From the initial search, a total of 2,287 articles were identified, from which 1,034 duplicates were removed, leaving 1,253 articles for further screening (Figure 1). After screening by titles and abstracts, 79 articles remained and were examined in full text. Finally, 25 studies were deemed eligible for inclusion in the review. Reasons for exclusion of articles examined in full text can be found in Figure 1.
Included studies were mainly cohort studies (n=19) [6, 20-37]. The remaining studies included five cross sectional studies [4, 38-41], and one case control study [42]. Methodological quality ranged from 60% [4] to a perfect score of 100% [22], with an average of 82% across all study types. Key data from each included study are presented in Table 2.
The studies were from a range of countries, with 19 from the United States [4, 6, 20, 21, 24-26, 28, 29, 31-36, 39, 41, 42], two from both the United Kingdom [22, 37] and one each from Norway [38], Greece [27], Ireland [30], and Slovenia [40]. Army was the most represented service, accounting for 18 studies [4, 6, 20-22, 24-33, 37, 39, 42], followed by Air Force, with four studies [23, 34, 36, 41]. Two studies were of the Armed Services more broadly [38, 40] and the remaining study involved Marines [35]. Investigations were most often conducted in basic training contexts, accounting for 14 of the studies [21-23, 28-33, 36, 41, 43-45], followed by active duty personnel for eight studies [4, 20, 25, 34, 37, 38, 40, 46], officer training for three studies [6, 27, 35], and Advanced Individual Training (AIT) or Initial Employment Training (IET) for two studies [26, 39]. All of the training performed in contexts outside the United States occurred in mixed sex training programs [27, 30, 37, 38, 43].
The most common method of obtaining injury data in the included studies was by reviewing medical records (n=19), while some studies used self-report surveys [4, 20, 38], or a mixture of methods [23, 26, 40]. Only four studies accounted for fitness levels when comparing injury rates between the sexes [4, 6, 21, 22].
Injury Rates
A total of 20 of the included studies reported that female personnel were injured at a higher rate than male personnel. One study reported that being deployed on operations was associated with lower injury rates for both sexes than pre-deployment training, however a significantly greater injury rate was found amongst female personnel when compared to male personnel, pre-deployment, while rates were similar during deployment [20]. Four studies reported that there were no differences in injury rates between male and female personnel [24, 30, 38, 40]. Two studies [6, 39] followed personnel through to later in their initial training, observing injury rates of officer cadets either at the end of their first semester and then second semester [6], or recruits after initial training and during Advanced Individual Training [39], and found there were no significant differences in injury rates between female and male personnel during those later training stages, despite initial higher rates in female personnel.
Table 2: Key data from included studies
Author, Year
|
Population
|
Reporting Method
|
Factors for which findings were adjusted
|
Key Findings
|
Injury Incidence Rate/1000/year*
|
RR (♀:♂)
|
Critical Appraisal Score
|
1. Anderson, 2017 [4]
|
Active-Duty US Army (363 females, 4384 males) over 12 months.
|
Self-reported survey
|
Gender, age, BF, smoking, fitness physical demand,
|
Injury incidence over 12 months
♀ = 53%
♂ = 42%
|
♀ = 530/1000/year
♂ = 420/1000/year
|
1.26 [1.14-1.40] p<0.01)
aRR=0.95 [0.86-1.05]
|
60%
|
2. Anderson, 2015 [20]
|
Active-Duty US Army (43 women, 727 men) over 12 months
|
Self-reported survey
|
None
|
Injury rate:
Pre-deployment
♀ 42.6/1000/month
♂ 36.2/1000/month
On-Deployment
♀ 14.0/1000/month
♂ 19.0/1000/month
|
♀ = 511.2/1000/year
♂ = 434.5/1000/year
♀ = 168.0/1000/year
♂ = 228.0/1000/year
|
1.18 [0.87-1.59]
p=0.33
0.71 [0.36-1.42]
p=0.33
|
83%
|
3. Bell, 2000 [21]
|
Trainees undergoing US Army Basic Training over 8 weeks (352 women, 509 men)
|
Medical Records
|
Fitness, Age and Race
|
Injury rate over 8 weeks of basic training:
♀ = 57%
♂ = 27%
|
♀ = 3721.8/1000/year
♂ = 1767/1000/year
|
2.1 [1.78-2.47]
aRR = 1.14 [0.48-2.72]
|
92%
|
4.Bijur 1997 [6]
|
558 West Point Officer Cadets in 6 weeks of basic training and initial 6 months of all training (85 women, 473 men)
|
Medical Records
|
Fitness
|
Injury rate in 6 weeks of basic training:
♀ = 67/85 cadets 78.8/100 cadets
♂= 152/473 cadets 32.1/100 cadets
After adjusting for fitness, difference between males and females was only 20/100 cadets, as opposed to 46.7/100 previously (including multiple injuries)
By end of 1st semester (subsequent 20 weeks of training):
♀ = 47.5/100 cadets 4
♂ = 40.5/100 cadets (p=0.42)
End of 2nd Semester (2nd 6-month period of training)
♀ = 32.3/100 cadets
♂ = 38.8/100 cadets (p=0.42)
|
♀ = 6829.3/1000/year
♂ = 2782.0/1000/year
♀ = 1235/1000/year
♂ = 1053/1000/year
♀ = 646/1000/year
♂ = 776/1000/year
|
2.45 [2.06-2.91]
p<0.001
1.16 [0.90-1.49]
p=0.25
0.82 [0.59-1.14]
p=0.23
|
92%
|
5. Billings 2004 [23]
|
Recruits in US Air Force Academy Basic Training over 12 months (224 women, 986 men)
|
Combination of 3 systems
|
None
|
Injury Incidence per annum
♀= 1,067 per 1000 cadets
♂= 615.6/1000 cadets
No difference in number of restricted duty days.
|
♀ = 1067/1000/year
♂ = 615.6/1000/year
|
1.73 [1.67-1.80]
|
75%
|
6. Blacker, 2008 [22]
|
Recruits in British Army Basic Training over 12 weeks (1,480 females, 11,937 males)
|
Medical Records
|
Fitness
|
Injury Incidence over 12 weeks basic training
♀=0.173/100 person days (202/1,480)
♂=0.061/100 person days (550/11,937)
HR 2.91 [2.48-3.43] p<0.001
Sex did not feature in the multivariate model predicting injury risk. 2.4km run time was a significant risk factor for injury (p<0.001).
|
♀ = 631.5/1000/year
♂ = 222.7/1000/year
|
2.96 [2.55-3.44]
p<0.001
|
100%
|
7. Cosio-Lima 2013 [24]
|
Active-duty US Army soldiers over 9.5 months
(6 females, 143 males)
|
Medical Records
|
None
|
Injury Incidence in 9.5 months
♀ = 33%
♂ = 50.3%
|
♀ = 422.6/1000/year
♂ = 644.8/1000/year
|
0.66 [0.21-2.08]
p=0.48
|
75%
|
8. Darakjy, 2006 [25]
|
Active-Duty US Armor Division soldiers during 37 days of continuous training
(413 females, 4,101 males)
|
Medical Records
|
None
|
Injury rate in 37 days
♀ =22.9/1000 soldiers/week
♂ =11.8/1000 soldiers/week
|
♀ = 1,190.8/1000/year
♂ = 613.6/1000/year
|
1.93 [1.54-2.32]
p<0.001
|
83%
|
9. Fadum, 2019 [38]
|
Active-Duty Norwegian Military Women over 12 months
(1,068 females, 8,100 males)
|
Self-reported Survey
|
None
|
Injury rates over 12 months
♀= 284/1068 (26.6%)
♂= 2034/8100 (25.1%)
|
♀ = 265.9/1000/year
♂ = 251.1/1000/year
|
1.06 [0.95-1.18]
p=0.29
|
85%
|
10. Grier, 2011 [26]
|
Trainees at US Army Ordinance school with courses ranging from 9-16 weeks (498 females, 3,757 males)
|
Self-reported survey & medical records (prospective and retrospective)
|
* Stratified
|
Injury rate for 9-16 weeks training:
♀ = 61%
♂ = 36%
|
|
1.70 [1.56-1.84]
p<0.001
|
83%
|
11. Havenetidis, 2011[27]
|
Greek Army Officer Cadets in training for 7 weeks.
(20 females, 233 males)
|
Physician Recorded
|
None
|
Injury Rate during 7 weeks of training
♀ = 35%
♂ = 31.7%
|
♀ = 2607.1/1000/year
♂ = 2365.8/1000/year
|
1.10 [0.59-2.06]
p=0.76
|
67%
|
12. Henderson, 2000 [39]
|
US Army Recruits undergoing Basic Training for 8 weeks (237 females, 371 males)
and trainees in US Combat Medics AIT (10 week) (287 females, 439 males)
|
Medical Records
|
None
|
BCT Cumulative Injury Incidence
♀ = 51.5% (122/237)
♂ = 26.1% (97/371)
AIT Cumulative Injury Incidence
♀ = 29.6% (85/287)
♂ = 23.7% (104/439)
|
BCT
♀ = 3355.2/1000/year
♂ = 1704.1/1000/year
AIT
♀ = 1544.3/1000/year
♂ = 1235.3/1000/year
|
1.97 [1.59-2.43]
p<0.001
1.25 [0.98-1.60] p=0.078
|
75%
|
13. Jones, 1993 [28]
|
Recruits in US Army Basic Training for 8 weeks (186 females, 124 males)
|
Medical Records
|
None
|
Injury rates
♀ = 50.5%
♂ = 27.4%
|
♀ = 3294.0/1000/year
♂ = 1787.2/1000/year
|
1.84 [1.34-2.54]
p<0.001
|
83%
|
14. Jones, 2017 [29]
|
Recruits in US Army Basic Training for 8 weeks (41,727 females, 143,398 male)
|
Medical Records
|
None
|
Injury rates:
♀ = 40.3%
♂ = 15.7%
|
♀ = 2626.7/1000/year
♂ = 1023.3/1000/year
|
2.57 [2.52-2.61] p<0.001
|
83%
|
15. Kerr, 2004 [30]
|
Recruits in Irish Army Basic Training for 16 weeks (40 females, 354 males)
|
Medical Records
|
None
|
Injury Incidence
♀ = 99.26 injuries/1000-man weeks
♂ =56.96 injuries/1000-man weeks
Injury rates
♀ = 54/40
♂ = 271/354
|
♀ = 5161.5/1000/year
♂ = 2961.9.1/1000/year
|
1.76[1.06-2.94]
|
75%
|
16. Knapik, 2002 [33]
|
Recruits in US Army basic training for 8 weeks
(Summer: 434 females, 733 males)
(Fall: 591 females, 810 males)
|
Medical Records
|
None
|
Summer injury rates
♀ = Summer 63.1%
♂ = Summer 37.0%
Fall injury rates
♀ = 44.8%
♂ = 18.9%
Total injury rates:
♀ =539/1025
♂=424/1,543
|
Summer
♀ = 4115.0/1000/year
♂ = 2409.7/1000/year
Fall
♀ = 2922.6/1000/year
♂ = 1231.2/1000/year
♀ = 3427.4/1000/year
♂ = 1791.0/1000/year
|
1.71 [1.52-1.92]
p<0.001
2.37 [2.00-2.81]
p<0.001
1.91 [1.73-2.11]
p<0.001
|
92%
|
17. Knapik, 2001 [31]
|
Recruits in US Army basic training for 8 weeks (247 females, 567 males)
|
Medical Records
|
None
|
Injury rates for ‘control group’
♀ = 1.07/100 person days (148/247)
♂ = 0.56/100 person days (178/567)
|
♀ = 3905.4/1000/year
♂ = 2046.2/1000/year
|
1.91 [1.63-2.24]
p<0.001
|
83%
|
18. Knapik, 2001b [32]
|
Recruits in US Army Basic Training for 8 weeks) (452 females, 733 males)
|
Medical Records
|
None
|
Injury Incidence Rates
♀ 1.16/100 person days
♂ 0.56/100 person days
|
♀ = 4234.0/1000/year
♂ = 2044.0/1000/year
|
2.07 [1.83-2.35]
p<0.001
|
92%
|
19. Kovcan, 2019 [40]
|
Active-Duty Slovenian Armed Forces over one year. (11 females, 118 males)
|
Injury Report Form
|
None
|
Prevalence:
♀ = 27.7%
♂ = 50.8%
|
♀ = 272.7/1000/year
♂ = 508.5/1000/year
|
0.54 [0.20-1.43]
p=0.21
|
75%
|
20. Nye, 2014 [34]
|
Active-Duty USAF monitored over 7 years. (6,398 females, 61,506 males)
|
Database
|
None
|
Injury Rates over 7 years:
♀ = 71.8% (4,597/6,398)
♂ = 67.1% (41,278/61,506)
|
♀ = 102.6/1000/year
♂ = 95.9/1000/year
|
1.07 [1.05-1.09]
p<0.001
|
92%
|
21. Nye, 2016 [41]
|
Recruits in USAF Basic Training (8.5 week) 14,550 females, 52,975 males)
|
Database
|
None
|
Overall Incidence Rate in 8.5 weeks training:
♀ = 29.4 [28.6-30.3] injuries /1000 person weeks (2862/14550)
♂ = 15.1 [14.7-15.4] injuries/1000 person weeks (5586/52975)
|
♀ = 1196.6/1000/year
♂ = 641.5/1000/year
|
1.87 [1.79-1.94]
p<0.001
|
80%
|
22. Piantanida, 2000 [35]
|
Cadets in US Marines officer Basic training for 6 weeks. (30 females, 450 males)
|
Medical Records
|
None
|
Cumulative Injury Incidence in 6 weeks
♀ = 80%
♂ = 59.5%
|
♀ = 6952.4/1000/year
♂ = 5175.7/1000/year
|
1.34 [1.11-1.63]
p<0.003
|
75%
|
23. Snedcor, 2000 [36]
|
Recruits in US Airforce Basic Training for 30 training days. (5,250 females, 8,660 males)
|
Outpatient Visits
|
None
|
Injury rate over basic training
♀ = 63.0/1000 person weeks [60.6-65.5] (1743/5250)
♂ = 27.8/1000 person weeks [26.4-29.2] 1329/8660)
|
♀ = 3276.0/1000/year
♂ = 1445.6/1000/year
|
2.16 [2.03-2.30]
aRR = 2.22 [2.09-2.37]
|
83%
|
24. Strowbridge, 2002 [37]
|
Active-Duty British Army Personnel over 1 year (178 females, 3,377 males)
|
Medical Records
|
None
|
Injury Rate over 1 year
♀ = 44.5/1000/month
♂ = 26.6/1000/month
|
♀ = 534.0/1000/year
♂ = 319.2/1000/year
|
1.67 [1.45-1.93]
p<0.001
|
67%
|
25. Sulsky, 2018 [42]
|
Recruits in US Army Basic Training for 8 weeks. (21,651 females, 139,020 males)
|
Medical Records
|
None
|
At least One injury in basic training
♀ = 61% (16,833/21,651)
♂ = 39% (54,784/139,020)
|
♀ = 5067.4/1000/year
♂ = 2568.5/1000/year
|
1.97 [1.95-1.99]
p<0.001
|
91%
|
RR= Relative Risk, aRR = Adjusted Relative Risk, BF = Body Fat, N/S = not significant. * Calculated injury incidence rate per 1000 personnel per year
A total of 12 studies reported on injury rates during basic training. A meta-analysis of key findings from these studies (Figure 2) yielded a higher incidence rate of injury during basic training in female personnel than in males (RR = 2.10 [95% CI = 1.89-2.33]), however with a high level of heterogeneity across the 12 studies (I2=99%).
Four studies reported on injury rates during officer training, including at the US Military Academy at West Point [6], the US Air Force Academy [23], the Greek Hellenic Army Academy [27], and Marine Corps Officer training [35]. Cadets were monitored for a duration ranging from six weeks [6, 35] to one year [23]. Meta-analysis of the four studies (Figure 3) found a higher incidence rate of injury amongst female personnel, with a RR of 1.70 [95% CI = 1.33-2.17].
Eleven articles reported on injury rates after the completion of basic training, including during AIT [26, 39], at the latter stages of training at the US Military Academy [6], during a Sergeant Majors’ course [24], in active duty United States military personnel [4] and British Army personnel [37], in a US armour division [25], in active duty personne in the US Air Force l [34] and Norwegian [38] and Slovenian Armed Forces [40], and both pre and during deployment in the study by Anderson et al [20]. Figure 4 shows a meta-analysis of these studies, which found the overall injury incidence rate was higher in female personnel than male personnel (RR=1.23 [95% CI = 1.05-1.43]).
Of the articles which took fitness into account, three of the four [4, 21, 22] found that differences between the sexes in injury incidence rates were no longer significant once adjusted for fitness levels. However, the fourth study, despite finding the difference in injury rates between sexes decreased substantially, still showed a significantly greater injury incidence rate among female cadets [6]. Both the study by Anderson et al., [4] and that by Bell et al., [21] provided risk estimates which were combined in a meta-analysis (Figure 5), which in turn found no difference in injury rates between female and male personnel (RR=0.95 [95% CI = 0.86-1.05]). Despite not providing adjusted risk estimates for each sex, the study by Blacker et al., [22] found that gender did not feature in a multivariate regression model which explored risk factors for injury, while fitness level did.