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Running to breaking point? The relationship between 1.5-mile run time and injury risk in female recruits during British Army basic training
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  1. Rosalie Heller1 and
  2. H Stammers2
  1. 1 Medical Reception Station, Army Training Centre Pirbright, Pirbright, UK
  2. 2 Primary Care Rehabilitation Facility, Army Training Centre Pirbright, Pirbright, UK
  1. Correspondence to Rosalie Heller, Poynatts Cottage, Henley-on-Thames, OXON, RG9 6TG, UK; rosalieheller{at}doctors.org.uk

Abstract

Introduction The 1.5-mile best-effort run is used in the British Army to assess the fitness of all recruits and trained service personnel by means of the physical fitness assessment (PFA). The 1.5-mile run is a basic measure of fitness and slower times have been associated with an increased risk of musculoskeletal injury (MSkI), particularly during this early stage of training. The aim of this study was to establish whether 1.5-mile run times were associated with subsequent MSkIs among female recruits during their 14-week basic training.

Method Retrospective data were analysed from female recruits who had undertaken basic military training between June 2016 and October 2017. This included retrieving the results of their week 1 PFA; recording the type, cause and week of MSkI if they had sustained one; and noting down their outcome from basic training. Run times were statistically analysed in relation to MSkI occurrence of 227 female recruits using binomial logistic regression with an accepted alpha level of p value <0.05.

Results 1.5-mile run time predicted risk of MSkI (χ2 (1)=12.91, p<0.0005) in female recruits. The mean run time for injury-free recruits was faster than for injured recruits (12 min 13 s compared with 12 min 43 s). Every 10 s increase in run time was associated with an 8.3% increase in risk of injury.

Conclusion Slower 1.5-mile best-effort run time, as a surrogate of aerobic fitness, is associated with increased risk of MSkI in female recruits during basic training.

  • musculoskeletal injury
  • military training
  • preventative medicine
  • sports medicine
  • military medicine

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Key messages

  • There is a high incidence of injury amongst female recruits during British Army basic training.

  • Slower 1.5-mile run times correlate with a higher risk of musculoskeletal injury and time out of training for female recruits.

  • This study provides further evidence that could be used to guide the entry standard with regard to aerobic fitness of potential recruits.

  • This study should aid the British Army’s ability to select and prepare recruits appropriately for basic training and provide easily understood guidance for training staff to be aware of those at higher risk.

Introduction

Musculoskeletal injuries (MSkIs) are a significant problem for the UK military and other militaries worldwide, posing both time and financial burdens.1–3 Low aerobic fitness has been recognised as a risk factor for MSkI4 5 and a common test of aerobic fitness is a short-distance run (eg, 1.5 miles in the British Army6 and 2 miles in US Basic Combat Training).7 Previous studies have found a significant relationship between 1.5-mile run time and risk of MSkI.1 7 8 A study of US Army recruits in 1993 found 19.4% of female recruits in the fastest quartile (for a 1-mile run) were injured compared with 36.4% in the slowest quartile.8 Equally, the incidence of injury in male recruits was 0% for the fastest quartile compared with 36.8% in the slowest quartile.8 In addition, a study of recruits undertaking British Army initial training at three Army Training Centres (ATCs) (Pirbright, Lichfield and Winchester) between January 2003 and March 2005 found that a recruit’s risk of referral to remedial instructor following injury was 6.64 times greater for the quintile of the slowest runners (of the 1.5-mile run) compared with the fastest quintile.1 It has also been identified that women are often at higher risk of MSkI than men1 7 9 and that a greater proportion of recruits are injured in basic training compared with subsequent Initial Trade training.10

Hall11 investigated MSkI in male British Army recruits between 2009 and 2011 and found that the 1.5-mile run time could predict both MSkI risk and training outcome.11 This paper builds on Hall’s research, but instead investigates the relationship between run time and MSkI risk in female recruits. Previous studies from the US Marine Corps12 and Israel Defence Forces13 have found a significant relationship between slower run times in female recruits and the incidence of stress fractures and other MSkIs. This study is one of the first that looks solely into run times and risk of MSkI in female recruits within the British Army.

In order to start training to be a soldier in the British Army, a recruit will have completed a 2-day assessment at an Assessment Centre involving a medical examination, physical and mental tests, team exercises, a short interview and a career discussion. During this assessment, potential recruits are also given a personal development plan and are directed towards a fitness app ‘100% Army Fit’ designed by British Army physical training instructors (PTIs) to help them prepare physically. The pre-employment physical tests include a power bag lift (15–40 kg from ground to a 1.45 m platform), a jerry can carry (two full 20-litre jerry cans for up to 150 m) and the 1.5-mile best-effort run.6 The entry standard for the run varies between roles but is 14 min for the majority of the roles recruits to Pirbright will be going into, including combat medical technician.14 This should provide the potential recruits with an understanding of the minimum physical requirements for training.

The British Army delivers basic training, also known as the common military syllabus, over 14 weeks. It is designed to transform civilians into soldiers and prepare recruits for their Initial Trade training by training in skills such as fieldcraft, skill at arms and battlefield casualty drills, and, as a consequence, is physically and mentally demanding. Once recruited, MSkIs also pose threats to retention as recruits may be downgraded or even discharged. MSkI is a multifactorial problem, with female recruits often suffering a higher incidence of injuries than male recruits.8 By investigating the contribution aerobic fitness makes to this, more strategies can be put in place to prevent MSkIs in female recruits and help reduce the failure rate at basic training.

The study was a service evaluation with the aim of determining whether the 1.5-mile best-effort run can significantly predict MSkI in female recruits during British Army basic training. The secondary outcomes were looking to identify the following aspects of the MSkIs: type, week of occurrence, and cause.

Method

Participants

Retrospective data were collected from recruits’ week one 1.5-mile run times using the Physical Recreational Training Centre records of the nine single-sex female Troops (each composed of between 25 and 30 recruits) that passed through Pirbright between June 2016 and October 2017. A total of 243 female recruits passed their initial medical assessment, where doctors evaluate recruits’ suitability to continue with basic training, between June 2016 and October 2017, and were selected for analysis.

All recruits were informed at the start of their training of the following:

“Data regarding your injury will be retained for statistical purposes. Your notes may be accessed for subsequent data collection in injury trend audits. All data will remain anonymous.”

Recruits were allowed to opt out if they did not wish to have their data used.

Study design

The definition of MSkI for this study was “time out of training due to physical harm or damage”. Time out of training included missed training due to light duties chits, a ‘fit note’ issued by doctors, nurses and combat medical technicians where a recruit’s training activities have specified restrictions or amendments due to illness or injury; and referrals to the Primary Care Rehabilitation Facility (PCRF).

Defence Medical Information Capability Programme (DMICP), which holds the medical records of all military personnel, was accessed to identify recruits who were injured during basic training. The cause, type and week of MSkI, and whether it was just medically managed or included a PCRF referral were recorded. Both cause and type of MSkI were initially documented using free text. Cause of MSkI was later categorised according to the various activities that recruits took part in (eg, marching, tabbing (speed walking with weight), assault course, running). The type of MSkI was later divided down according to anatomical location. As this study was mainly focused on comparing injured recruits with injury-free recruits, only the first MSkI from a recruit’s time in basic training was included in the analysis. The recruits’ 1.5-mile run times from their Assessment Centre were also recorded to compare to their week 1 times.

Data analysis

All the data were entered onto an Excel spreadsheet split down into their Troops. The following data was recorded: selection and week 1 run times; ‘yes’ or ‘no’ for MSkI occurrence and PCRF referral; training outcome; and cause, type and week of MSkI.

Data were analysed using SPSS V.25 for Mac. A binomial logistic regression was used to examine whether run times were a significant predictor of MSkI occurrence, with the alpha value set at p <0.05. The Omnibus Test of Model Coefficients and Hosmer-Lemeshow Tests were performed to assess the fit of the data to this model. Receiver operating characteristic curves were used to compare sensitivity and specificity of the 1.5-mile run times in predicting MSkI occurrence.

MSkI types were classified as upper limb, lower limb, back or other. Lower-limb MSkI were further subdivided into upper leg, knee, lower leg, and foot or ankle. The recruits were split in half and also into quintiles based on their run times to compare injury rates in the different groups.

Of the 243 recruits selected for analysis who had week one 1.5-mile run times recorded, two recruits could not be located on DMICP, as they had no current or previous DMICP record; seven recruits had left basic training without documentation of MSkI but before completing it; and seven recruits were still in basic training. This left a total of 227 female recruits for further analysis. Eleven recruits were included who had a documented MSkI but did not complete basic training. There were five recruits who did not have a week 1 run time recorded and their selection 1.5-mile run times were used for analysis instead.

Results

The logistic regression showed 1.5-mile run time to be a statistically significant predictor of MSkI, χ2 (1)=12.91, p<0.0005. The model explained 7.4% (Nagelkerke R2) of the variance in MSkI and correctly classified 63.9% of cases. Sensitivity was 59.3%, specificity was 68.1%, positive predictive value was 62.7% and negative predictive value was 74.3%. Increasing run time was associated with an increased risk of MSkI and every second increase in time increased the odds of being injured by 1.008 (95% CI 1.003 to 1.012, p<0.05).15 Thus, for every 10 s increase in run time, the odds of MSkI increased by 8.3%. Mean 1.5-mile run time for injury-free recruits was 12 min 13 s and 12 min 43 s for injured recruits.

One hundred eight of the 227 recruits (47.6%) were injured during basic training. Moreover, 79.2% of injuries were lower limb and 36.9% of those were foot or ankle, which equated to almost 30% of all MSkIs (Figure 1). Four of the recruits’ MSkIs were stress fractures of the lower limb—two fractured necks of femur, one pubic ramus and one tibial plateau. With regard to week of injury, 72% (78 recruits) sustained an MSkI during the first 7 weeks (Figure 2). MSkIs were most commonly reported in weeks 5 (21%) and 3 (14%).

Figure 1

Anatomical location of musculoskeletal injuries.

Figure 2

Number of female recruits injured per week of basic training between June 2016 and October 2017.20 IMA, initial medical assessment; PFA, physical fitness assessment.

The data were also analysed for discrete time cut-offs associated with incidence of MSkI. Among the faster half of recruits (8 min 37 s to 12 min 34 s), 34% of the 114 recruits were injured. Within the slower half (12 min 35 s to 15 min 58 s), 61% of the 113 recruits were injured. When grouping the recruits into quintiles (Figure 3), the slowest quintile (13 min 20 s to 15 min 58 s) had double the rate of MSkI compared with the fastest quintile (8 min 37 s to 11 min 29 s). From 12 min 46 s (the third quintile and slower), the incidence of MSkI was greater than 50%.

Figure 3

Percentage injured compared with 1.5-mile run quintiles.

Fewer than 5% (4.8%) of the 227 recruits failed to complete basic training but were included in analysis due to documented MSkI. Of these 11 recruits, all except one stopped within the first 7 weeks. Their average 1.5-mile run time was 12 min 55 s, compared with 12 min 13 s for the injury-free recruits and 12 min 43 s for the injured recruits who did all complete basic training.

Discussion

This study found that the 1.5-mile best-effort run time significantly predicted the incidence of MSkI in British Army female recruits during basic training. The incidence of MSkI in British Army female recruits was also found to be much higher than in male recruits, 47.6% and 19.4%,11 respectively. The incidence in this study was, instead, comparable with the rates from a 2015 prospective study of MSkI during infantry training (48.65%).16 The higher MSkI rates in female recruits is supported by a number of studies1 4 5 7 that describe an increased risk of MSkI among female recruits compared with male recruits.

The disparity in incidence rates between female and male recruits might also be explained by the differing terrains of ATC Pirbright compared with ATR Bassingbourn of Hall’s 2017 study. The differing definitions of MSkI between these two studies should also be considered. In Hall’s article, MSkI occurrence was defined as MSkI leading to a subsequent PCRF referral.11 There was concern that this underestimated MSkI rates and so the definition was expanded for this study to include any MSkI that resulted in time out of training.11

The endurance, frequency and load carrying of physical training (PT) escalates substantially during weeks 2 to 3, and previous research has shown the peak week for MSkI in British Army basic training to be week 3 (O’Leary TJ, Saunders SC, McGuire S et al. Sex differences in the physical demands, physical adaptations and training outcomes in British Army Regular Soldier Training. Army Recruiting & Training Division; May 2017), coinciding with the initial peak shown in Figure 3. MSkIs caused by sudden increases in training intensity is supported by a 1999 study of the epidemiology of MSkI in US Marine Corps.17 The peak identified in week 5 corresponds with the first exercise on which the recruits undertake; they are working in variable terrain and suffering from a lack of sleep, which are all likely to increase their risk of MSkI.18 It may also signify the recruits’ unwillingness to report MSkIs before this milestone exercise to avoid the risk of being removed from it.

This study does not recommend using a run time cut-off, such as 12 min 30 s, as a new entry fitness standard for the British Army, as the high MSkI rates do not directly translate to failing basic training. Although the 11 recruits who did not pass basic training had, on average, slower run times, there were still 50 female recruits with run times greater than 12 min 30 s that passed basic training without acquiring a MSkI. The 1.5-mile best-effort run time could, however, be used as a guideline to indicate which recruits would benefit from further PT and possible referral to a pre-conditioning course prior to basic training. Furthermore, these recruits could be highlighted to PTIs and training teams with the focus on observing for early signs of overuse injuries.

When comparing selection and week 1 run times, it was noted that 19 recruits were more than 1 min slower in their week 1 run time compared with their selection run time, and four of these recruits were more than 2 min slower. It was beyond the scope of this study to establish causes for this discrepancy, but it would be helpful to know the length of time each recruit had to wait between their attendance at an Assessment Centre and starting basic training. Perhaps the increase in 1.5-mile run times was due to a reduction in aerobic fitness as candidates reduced or stopped training if they had passed their physical entry test. Going forward, it would be interesting to investigate whether there is a relationship between waiting time from Assessment Centre to basic training and risk of MSkI. A potential follow-on from this would be specifying a time limit between Assessment Centre and the start of basic training, longer than which the recruit would need to revalidate their physical tests. Regardless, these data support a review of the current advice provided during the recruitment process, with potential improvements to pre-basic training fitness through training plans and classes.

‘Gender streaming’, where male and female recruits follow the same syllabus but are trained in single-sex platoons, was introduced in 2006 at ATC Pirbright to reduce the incidence of overuse injuries in female recruits.19 An audit the year following its implementation showed a 47% decrease in medical discharge from overuse injuries.19 This implicates mixed sex training as another risk factor for MSkI but also suggests that investigating with the single-sex platoons at ATC Pirbright has eliminated it as a confounding factor in this study. Hall’s study was conducted at a male training establishment and thus also eliminates the confounding factor. However, as mixed-sex training still takes place within British Army training establishments, it would be interesting to add data from such environments for further comparison that might guide future policy.

Limitations

This study investigated solely the relationship between 1.5-mile run times and risk of MSkI. Due to time and financial constraints, it did not analyse confounding and potential contributory factors such as age, smoking, pre-existing injuries, obesity or previous levels of physical activity.

It was not possible to ascertain the most common cause of MSkI due to documentation and recall bias. From information gathered during DMICP note reviews, recruits would often present with a progressive complaint after a prolonged period of discomfort. It is also important to note that the recorded week of MSkI is the week that the recruit presented to the MTF, but may not necessarily reflect the week that the injury developed. As previously mentioned, they may well have had problems before, but only in the week stated did they feel it was bad enough to seek medical attention.

This study recorded each recruit’s outcome from basic training. With a larger cohort of female recruits, it would be judicious to investigate the relationship between basic and Initial Trade training outcome and 1.5-mile run time as Hall did with the male recruits at ATR Bassingbourn.11

Conclusion

This study indicates that almost half of the female recruit population in the British Army sustain an MSkI during basic training. In this service evaluation, most were lower-limb MSkIs (79.2%) and 72% of MSkIs occurred within the first 7 weeks of training. Moreover, 1.5-mile run time was a significant predictor of the probability of MSkI among female recruits during basic training and, in the future, may be used to guide potential injury prevention schemes and reduce time out of training.

References

Footnotes

  • Contributors RH was the lead on data collection, analysis and discussion. HJS provided support and guidance regarding data collection and analysis and proof-read the publication.

  • Funding Both authors are employed by the British Army. No additional funding from external sources was given to conduct the study.

  • Competing interests None declared.

  • Patient consent Not required.

  • Ethics approval Ethical approval for retrospective data retrieval and analysis was obtained from the Caldicott Guardian at ATC Pirbright Medical Treatment Facility (MTF).

  • Provenance and peer review Not commissioned; externally peer reviewed.