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Abstract
Introduction Musculoskeletal injuries to the lower extremities are major factors contributing to drop out from military tasks. The aim of the present study was to determine the incidence of musculoskeletal injuries and the parameters that differentiate between the soldiers who incurred these injuries and those who did not along 14 weeks of an infantry commanders course.
Methods One-hundred and sixty-eight participants were recruited from an infantry commanders course. The soldiers were tested before (pre), in the middle (middle) and at the end (last) of the course for anthropometric measurements, proprioceptive ability and dynamic postural balance (DPB), and filled out an ankle stability questionnaire (Cumberland Ankle Instability Tool (CAIT). A physiotherapist followed and recorded all musculoskeletal injuries incurred by the participants during the course.
Results Fifty-eight participants out of the 168 (34.5%) reported some pain/injury. Time effects were found for body mass index, DPB asymmetry, DPB in posterior-medial (P-M) direction and proprioception ability. Injury effects were found for DPB asymmetry, DPB in P-M direction, CAIT and proprioception ability. An interaction was found for proprioception ability. The Cox regression showed that the variables that are mostly effecting injuries were pretesting proprioception ability, DPB asymmetry and CAIT.
Conclusions More than one out of three participants incurred musculoskeletal injuries, with deficits in proprioception ability, DPB and ankle stability in pretesting as major factors contributing to injuries. Further studies should look at the effect of specific exercises such as proprioception, DPB and ankle stability exercises for prevention and treatment of musculoskeletal injuries among combat soldiers.
- combat soldiers
- musculoskeletal injuries
- dynamic postural balance
- proprioceptive ability
Data availability statement
All data relevant to the study are included in the article or uploaded as supplementary information.
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Key message
The study aimed to determine the incidence of musculoskeletal injuries and the parameters that differentiate between the injured and healthy soldiers.
One hundred sixty-eight soldiers from commanders course were tested for anthropometric measurements, proprioceptive ability, postural balance and ankle stability.
A physiotherapist followed and recorded all musculoskeletal injuries incurred by the participants during the course.
The percentage of soldiers who were injured was 34.5%.
Deficits in proprioception, in postural balance and in ankle stability in pretesting were major factors contributing to injuries.
Further studies should look at the effect of specific exercises such as proprioception and ankle stability exercises for prevention/treatment of injuries among combat soldiers.
Introduction
The lower extremities are one of the most common anatomical locations of injuries in the military,1 2 with the ankle/foot and knee injuries reported as one of the most frequent injuries impacting the entire scope of military readiness.1 2 The Israeli Defense Forces (IDF) manifested that 43% of all medical dropouts from basic training in the infantry were due to orthopaedic reasons,3 with lower extremity and low back injuries accounting for 71.5% of all orthopaedic injuries.4 Nye et al 5 showed that in Air Force basic military trainees the majority of all musculoskeletal injuries involved the lower extremities (78.4%). Injury epidemiology of US Army Special Operations Forces shows 24.5 injuries per 100 subjects per year for total injuries, with 15% preventable ankle injuries and 25% preventable knee injuries.6 In 2016, the US Army’s 101st Airborne Division reported about 14.9%–16.3% of all injuries to be foot and ankle injuries and 12.9%–13.3% ankle sprains/strains.7
Factors related to injuries, such as body morphology, reduced postural control and proprioception deficits, have been widely reported among athletes.8 In the military literature, musculoskeletal injuries may be related to training loads and regimens9 10; however, very little is known about specific injury-related factors among soldiers.11 12 Deficits in or a reduction of postural balance and proprioception abilities were reported to be related to injuries both in athletes8 and among military soldiers.1 When it comes to predicting injury risk in athletes, functional movement tests that include dynamic postural balance (DPB and proprioception abilities were found to have substantial benefit.13 It should be understood that an injury such as an ankle sprain during warfare of an untrained soldier can be lethal for the force. Furthermore, soldiers who developed an injury within a year following baseline tests were found to have significantly lower baseline balance, and were found to be at higher risk of further musculoskeletal injuries.14 Thus, the lower extremities—and ankle joints in particular—require optimal functioning during repeated movements on unstable surfaces, especially when accompanied by a significant amount of external weight.3
At present, there is a lack of sufficient studies that consider the incidence of injuries during difficult training activities, such as a commanders course, or studies that investigate whether parameters such as body morphology, reduced DPB, reduced proprioception ability or reduced subjective ankle stability are related to injuries in soldiers. Therefore, the purpose of this study was to determine the incidence of injuries, and the parameters that differentiate between the injured/non-injured soldiers, along a 14-week infantry commanders course.
Methods
Participants
One-hundred and sixty-eight male subjects (combat soldiers), aged 18–20 years, taking part in an infantry commanders course, were included in the current study. The infantry commanders course lasts 14 weeks. The main goal of the course is to develop the next generation of IDF commanders. Combat fitness is a major topic in the course, both physically during the training programme as well as teaching the candidates tools of proper training for later on. Each candidate has to pass a classification process before entering the course. The course included only male operators from the infantry brigades and special units of the ground forces.
Recruitment process
All soldiers who started the course (n=204) attended our presentation and explanation process (as it was part of the opening activity of the course). In that talk, we explained to the soldiers the research aims, the procedure of the research, the risk to the participants, etc. Special attention was given to the following conditions: (1) the subjects can choose whether they wanted to join the research or not; (2) their commanders will not have any access to their private results or injury report (written by the physiotherapist); (3) soldiers who choose not to participate in the research will not be affected and (4) the subjects can withdraw their voluntary participation at any point along the course without any required explanation to their commanders or to the research team.
Data collection
Data were collected at three time points along the 14-week course—in the first 2 days of the course (pre), at the beginning of the eighth week of the course (middle) and in the last 3 days of the course (last). The first two testing took place following the weekend (2 days of full rest) and the last testing took place following 2 days of theoretical learning (with no physical activity). Each testing included anthropometric measurements, proprioceptive ability and dynamic postural balance (Y balance test (YBT)), and each subject was asked to fill out an ankle stability questionnaire (Cumberland Ankle Instability Tool (CAIT)). A physiotherapist (specialising in musculoskeletal injuries and in military medicine) evaluated the subjects six times during the study period. The physiotherapist asked each participants to report musculoskeletal complaints or pain. Participants who reported they had complaints/pain underwent an additional individual examination by that physiotherapist.
Anthropometric measurements
Each subject’s anthropometric features were recorded. Body mass index (BMI) was calculated.
Dominant leg
All participants were tested for their dominant leg (when kicking a ball).
Dynamic postural balance
The YBT for each leg was performed according to the technique previously published by Coughlan et al.15
Proprioception ability
The active movement extent discrimination assessment (AMEDA) provides ankle discrimination ability scores representing participants’ sensitivity to small differences in the extent of ankle inversion movements made in a normal weight-bearing stance.16
Cumberland Ankle Instability Tool
This self-reported ankle instability questionnaire was completed by all participants. The CAIT is scored for both legs, and the maximum score is 30, with a low score indicating more severe CAIT.17
The tests took place on the first day of the course and on the day before it ended. All tests were performed by the participants in bare feet, but wearing their army uniform.
Injury examination
Each participant was asked by the physiotherapist to describe his major complaint (pain/swelling/instability/changes in sensation/other) and to report the region of his pain (ankle/shin (tibia and fibula)/knee/hip/lower back/thoracic spine/shoulder/other). The participants were asked to report their level of pain (at the specific site of the complaint) at rest and during activities (on a ruler Visual Analogue Scale (VAS) scale), and if they experienced night pain (yes/no). The participants were also asked whether their injury is chronic/recurrent, how the injury occurred (trauma/overuse injury/cannot recall/other) and what triggers the pain (running/walking/specific movement/weight bearing/cannot tell exactly/other). Following additional clinical examinations specific for each subject (such as the McMurry test), the type of injury for each soldier was identified (ankle sprain/shin pain/anterior knee pain/Achilles tendinitis/non-identified type/other) by the physiotherapist.
It should be noted that the injury diagnosis/examinations by non-military physiotherapist were for research purposes only. The subjects were diagnosed/examined by the physiotherapist six times along the course, at six different time points that were convenient along the course programme (three times during the first half of the course and three times during the second half of the course). Due to ethics requirements, no medical care was provided to the injured subjects by our physiotherapist. The physiotherapist could only provide some suggestions to the injured subjects (such as a recommendation for stretching exercises or some bandaging advice), and to recommend that the injured subjects turn to the army physician for further diagnosis/examination (however, as the army physician was part of the military staff, only very few of the subjects chose to report to the physician about their pain, due to their fear of losing their position in the course).
The physiotherapist and the researchers that carried out the proprioception ability measures, DPB measures and all other measures were blinded to each other, meaning that the researchers that carried out the measurements did not know whether any participants were diagnosed/examined or not by the physiotherapist, or whether the subject was classified in the injured or non-injured group.
Data analysis
The participants’ description of injuries (such as region of pain, types of injuries, cause of injury and triggers for pain) was used in the analysis. Participants were divided into three groups: not injured, injured in the first half of the course and injured in the second half. The findings were compared using analysis of variance (ANOVA) with repeated measures (three groups (not injured, injured first half, injured second half)×three times (pre, middle, post)). Post hoc tests were adjusted using Bonferroni correction. For each variable, only subjects that were tested along the whole three testing were included in the ANOVA analysis. Therefore, the number of subjects was different between different variables (due to typing errors, absence of a specific test for a specific subject, etc). Leg length asymmetry was characterised by absolute values of the difference between the right and left leg. A χ2 test was performed in order to compare the incidence of asymmetry in the three groups. In order to determine the variables that mostly differentiate between the three groups, Cox proportional hazard regression (forward entry) was performed using the variables showing a significant difference between the groups.
The data were analysed using SPSS V.26 software (SPSS, Chicago, Illinois, USA). The a priori level of significance for all statistical tests was alpha level of 0.05.
Results
Injury description
Fifty-eight participants out of 168 (34.5%) reported some pain/injury to the physiotherapist (in one of his six visits) along the 14 weeks of the course. Pain was the main cause (93.1%) for their request to see the physiotherapist. The mean (±SD) level of pain at rest (as identified by VAS scale) was 4.0±1.4 and the pain level during activities was 7.3±2.6. Night pain was reported by 51.7% of the participants. As for the region of pain, the ankle area, the knee area, the shin area and other areas of the lower extremity were reported by 32.8%, 17.2%, 17.2% and 32.8% of the soldiers, respectively. The types of injuries that were identified by the physiotherapist were: ankle sprain (19.0%), shin pain (20.7%), anterior knee pain (13.8%), Achilles tendinitis (6.9%) and non-identified type (39.7%). Trauma was reported as the main cause of injury (56.9%). The trigger for pain was reported as running (44.2%), walking (16.3%), specific movement (16.3%), weight bearing (9.3%), could not determine exactly (9.3%) and other (4.6%).
Considering the time of injury, 32 participants (55.2%) reported their injury between pretesting and middle testing (first half of the course) and 26 (44.8%) reported their injury between middle testing and last testing (second half of the course).
Due to the small number of injured subjects in each area/type of injury, all injured subjects were further analysed as ‘injured’ subjects.
Injured in the first half, injured in the second half and non-injured participants
Three groups of soldiers (no-injury/injured in the first half of the course/injured in the second half of the course) were compared. Leg length (measured at pretesting) asymmetry was not found to be related to injuries (χ2=2.98, p=0.23). Table 1 presents the mean (±SD) BMI, dynamic postural balance (YBT), proprioception ability (AMEDA) and chronic ankle instability questionnaire (CAIT) at pretesting, middle testing and last testing for non-injured, injured in the first half of the course and injured participants in the second half of the course.
Mean ± SD of anthropometric parameters, postural balance, chronic ankle instability and proprioception ability in pre, mid and last testing, of soldiers that were not-injured/injured 1st half/injured 2nd half of the course.
Time effects (pretesting, middle testing and last testing) were found for BMI (F(2,190)=8.99, η2=0.086; p<0.001); YBT asymmetry in the posterior-medial direction (F(2,188)=4.65, η2=0.047; p=0.011); YBT in the posterior-medial direction for the dominant leg (F(2,188)=18.63, η2=0.165; p<0.001); YBT in the posterior-medial direction for the non-dominant leg (F(2,190)=9.416, η2=0.090; p<0.001); YBT composite score in the dominant leg (F(2,188)=17.60, η2=0.158; p<0.001); YBT composite score in the non-dominant leg (F(2,190)=10.67, η2=0.101; p<0.001) and AMEDA testing (F(2,186)= 15.46, η2=0.143; p<0.001). Non-injured/first-half injured/second half injury effects were found for YBT asymmetry in the posterior-medial direction (F(2,94)=4.13, η2=0.081; p=0.019); YBT in the posterior-medial direction for the dominant leg (F(2,94)=3.87, η2=0.096; p=0.024); CAIT in the dominant leg (F(2,50)=5.02, η2=0.090; p=0.029); CAIT in the non-dominant leg (F(2,50)=3.99, η2=0.138; p=0.025) and AMEDA testing (F(2,93)=4.84, η2=0.094; p=0.010). An interaction (time×injury) was found for AMEDA testing (F(4,186)=3.18, η2=0.133; p=0.044).
Cox regression
Cox proportional hazard regression was performed to show the significant factors differing between injured and non-injured subjects at pretesting. The three variables that were entered were the pretesting results of the proprioception ability (AMEDA), dynamic postural balance (YBT) asymmetry in the posterior-medial direction and CAIT in the non-dominant leg (Table 2; Figure 1).
Survival curve (Cox regression model) of injured soldiers along the course.
Cox proportional hazard regression model of factors differing between injured and non-injured soldiers.
Discussion
This study evaluated the incidence of injuries and factors (such as postural balance and proprioception ability) that might be related to injuries among soldiers participating in an infantry commanders course.
Incidence and distribution of injuries
In total, 34.5% of the 168 screened participants reported some discomfort/pain and underwent clinical examination by the physiotherapist. The ankle area followed by the knee and shin areas were the main regions of pain, with ankle sprains, shin pain and anterior knee pain as the main types of injuries. The high incidence and distribution of injuries among our soldiers is consistent with previously published studies,4–6 for example, Bulathsinhala et al reported that ankle sprains are one of the most common military-related injuries18; Nye et al explained that 12.5% of Air Force trained recruits sustained one or more musculoskeletal injury along basic military training5 and Schwartz et al 4 reported that most orthopaedic diagnoses were caused by overuse injuries (90%), whereas trauma injuries accounted for 10%. Considering the location of the injuries, similar to our results, special forces operators were reported to suffer a wider distribution of injuries across multiple joints, including the knee and ankle.6 Nagai et al 14 reported that most musculoskeletal injuries occurred in the knee, lumbo-pelvic area and ankle, with types of injury including low back pain, ankle sprain, patellofemoral pain, knee pain, shoulder pain and shin splints. As for the trigger of pain, in the current study most participants reported that running, walking, specific movements and weight bearing were the main factors that were the trigger for their pain. Investigating the effects of different physical training elements on injury rates, Knapik et al 19 reported that running was found to be associated with most of the physical training injuries during basic training (62% of male physical training injuries). It was explained that a prevention programme reducing running mileage led to 10%–24% fewer injuries, while maintaining the soldier’s running speed.19 It should be noted that musculoskeletal injuries remain a major contributor to morbidity, missed training time, discharges and fiscal burden; they are also associated with lost time from duty and impacting military readiness.5 As injuries such as ankle sprains have the potential for long-term functional deficits, and may lead to limited duty days in both training and combat soldiers,20 most authors suggested specific prevention and rehabilitation programmes for reduced risk of injuries.
Factors related to injuries
An additional aim of the current study was to determine the anthropometric parameters and the biomechanical factors (dynamic postural balance, proprioceptive ability and subjective ankle stability) that may predict musculoskeletal injury among combat soldiers. In the current study, parameters such as smoking habits, alcohol use, boot type, gait pattern and physical fitness level were not assessed, although previous studies showed those factors as significantly related to musculoskeletal injuries among combat soldiers.11 21 22
Our hypothesis that all the factors measured in the current study will differ between injured and non-injured soldiers, was partially supported by our results. Considering BMI, although time effect (pretesting, middle testing and last testing) was found for BMI, no injury effect and no interactions (time×injury) were found. Interestingly, although not significant, in the pretesting the non-injured group had the ‘average’ BMI compared with participants who were injured in the first half of the course (had higher BMI) and compared with participants who were injured in the second half of the course (with lower BMI). Similar to our results, Heebner et al 23 and Teyhen et al 12 found no difference in BMI in soldiers who sustained an injury and those who did not. Looking at the army literature, there is no consistency considering the relation between high/low BMI and injuries. Jones et al 24 showed that injury risks were highest in the army trainees groups with the lowest BMI, and a 6-month prospective follow-up study in the Finnish army reported that being underweight according to BMI increased the hazard rate for overuse injuries.11 It was suggested that a higher BMI may have a protective effect against injury, due to the greater absolute amounts of muscle among soldiers with higher BMI that enable those soldiers to cope better with load-carrying tasks than their lighter counterparts.11 24 In contrast, Heebner et al 23 reported that army operators who sustained a spinal injury had a significantly greater BMI than those who did not. Additionally, several studies of military trainees have demonstrated either a J-shaped or a bimodal relationship between BMI and injury risk (high injury risk at the high and low extremes of BMI, and lowest among those with ‘average’ BMI).21 Interestingly, in the present study leg length asymmetry was not found to be related to injuries. In most previous studies, asymmetry was found to discriminate between injured and healthy athletes, as length differences mostly disturb the correct kinematic chain along lower limb joints, creating abnormal stresses and leading to lower extremity injuries.25
Cox regression demonstrated that participants who had at pretesting reduced proprioceptive ability (examined by the AMEDA device), reduced DPB asymmetry in the posterior-medial direction (examined by the YBT device) and low subjective ankle stability (reported by the CAIT questionnaire), were at higher injury risk. The literature already reported that residual sensorimotor deficit, decreased postural stability and reduced functional movement are factors related to musculoskeletal injuries among healthy, physically active athletes.26 Yet, only a limited number of studies looked for those parameters in relation to injuries among soldiers. Nagai et al,14 for example, reported that soldiers who developed an injury along a 1-year follow-up had significantly inferior static balance variables at baseline measurement.14 In the present study, we found that reduced DPB in the posterior-medial direction (specifically) may differentiate between injured and non-injured participants. Similar to our results, Hartley et al 27 found greater posterior-medial asymmetry in injured female athletes compared with non-injured, suggesting that asymmetry might be dependent on multiple factors such as gender, sport and injury definition. On the other hand, Bansbach et al 20 reported no significant differences in dynamic postural stability between injured and non-injured Special Operations Forces Operators. Surprisingly, in the present study, no interactions (time×injury) were found for DPB parameters. It might be that the postural stability of the injured participants had either returned to baseline levels through daily military tasks, or their single injuries did not alter their postural stability. It should be noted that although injuries such as ankle injuries are mostly reported to have long-term effects, few studies showed that an injured subject might not demonstrate deficits in postural stability, proprioception or strength.20
Considering proprioception ability, Mohammadi et al 28 measured whether military exercise was associated with a reduced proprioception ability, and tried to look for a relationship between proprioception and lower extremity injury. The authors found that proprioception ability was reduced after military exercises; they explained that fatigue may cause dysfunction of muscle mechanoreceptors, the receptors around the joints and other mechanisms in the proprioception pathway.28 As studies5 7–11 mostly suggested that reduced proprioceptive ability due to fatigue may be a risk factor for lower extremity musculoskeletal injuries, it is likely that the effect of fatigue (along the course) was greater for our soldiers who manifested reduced proprioceptive ability at pretesting—a deficiency that increased their risk of being injured.
As expected, the subjective ankle stability score (identified by the CAIT questionnaire) was found to be related to injuries among our soldiers. Similar to our results, Vaulerin et al 29 reported that CAIT scores significantly differ between injured and non-injured firefighters. No previous study measured whether the subjective assessment of ankle instability among soldiers is related to the risk of injuries. In the literature, it is commonly explained that athletes and non-athletes with chronic ankle instability have delayed peroneal reaction time, proprioceptive deficits and lower extremity kinematic deficits, all as possible key risk factors for future musculoskeletal injuries.30 31
Limitations of the study
There are several limitations to this study: (1) the relatively small number of participants (only 204 subjects started the course); (2) not all the soldiers who started the course agreed to participate in our research (168/204=82.4%), a fact that might affect the external validity of the results; (3) the results could have been influenced by a number of unmeasured factors that might be related to injuries among combat soldiers, such as smoking/alcohol/boot type/height/stride length/physical fitness/physical activity type, etc; (4) the CAIT questionnaire was missing for large number of participants in the second testing due to reasons beyond our control, a fact that probably affected the results; (5) recruits who started the course might have been suffering from pre-existing and undeclared injuries; (6) as the physiotherapist was non-military personnel he could not be reached by the subjects on a daily basis (the subjects were diagnosed/examined only six times along the course); (7) the subjects might under-report their injuries due to the fear of losing their position in the commanders course and, (8) injured soldiers who did not report their injury and were categorised in the ‘non-injured’ group, probably influenced the results of the study.
Conclusions
A high incidence of soldiers with musculoskeletal injuries was found (one out of three), with deficits in proprioception ability, in DPB and in ankle stability at pretesting as major factors contributing to these injuries. Our results might suggest potential evidence-based data for improved exercise protocols for injury prevention and rehabilitation during combat training in military setting. Further studies should look at the effect of specific exercises such as proprioception, DPB and ankle stability exercises for prevention and treatment of musculoskeletal injuries among combat soldiers.
Data availability statement
All data relevant to the study are included in the article or uploaded as supplementary information.
Ethics statements
Patient consent for publication
Ethics approval
This study was approved by the Institutional Review Board (IRB) of the IDF Medical Corps (1813–2017).
References
Footnotes
Contributors All authors contribute to that manuscript.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.