military physical training with greater^12,13 and poorer³ muscular power reported following this training regime compared to traditional physical training.

In an attempt to synthesize the research, several narrative reviews have examined the physical performance implications of specific, nontraditional military physical training (e.g., combined high-intensity muscular strength and aerobic endurance training or high-intensity fitness training).^5,21 These reviews concluded that combining high-intensity strength and endurance training,⁵ as well as high-intensity fitness training,²¹ induced superior improvements in muscular strength and aerobic fitness than traditional training. However, these studies failed to review the effects of these training programs on other fitness domains, such as flexibility and muscular power.^5,21

Based on the inconsistencies in the research to date and limited systematic reviews on the effects of nontraditional military physical training on fitness domains, it is extremely difficult for physical training instructors to design the ideal training program for performance optimization and injury prevention in the military. A greater understanding of the impact of training would assist the military with the appropriate allocation of time, resources, and expertise to enhance soldier performance.¹¹ The aim of this systematic review was to identify the effects of physical training on various fitness domains (i.e., aerobic fitness, flexibility, muscular endurance, muscular power, muscular strength, and occupationally specific physical performance) that contribute to occupational performance and MSKI risk in military personnel. We hypothesized that nontraditional military physical training would result in greater increases in fitness domains compared to traditional military physical training.

METHODS

This systematic review and meta-analysis was conducted in accordance with the guidelines provided by the Preferred Reporting Items for Systematic Review and Meta-Analyses statement²² and followed the population, intervention/exposure, comparison, and outcome (PICO) approach. The review protocol was registered in PROSPERO (CRD42021234402).

Studies that met the following PICO criteria were considered eligible and were included in this review:

1. Population: Healthy military personnel in the army, navy, air force, or marines.

2. Intervention or exposure: Nontraditional military physical training program of at least 4 weeks’ duration.

3. Comparison: Traditional military physical training program of at least 4 weeks’ duration.

4. Outcome: The effects of the training program on fitness domains (i.e., aerobic fitness, flexibility, muscular endurance, muscular power, muscular strength, and occupationally specific physical performance).

Studies were excluded if: (1) the participants were injured before the commencement of the study; (2) the results of physical fitness assessments were not reported before and following the intervention; (3) the training for the traditional training or control (CON) group was unspecified; (4) the training for the nontraditional training or experimental (EXP) group was similar to that of traditional military physical training; (5) they were published in a language other than English; and (6) they were reported as abstracts, reviews or case reports.

For the purpose of this review, traditional military physical training (CON) was defined as an exercise training program consisting of aerobic running/walking exercises, calisthenics, and/or muscular endurance exercises (i.e., involving resistance training when intensity <70% 1 repetition maximum (RM) or with a load that permitted >12 repetitions). In contrast, nontraditional military physical training (EXP) was defined as an exercise training program that consisted of training with mode altered (e.g., adding resistance or power-oriented exercises in conjunction with traditional aerobic and muscular endurance exercises) or resistance training with an intensity ≥70% 1 RM or with a load that permitted ≤12 repetitions.

The outcome measures for this systematic review were reported for each of the fitness domains, including: (1) aerobic capacity (i.e., multistage fitness test, time trial, treadmill maximal oxygen consumption (VO_2max); (2) flexibility (i.e., joint range-of-motion, sit and reach); (3) muscular endurance (i.e., push-ups); (4) muscular power (i.e., countermovement jump); (5) muscular strength (i.e., maximum voluntary contraction [MVC], RM); and (6) occupationally specific physical performance (i.e., load carriage, 30-m run with combat gear and dummy rifle).

Given that time trial results are interpreted in the opposite direction to other valid measures of aerobic fitness (i.e., a smaller time trial value indicated better performance), time trial results were converted into the completed time trial speed (ms⁻¹) to enable comparison with other relevant and positive measures of aerobic fitness (i.e., VO_2max). Additionally, the relationship between mean and standard deviation for time trial results was maintained for the converted speed results (e.g., a time trial mean of 804s ± 114 s was converted to speed as follows: 3,218 m/804 s  = 4.00 ms⁻¹, while the standard deviation was 114 s/804 s × 4.00 ms⁻¹ = 0.57 s).

A literature search was performed on January 25, 2021, and subsequently updated on July 6, 2021, and December 6, 2021, across five major electronic databases (CINAHL, Medline, Scopus, SportDiscus, and Web of Science). For the Medline search, four groupings of MeSH terms were utilized in combination. A free text search was also conducted in Medline (January 2019 to current) for studies that were “in-press” or did not have assigned MeSH terms. The MeSH and free text search terms can be found in Supplementary Table S1. Equivalent free text searches, without a time limit, were conducted in CINAHL, Scopus, SportDiscus, and Web of Science. The reference lists of all included studies were also screened for any studies that could be considered for possible inclusion in the review. An independent reviewer completed the computed literature search with a random sample of 40% of the titles/abstracts screened by a further two independent reviewers to assess interrater reliability. A weighted Kappa statistic value of 0.79 (95% confidence interval: 0.70-0.88) was obtained, which was considered acceptable for interrater reliability.²³

The methodological quality of each included study was screened using the PEDro scale with the overall score per study reported as a sum.²⁴ The PEDro criterion, “There was blinding of all subjects,” was removed from methodological evaluation as a participant could not be blinded when assigned an exercise training intervention. Therefore, methodological quality was scored out of a maximum of 10: excellent (8-10); good (5-7); fair (3-4); and poor (<3).²⁴

A meta-analysis was conducted using Reviewer Manager Software 5. All data from included studies were reported as mean ± standard deviation with the level of statistical significance set at <0.05.²⁵ The heterogeneity of studies was assessed via I² statistic tests with I² values of 25%, 50%, and 75% classified as low, moderate, and high heterogeneity, respectively.²⁶ To evaluate the effectiveness of the training programs, postintervention data were compared between EXP and CON groups and reported via forest plots using a random-effects model.²⁷ The standardized mean difference (SMD) was calculated to determine the magnitude of between-group differences (i.e., EXP vs. CON) with values of <0.2, 0.2, 0.5, and 0.8 classified as trivial, small, medium, and large, respectively.²⁸

RESULTS

The search identified 7,350 records with the removal of duplicates, screening of title and abstract and full-text screening resulting in 15 original articles included for evaluation (Supplementary Table S2).

A total of 1,613 participants were included in the analysis, with 1,037 and 576 participants comprising the EXP and CON groups, respectively (Supplementary Table S3). The mean age of participants ranged from 18 to 37 years in each of the EXP and CON groups. The mean body mass index ranges were 21.3-26.9 kg.m⁻² (EXP) and 22.0-28.1 kg.m⁻² (CON), while the mean height ranges were 1.71-1.84 m (EXP) and 1.73-1.86 m (CON). Overall, the physical characteristics of each group were similar. Included participants were employed from all three forces (i.e., army,^{1,3,6,12,13,16,29} air-force,^17,30 and navy³¹) across a range of expertise levels (i.e., recruits to experienced) and from a number of different countries (i.e., Australia,^1,6 Brazil,^12,13,16 Canada,³⁰ Denmark,^20,32 Finland,^18,19,33 Thailand,¹⁷ and the United States^3,29,31) (Supplementary Table S3). Of the 11 studies that included details of sex,^{1,6,16–20,29,31–33} 1,286 participants were male and 60 participants were female. Eight of these studies included male participants exclusively.^{1,16–19,29,31,33}

A variety of nontraditional military physical training programs were evaluated in the included studies and involved increasing resistance training intensity (n = 5),^{6,12,13,18,19} parodied resistance training (n = 2),^1,29 the addition of resistance training to traditional aerobic and muscular endurance exercises (n = 5),^{3,17,20,32,33} targeted cervical resistance training (n = 2)^30,31 and passive flexibility training (n = 1)¹⁶ (Supplementary Table S4).

The PEDro scores for included studies ranges from 4 (fair) to 6 (good). with the average score being 5.3, indicating good quality (Supplementary Table S4). The average quality of the studies included for each fitness domain was also rated as “good” as follows: aerobic fitness (5.7); flexibility (5.5); muscular endurance (5.3); muscular power (5.3); muscular strength via MVC (5.3); muscular strength via RM (5.4); occupationally specific physical performance (5.6). The following quality criteria were included in all studies: (1) eligibility criteria were specified; (8) all subjects for whom outcome measures were available received the intervention or control condition as allocated; and (10) the study provided both point measures and measures of variability for at least one key outcome. None of the following criteria were identified in the included studies: (3) allocation was concealed; (6) blinding of all therapists; and (7) blinding of all assessors who measured at least one key outcome (Supplementary Table S5).

Measures of aerobic fitness, flexibility, muscular endurance, muscular power, muscular strength, and occupationally specific physical performance were included in the meta-analyses.

Nontraditional military physical training resulted in greater posttraining values for muscular endurance (small SMD; P = .004; Fig. 1A), power (large SMD; P < .0001, Fig. 1B), strength via RM testing (large SMD; P < .00001; Fig. 2A) and occupationally specific physical performance (medium SMD; P = .007; Fig. 2B) with moderate-high heterogeneity noted (66%-91%). In contrast, there was no significant difference between EXP and CON groups for aerobic fitness (small SMD; P = .23; Fig. 3A), flexibility (medium SMD; P = .16; Fig. 3B), and muscular strength via MVC testing (trivial SMD; P = .28; Fig. 3C). Moderate-high interstudy heterogeneity was identified for these latter outcomes (66%-86%).

FIGURE 1.

A forest plot of meta-analysis of (A) muscular endurance and (B) muscular power.

FIGURE 2.

A forest plot of meta-analysis of (A) muscular strength via RM and (B) occupationally specific physical performance.

FIGURE 3.

A forest plot of meta-analysis of (A) aerobic fitness, (B) flexibility, and (C) muscular strength via MVC.

DISCUSSION

The current systematic review quantitatively identified the effects of traditional and nontraditional military physical training on various fitness domains (i.e., aerobic fitness, flexibility, muscular endurance, power, strength, and occupationally specific physical performance) that contribute to military-specific performance and MSKI risk in military personnel. The meta-analyses showed that nontraditional training resulted in significantly greater posttraining values for muscular endurance, power, strength measured via RM, and occupationally specific physical performance compared to traditional training. However, no significant group differences were noted for the posttraining values of aerobic fitness, flexibility, and muscular strength measured via MVC. These results partially supported our hypothesis that nontraditional training would result in greater increases in fitness domains compared to traditional training. The results of this review have shown that nontraditional training can induce several benefits to support the development of training programs to enhance soldier performance and to potentially reduce MSKI risk.

Nontraditional military physical training resulted in a significantly greater posttraining muscular strength measured via RM, aligning with previous qualitative reviews.^5,21 Classically, traditional training incorporates circuit activities (e.g., push-ups, jerry carry, and box lift)⁶ and calisthenics for the purpose of muscular endurance development.^6,12,13 However, this focus may not be specific to enhance strength. In contrast, incorporation of heavy resistance training exercises, such as deadlifts, squats, and chest presses may be more appropriate to specifically develop strength with clear benefits (i.e., large SMD) noted primarily in the included studies that integrated these exercises.^1,6,12,13 For example, Heilbronn et al.¹ specifically prescribed a resistance training program to induce myofibrillar hypertrophy, increase motor unit recruitment, and alter neural recruitment patterns in order to develop muscular strength.¹ The current findings provide further support that targeted and advanced resistance training can significantly improve strength, a key component reported for military performance.^1,2,4 In contrast, traditional training examined in the current review resulted in lower posttraining strength assessments and therefore may be sub-optimal for strength development and reduction of MSKI risk.³⁴

While muscular strength via RM was demonstrated to be greater for the nontraditional training group, the current meta-analysis identified no differences for MVC measures between groups.^{18–20,30,33} A possible explanation for this lack of difference may be because of the inherent nature of muscular strength assessment protocols. Maximal voluntary contraction tests are typically isometric assessments, whereby the contraction velocity is controlled and performed using monoarticular motion (e.g., elbow flexion or knee extension). This muscular strength measurement does not effectively reflect the entire movement pattern observed during resistance training, which consists of concentric and eccentric contractions and multiarticular actions. This limitation is further demonstrated during isometric strength assessments,^{18–20,30,33} where the muscle force generation capacity does not vary during the entire joint range-of-motion. On the other hand, RM testing replicates the movement patterns of resistance exercises and, thus, may be more pertinent to detect improvements in muscular strength with military-based resistance training.³⁵ Despite these results, the significantly greater RM in the EXP group reinforce the positive effects of heavy resistance training on strength development and its potential to enhance benefits beyond traditional military training.

Given that strength (via RM) was greater following nontraditional training, it was no surprise that greater muscular power was also noted posttraining.¹² Nontraditional training that incorporated resistance training in combination with power exercises, such as jump squats and countermovement jump, produced the greatest posttraining results and SMD (Fig. 1B) compared to studies that incorporated resistance training alone.^12,13 Previous research has identified the overriding importance of strength and power in the modern battlefield where the execution of sprinting, lifting, pulling, crawling, and climbing swiftly, while carrying heavy loads is of particular significance.³⁶ Therefore, the implementation of heavy resistance training together with power exercises (i.e., conduction of nontraditional training) may enhance the development of both muscular strength and power for improved combat readiness. Future studies are encouraged to specifically examine the impact of nontraditional training on combat performance to provide greater support for its inclusion in the preparation of military personnel.

Similarly, it was no surprise that greater muscular endurance was also noted following non-traditional training. While the focus on muscular endurance training was similar between training groups (Supplementary Table S4), the ability of heavy resistance training to induce improvements in muscular strength and endurance may explain the observed posttraining outcomes. Previous research has demonstrated that the performance of heavy bench presses for 6 weeks increases the number of push-ups one can perform.³⁷ This observed improvement is likely due, in part, to the greater strength developed from resistance training which results in endurance tests being performed at a lower submaximal level. This being said, our meta-analyses also showed that nontraditional training that incorporated heavy resistance training also resulted in no significant difference for aerobic fitness between traditional and nontraditional training groups.^6,29,32 This result highlighted that resistance training improved muscular strength, power, and endurance without diminishing aerobic fitness development, highlighting yet another advantage of nontraditional training to prepare military personnel.

Furthermore, our meta-analysis also found better performance of occupationally specific physical assessments (i.e., load carriage, 30-m run with combat gear, and dummy rifle) for the nontraditional training group compared to the traditional training group. Regardless of their military occupational specialty (e.g., maintenance, stores, driving, combat, ship maintenance, and medical), soldiers must be capable of performing physically demanding tasks.^38,39 Hence, a crucial aim of military physical training is to prepare soldiers for this, with regular monitoring needed for this preparation.⁷ However, occupationally specific physical performance, which encompasses multiple fitness domains, may not be assessed regularly during annual physical fitness tests.^2,4 For example, annual physical fitness tests across the armed forces typically comprise of a timed run, push-ups, and sit-ups, which focus on aerobic capacity and muscular endurance only.² The link between these fitness tests and traditional military physical training, which is heavily geared toward aerobic and muscular endurance development, is obvious.^2,4 However, the link between these tests, and subsequently training, and the occupationally specific physical demands of the military is less clear. These simple physical fitness tests do not represent the multidimensional reality of the military, where high levels of other domains such as strength, speed, power, and agility are imperative for soldier performance in any occupational specialty.^2,7,38,39 The results of the meta-analysis further reinforce. the importance of nontraditional training for greater occupational performance in military personnel.

According to the meta-analysis, posttraining values for flexibility were similar between nontraditional and traditional training. However, only one of the analyzed studies specifically incorporated flexibility training as part of their nontraditional training program.¹⁶ Soares et al.¹⁶ reported that joint range-of-movement was significantly greater following the addition of stretching exercises in EXP group one (training large to small joints for passive flexibility) (+12.6%) and EXP group two (training small to large joints for passive flexibility) (+12.1%), compared to traditional training without flexibility training.¹⁶ These results highlight the importance of including flexibility-specific training to improve flexibility, with further studies needed to confirm this benefit.

While some military groups are advancing their training practices to adopt a more nontraditional approach, large variations in standard military physical training continue to exist between countries, forces, and individual units as shown in the current studies of this review (Supplementary Table S4) and anecdotal discussions. As this area continues to evolve, unit commanders and physical training instructors may utilize the findings from this review in their military practice with the prescription of nontraditional military physical training that combines heavy resistance with power exercises to improve strength, power, and endurance without compromising aerobic fitness. This advanced training regime is likely to benefit military personnel to not only optimize physical performance but also reduce MSKI risk. Previous studies reported that poor performance in multiple fitness domains, such as aerobic fitness, flexibility, muscular endurance, etc., was strongly associated with an increased incidence of MSKI in military populations.^8,34 The current results reinforce that targeted military physical training can alter physical fitness domains that potentially contribute to reduced MSKI risk.³⁴ For example, the positive muscular strength adaptations from heavy resistance training can protect bones, joints, and muscles when under stress/impact.⁴⁰ Prior research reported a significant reduction in MSKI risk for military personnel undertaking resistance training.⁴¹ Ultimately, our recommendations for practice are to incorporate heavy resistance training with power exercises as part of traditional training in order to optimize physical performance and potentially reduce MSKI risk. In addition, the inclusion of flexibility-specific training in the military also has the potential to reduce MSKI risk through the prevention of muscle strains during activities involving the stretch-shortening cycle.^8,16 Improvements in flexibility have been shown to be of benefit to the military with reductions in lower extremity overuse injuries obtained from jumping and sprinting activities, which are exacerbated by uneven terrain and heavy load carriage.⁸ We encourage future studies to elaborate on the findings of this review by researching the effects of nontraditional military physical training including flexibility-specific training on MSKI incidence in military personnel.

Although the current review examined a range of studies, a number of limitations within the included studies should be acknowledged. Firstly, the majority of studies had modest sample sizes, with 10 of the 15 included studies (66.7%) having ≤20 participants per group, an unpowered sample size for most investigations.⁴² Modest sample sizes may give rise to a false-negative result,⁴² with future research encouraged to incorporate larger sample sizes. Secondly, only 4.5% of participants in the studies were female (of studies that reported sex). Given the greater role that women are playing in the military, it is important for future research to consider sex differences, such as hormonal variations and lower baseline physical fitness in females, that will impact the efficacy of physical training.⁴³ Thirdly, the ages of the military personnel reported in these studies varied by 19 years between the youngest and oldest participant (18-37 years old). This large variation in age may have an effect on the military experience and previous physical training history of participants, in turn, influencing the effectiveness of the studied training programs.

A novel and important strength of this systematic review was that we assessed a broad range of fitness domains, unlike previous reviews that only focused on one or two fitness domains (e.g., aerobic fitness and muscular strength).^5,21 As a result, the transferability of findings to physical training within the military context was enhanced with the effects of military physical training on multiple fitness domains evaluated at once. Although a broad range of fitness domains were assessed that have been found to contribute to MSKI risk,^2,3,8 MSKI risk was not an outcome in the included studies and this review. Hence, the relationship between traditional and nontraditional training and MSKI risk can only be hypothesized. To confirm this, future studies are encouraged to look at the direct impact between training, the fitness domains, and MSKI risk. Other limitations of this review that require further consideration include the pretraining differences noted between EXP and CON groups for some studies.^20,32 Consequently, while the meta-analysis may have found little between-group differences posttraining, there may have been greater improvements for one group. Additionally, high heterogeneity was evident within the meta-analyses owing to the variations in populations of, as well as the differences in, training programs’ length and type between forces, occupations, and countries in the included studies. The lack of standardization of military physical training in the world made comparisons, particularly of the CON (traditional training) group, difficult. The current study focused on a definition of military physical training based upon fitness domains historically. However, these definitions may be evolving with nontraditional training, as defined in the current study, becoming the standard or traditional form for some countries and/or forces. Future studies can expand upon the current results and explore the intricacies of evolving military training practices worldwide.

In conclusion, the current systematic review identified that nontraditional military physical training had a greater posttraining effect on muscular endurance, power, strength measured via RM, and occupationally specific physical performance. These findings demonstrate the benefits of nontraditional training to optimize occupational performance while potentially reducing MSKI risk. This will assist the military with the appropriate allocation of time, resources, and expertise when designing the ideal training program to enhance soldier performance. Future research should focus on determining the effects of nontraditional training on MSKI incidence to provide insight into the ideal military training program for performance and well-being.

ACKNOWLEDGMENTS

None declared.

SUPPLEMENTARY MATERIAL

SUPPLEMENTARY MATERIAL is available at Military Medicine online.

FUNDING

None declared.

CONFLICT OF INTEREST STATEMENT

None declared.

Author notes

Previously presented as an oral poster presentation at the Defence Health Sciences Symposium 2021, Defence Science and Technology Group, Melbourne, Australia, November 29, 2021.

The views, opinions, and/or findings contained in this review are those of the authors and should not be construed as official Australian Defence Force position, policy, or decision.