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Incidences and Causative Factors of Lower Limb Injuries in the New Zealand Army

By April 13, 2021No Comments

By Mr J. Rousseau

Abstract

Objective

To examine long-term musculoskeletal injury trends in the New Zealand Army and determine the most common injuries and the activities as a causative factor. This will be useful for identifying activities causing injuries.

Methods

Eleven years of reported musculoskeletal injury data for the New Zealand Army (n=20461) was examined. Activities as causative factors were derived from a filtered narrative, completed by the soldier at time of injury. Frequency data and Chi square analysis was carried out on injury anatomical site by activity.

Results

Analysis shows a significant proportion of injuries are lower limb injuries (45%) and this rate has remained static over 11 years. The ankle has the highest injury rate, with most of these occurring during military physical training and individual sports during sporting activities. 

Conclusion

Regardless of several interventions over 11 years, the overall injury rates did not change. Of the physical training and individual sport activities, running resulted in the most ankle injuries. These results will help inform defence forces when injuries are likely to occur. Future research is required to determine ankle injury aetiology.

Key Words: ankle, army, injury, lower limb

Introduction

Musculoskeletal injuries are a significant problem in military populations[1-3]. For New Zealand, this results in approximately 20% of the total number of New Zealand Army (NZA) Regular Force personnel not able to be deployed at any given time (Annual Report on the Health of the New Zealand Army 2013, unpublished). Additionally, they are the most common cause of discontinuing military service[4]. The overall costs of injuries to any armed forces are loss of manpower, loss of training time, loss of duty time, reduction in force effectiveness or readiness and an increase in medical costs[5]. Data extracted from the New Zealand Defence Force’s Accredited Employer Programme (AEP) indicates that minor (Grade 1) to moderate (Grade 2) musculoskeletal injuries, such as a sprain or strain, are estimated to cost the NZA between $1 – 1.5 M per annum. However, this does not take into consideration the true cost to the NZA in terms of loss of manpower and training days.  

Military commanders may well accept that current injury rates are an inevitable outcome of military training, as it includes many activities such as loaded running, walking, climbing and marching, often performed at high intensity levels, predisposing soldiers to injury[2]. Nonetheless, injuries remain one of the NZA’s greater challenges, as the potential exists for injury related factors to adversely affect deployability of combat personnel in a tactical or kinetic operation. Military injury statistics have been reported previously, for example a snapshot of New Zealand Defence Force regular force members’ lower limb injuries from 2002-2003[2] reported the largest number of lower limb injuries were at the ankle (37.1%). Studies of between 8 – 52 weeks duration examining injuries in defence forces of other nations have reported similar results[3]. As a result of injuries in the US Army, physical training programmes were redesigned[6]. The US Army implemented the Physical Readiness Training and Fitness Assessment Programme (PRT)[7] during US Army Basic Combat Training and the Ordnance Advanced Individual Training for an operational infantry unit.  The programme was evaluated by the US Army Centre for Health Promotions[6]. These programmes have been successful in reducing the number of musculoskeletal injuries during initial military training; however, the long-term effects of these programmes are not known. 

Few longitudinal studies document ankle and foot injury trends over many years.  Knapik, Montain, McGraw, Grier, Ely, Jones[8] completed a study examining the demographics and physical risk factors for stress fractures in US Army recruits over the period from 1997 to 2007. A report by the US Army Public Health Centre[9] documents the number of musculoskeletal injuries in US Army soldiers with more than half of all injuries being lower extremity injuries. However, they did not document or highlight when or if interventions had taken place during that time, as numerous studies have suggested[10-12], and if the trends changed accordingly.

A review by Andersen, Grimshaw, Kelso, Bentley[1] reported that musculoskeletal injury rates in the military are still a significant issue despite substantial research regarding intervention strategies to mitigate injuries. Therefore, there is a need to track possible patterns or trends over a number of years, particularly noting when any intervention strategies were put in place to reduce injury to properly assess their efficacy. Furthermore, a long-term understanding of any injury trend is required before investigation in aetiology, changes in policy, or further interventions to improve the number of military personnel for deployment and reduce associated costs are implemented. This study aims to identify long-term trends in the occurrence of lower limb injuries in the NZ military population over 11 years, and whether any alteration in the trend was due to an implemented intervention directed at reducing lower limb injury risk.

3.4 Methods

Data from 2005 to 2015 were extracted from the New Zealand Defence Force (NZDF) Accredited Employer Programme (AEP), which is affiliated to the New Zealand Government Accident Compensation Corporation Act (ACC) 2001. The AEP allows an employer to act on behalf of the ACC, managing workplace injuries for their employees and providing entitlements under the 2001 Act for work-related personal injuries and illnesses. All injuries are legally required under the Act to be recorded through the AEP process.

All injuries requiring medical attention are captured and classified according to the Read Classification System[13] used by the NZDF. Lower limb injuries are defined in the Read System as injuries to the hip, thigh, knee, lower leg, ankle, foot and toe. Although the AEP data does include hip and thigh injuries, for the purposes of this study, “lower limb” was confined to the knee, lower leg, ankle and foot. Each AEP entry is recorded as a new injury event; therefore, an individual may have more than one AEP event within a year. Reoccurring injuries to specific anatomical sites is indicative of not addressing the aetiology of the injury. It is also important to note these data refer to reported injury events and not a different individual for each injury as the outcome remains that the person is not available for deployment.

In order to determine causal factors of the injuries, the narrative given on the AEP form completed by the soldier was searched to identify the activity at the time of the injury. The narrative includes statements such as “Running during unit PT and stepped into a hole and felt pain in left calf” and “Rolled my left ankle while doing an 8 km pack march”. Narratives were filtered using Microsoft© Excel 2016 to uncover the contributing activity, e.g.:

  1. “Running during unit physical training (PT) and felt pain in left calf”.
  1. Injury – lower limb (left calf)
  2. Contributing activity in which it occurred – running
  1. “Rolled my left ankle while doing an 8 km pack march” 
  1. Injury – lower limb (ankle)
  2. Contributing activity – military training (pack march)

Three main activity categories were identified from the narratives, 1. Military Training; 2. Sport; and 3. Other (Table 1). 

Military training activities were further divided into six categories (Table 1): combat drill, military core skills training, pack marching, physical training, walking/patrolling and accident/other. This categorisation is useful in that it identifies specific groups of training activities, which may be causative. Each of these categories are distinctly different activities. For example, pack marching, which is a loaded activity versus combat drill, which includes contact activities. Patrolling is walking with boots, wearing full uniform and carrying various loads, while physical training is structured exercise in sport fatigues (T-shirt, shorts and training shoes) to enhance aerobic and anaerobic fitness and includes calisthenics to increase body strength. The physical training category also includes running in sport fatigues as a form of exercise to maintain and improve aerobic fitness. This is often performed at the soldiers’ leisure or during structured physical training sessions.  Pack marching involves wearing full military uniform, military boots and carriage of equipment and weaponry in urban and field environments. Military training encompasses training in individual military core skills such as parade ground drill, whereas combat drill involves fully equipped soldiers engaged in physical contact activities such as close quarter combat and riot training. Accidents/other refer to incidental or unplanned events; examples of accidents include unintended collisions, trips or falls during military training. These injuries are usually acute by nature. “Other” refers to injuries having no reported activity or contributing factor reported in the AEP narrative given by the soldier e.g., “hurt my left leg”, and “woke up this morning and felt pain in my ankle”.

To determine during which sporting activity ankle and other lower limb injuries occurred, different sports were grouped together according to the sport type (Table 1): indoor court, contact, individual, field and other sports. The sports identified are common to New Zealand; NZA personnel participate in them on an official basis. Some of these sports may be different in other countries e.g., netball, rugby union and rugby league, touch rugby and cricket. Categories also allowed for sufficient sample size analysis in this study. “Other” sports refer to minor unofficial NZA sporting codes with few participants. These include sports such as rock climbing and ultimate frisbee.

Table 1.  Main Categories and Activities

Military Training (N = 3262) Sport (N = 2769) Other (N = 860)
Assaulting Basketball Accident
Battle drill Boxing Falling/Tripping/Slipping
Battle efficiency testing Cricket Collision
Combat fitness testing Cycling Exposure
Close quarter combat Handball Lifting
Parade ground drill Rugby League Motor vehicle accident
Jumping Rugby Union Overuse
Pack marching Running
Parachuting Skiing
Physical Training Snowboarding
Patrolling Soccer
Rappelling Softball
Required fitness testing Squash
Riot training Swimming
Ropes Tennis
Walking Touch
Volleyball

The time versus lower limb injury rate plots (Figure 1) denote when the NZA implemented specific physical training or footwear-related interventions. Cross tabulation analysis was carried out using Chi-Square statistical analysis and significant relationships (p < 0.001) were determined between: (A) Injury Site and Main Activity; (B) Injury Site and Military Training Activities; and (C) Injury Site and Sporting Activities.

Results

Data from the past 11 years (Figure 1) revealed there has been little change in injury rates over this period despite changes to footwear (2009, 2010, 2014, 2015) and the implementation of a balance and proprioceptive training programme during initial military training (2013). On average, 40% of all NZA soldiers were injured annually, and 45% of these injuries are to the lower limb. The ankle joint had the highest proportion of all the lower limb injuries (Figures 2a-c).

Cross tabulation analysis of: A. Injury Site and Main Activity showed the ankle had the highest incidence of injury in military training and sport activities whereas the knee had the highest number of injuries in the category ‘Other’ (X2 (6, N = 860) = 91.56, p < 0.001) (Figure 2a); B. Injury Site and Military Training Activities revealed the ankle joint had the greatest amount of injuries (X2 (15, N = 3262) = 142.49, p < 0.001) (Figure 2b); C. Injury Site and Sporting Activities revealed that the ankle joint was the site with the highest amount of injuries for indoor court sports, individual sports and field sports. The knee is the site with the greatest number of injuries for contact sports; the lower leg and knee are the injury sites with the greatest number of injuries in other sports (X2 (12, N = 2769) = 221.62, p < 0.001) (Figure 2c).

Figure 1. Trends from New Zealand Defence Force AEP data from 2005-2015 for total number of regular force (RF) personnel, total number of musculoskeletal injuries, and total number of lower limb injuries.

 

Figure 2a – c. Injury sites and causative activities. Main activities identified (a), military training activities (b), and (c) sports.

Discussion

This research represents one of only a few to describe long-term lower limb injury trends in the military and is the first to do so for the NZA. It is also the first to note when interventions were implemented over the period. Eleven years of NZA injury data show an unchanging trend of high numbers of lower limb injuries. Moreover, the ankle joint is still the most injured joint during military and sporting activities, with running as the most common activity leading to ankle injury in the NZA. Lower limb injuries, particularly the ankle joint, account for the highest proportion of all injuries sustained in the NZA regardless of multiple interventions over the intervening years. 

At present, approximately 40% of all NZA soldiers are injured or re-injured annually, of this 40% total injury rate, 45% are to the lower limb. Similarity can be found in the recent study by Schwartz et al. (2014) who reported that lower limb injuries accounted for 44.5% of all injuries in the Israeli Defence Force during up to 7 months basic training. Furthermore, our findings are in agreement with Davidson, Chalmers, Wilson, McBride [2] who identified the ankle as the most common lower limb musculoskeletal injury site in the NZDF. Of the three military services making up the NZDF, the Royal New Zealand Navy (RNZN); the New Zealand Army; and the Royal New Zealand Air Force (RNZAF), the NZA is the service having the highest proportion of ankle injuries. Previous research[1,4,5,14,15] has also highlighted the ankle joint as the most common injury site in military populations. These injuries were attributed to challenging training programmes and other physical activities of military training and operations, but did not specify which specific military training activities resulted in ankle injury.  A report by the US Army Public Health Centre[16] noted the ankle was the most common site for acute and traumatic injuries, but was the third most injured site (behind the knee and hip) for all injuries. However, these are only reported for new recruits in a calendar year. Further studies are situation specific for injuries, for example Roy, Knapik, Ritland, Murphy, Sharp[17] reported low back and other body regions have higher injury rates in a combat team on deployment; or vehicle mechanics had a greater number of lower back and knee injuries[18] noting these injuries are trade specific and do not take total number of injuries for US Army personnel into consideration, neither are any interventions discussed. Our current study did not distinguish between new recruits, situation or time in service. Moreover, it has shown additional information that running during PT, or as an activity on its own, was the most common contributor to ankle injury.  

It was thought that lower limb injuries and particularly ankle injuries would be most prevalent in contact sports such as rugby union and rugby league, or those with a high degree of running and jumping on hard surfaces such as basketball as previously reported by Borowski, Yard, Fields, Comstock[19] and Cumps, Verhagen, Meeusen[20]. While these factors were prominent, the significant injury inducing activity was simply running. Furthermore, investigations of injury trends in rugby have reported the head/neck/shoulder region of the body similar numbers of injuries as the lower limb albeit lower limb injuries required the longest average absence from play[21].

One could assume that the ankle joint has been compromised or the nature of training is damaging the ankle joint. The ankle may be unstable due to a lack in strength and/or proprioception therefore predisposing it to injury. Training regimes do not appear to have strengthened the joint sufficiently to meet the demands of military training. It is possible new recruits are arriving for military training with already “weakened” ankle joints which predisposes them to lower limb and ankle injuries early in the military training programme. This is supported by reported attrition rates due to lower limb musculoskeletal injury of potential new recruits who underwent a medical examination up to one year prior to the day of compulsory recruitment in the Israeli Defence Force[22]. Similarly, Goodall, Pope, Coyle, Neumayer[23] reported injuires to the knee (24%) and ankle (14%) during Australian Army recuit training.

Despite research reporting that soldiers may be predisposed to ankle injuries[24,25], no definitive evidence exists regarding the possible mechanisms that if identified in this population, could be reversed. Our data show unequivocally that the trend of lower limb injuries has been static over the past 11 years (Figure 1), and this is despite interventions such as changes to footwear (military boots and running shoes). This is supported by studies whereby examining changes to training shoes as an intervention confirmed that assigning running shoes based on the shape of the plantar surface of the foot had little influence on injury risk[26]. This included assigning to recruits motion control, stability, or cushioned shoes based on static foot type. Finestone, Milgrom[27] also found that shoe modifications and orthoses were not effective in lowering the incidence of injuries in the Israeli Army. A systematic review by Wardle[28] investigating the mitigation of musculoskeletal injuries in military populations found no clear indication of the effectiveness of footwear modifications. 

Changes to physical training programmes such as the introduction of more controlled warm-up and cool-down protocols, the inclusion of balance board training[29] or combined balance and agility training have all been used as interventions to decrease lower limb injuries in military populations[23,30,31]. None of the above-mentioned interventions were followed up for long term effectiveness, or lower limb injury data examined for decreasing trends. Knapik, Rieger, Palkoska, Van Camp, Darakjy[6] documented a training programme intervention that included proprioception-balance-agility components to reduce injury. The inclusion of balance and agility training as an intervention in the military is normally implemented as an extra training requirement. This type of additional structured balance and agility training for recruit physical training did not significantly reduce lower limb, knee and ankle, or knee and ankle ligament injury rates, probably due to an existing high training load, and may even have hastened the onset of injury[23]. The NZA implemented a similar such intervention in 2013 (see Figure 1); it did not affect the overall trend in ankle injury rates. 

Further training related interventions include the reduction of the cumulative training load and marching distance, and a gradual increase in training intensity to reduce the incidence of injuries[3]. The modification of intensity, frequency and duration of basic military training activities and an improvement in equipment resulted in a reduction of overuse and stress fracture injury while achieving recruit fitness levels[32]. The US Army Physical Readiness Training programme has been successful in reducing injury rates when compared to traditional Army physical training programmes[6]. However, most physical training intervention programmes are implemented during basic military training and no long-term effect has yet been established.

Shock absorbing insoles and foot orthotic devices have had substantial attention as a mitigating factor in preventing lower limb injuries in military populations; however, results are inconclusive as to their effectiveness. The NZA undertook a trial in 2010 to assess the feasibility of addressing lower limb injuries by issuing orthotics[33]. During this trial, shock-absorbing insoles seemed to be of benefit for reducing stress fracture-type overuse injuries, but not a reduction in all lower limb injuries[28]. In an earlier study investigating the use of insoles, a potentially successful insole was identified, however the study also acknowledged that further research was required to determine any definitive results in the use of an insole for injury mitigation[34]. Similarly, a trial was conducted by Franklyn-Miller, Wilson, Bilzon, McCrory[10] to investigate the use of customised orthoses to reduce the incidence of lower limb injuries during military training. The trial demonstrated a significant decrease in exercise-related lower limb injuries during a single training period. However, a critical analysis by Richter, Austin, Reinking[35] of a review by Collins, Bisset, McPoil, Vicenzino[36] regarding the use of orthoses, found there was insufficient evidence to recommend foot orthoses for the prevention of lower limb injuries. Furthermore, the systematic review by Baxter, Ribeiro, Milosavljevic[33] investigating the use of orthotics as an injury prevention strategy for military personnel indicated that no strong conclusions can be made on the use of orthotics as a preventive measure for overuse injury in the military. It must be stated that an overriding factor with studies investigating the use of orthotics is they are conducted over short time periods, typically between 12 and 15 weeks[33], which may mean the efficacy of their use could be ‘lost’ in injury statistics covering longer periods of +52 weeks.

It is important to note that the proportion of time spent in each category of training has not been quantified. This is recognised as a limitation; however, this is an observational study to understand trends and associated activities, and not to determine specificity in those activities. Nevertheless, one of the outcomes of any significant injury pattern findings will be to construct future research around the proportion of training categories. A change in exercise duration for certain categories may well reduce injury rates.

A further limitation is not distinguishing whether the injuries were sustained by recruits or trained servicemen. As stated in the discussion, several studies report higher injuries in recruits which may bias the result.  Nevertheless, the aim of this study was to report the trend of lower limb injuries over a number of years, and it does not appear to change.

This study did not list the underlying factors which precedes an injury initiating event, as set out in Meeuwisse’s multifactorial injury causation model[37].  It reports the trend of events that lead to the injury and therefore provides scope to work backwards to identify those susceptible via internal risk factors or identify external factors.

3.7 Implications for Public Health

This paper has important implications as it addresses the fact that interventions have been carried out regarding the high rate of ankle injuries in the NZA, yet no significant decreases in the number of ankle injuries has taken place over 11 years. Before any further appropriate injury prevention strategies and training programmes can be put in place, it is important that the causes and mechanisms of injuries are understood and thoroughly assessed. A case in point is that the activity inciting the greatest number of ankle injuries was running.  However, it does not mean that soldiers should run less, but rather that further research is required to understand why.  

Continued efforts to ameliorate preventable ankle injuries in the military setting, will have personal, economic and organisational benefits.

3.8 Conclusion

This study is unique in identifying unchanged lower-limb injury data for over a decade, despite interventions of changes to boots or running shoes and alterations to recruit training. Furthermore, this study confirms that injury occurrences to the lower limb and particularly the ankle joint are the most prevalent injuries in the NZA.  The majority of these injuries occur during running whether it is military physical training or sports. Further investigations are required to define the aetiology of these lower limb injuries, particularly the ankle joint, are recommended before initiating future risk reduction interventions.

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