Association Between Reproductive Health Factors and Musculoskeletal Injuries in Female Canadian Armed Forces Members

Abstract

Background:

Musculoskeletal injuries (MSKi) play a role in member retention in the military. In general, female military members have higher rates of MSKi than males and female reproductive health characteristics may be contributing to these disparities. This study seeks to characterize reproductive health factors in female Canadian Armed Forces (CAF) members and their relationship with MSKi.

Materials and Methods:

An electronic survey (SurveyMonkey^®) was made available to present and former CAF members 18–65 years of age. Responses were collected between September 2020 and February 2021. Seven female reproductive characteristics were assessed: age of menarche, menstrual cycle regularity, birth control use, having given birth while serving, endometriosis, early menopause, and secondary oligomenorrhea/amenorrhea. Binary logistic regressions were used to analyze associations between reproductive characteristics with repetitive strain (RSI) and acute injuries.

Results:

A total of 2,001 participants consented to the survey with 855 respondents being female. Females reporting menstrual cycles as never regular, irregular for a few months, who never had a period, and whose periods stopped while serving presented a greater likelihood of reporting RSI compared to their peers who reported regular menstrual cycles (adjusted odds ratio [aOR]: 1.898, confidence interval [CI]: 1.138–3.166). Participants who have experienced endometriosis presented a higher likelihood of reporting acute injuries than those who did not (aOR: 2.426, CI: 1.030–5.709).

Conclusion:

This examination of females within the CAF suggests that irregular menstrual cycles or absent periods increase the likelihood of experiencing MSKi, namely those categorized as RSI injuries and reporting endometriosis were associated with greater rates of acute injuries.

Introduction

The Canadian Armed Forces (CAF) aims to increase the presence of female members from 16% (August 2020) to 25% representation by 2026.¹ With the goal of meeting this rate of female representation, the CAF is committed to developing solutions to increase the recruitment and retention of its female members.² To assist the CAF in meeting their target, one must gain a better understanding of the obstacles and barriers female members face. This discrepancy between male and female recruitment and retention rates is not unique to the CAF, it is common in militaries throughout the world. For example, the US military has identified several barriers to retention of female members, including policy surrounding reproductive health care and injury.^3,4

The Canadian Surgeon General recognizes musculoskeletal injuries (MSKi) as a fundamental factor impacting operational readiness and military members' health.⁵ In 2017, Canada allocated ∼$200 million toward building strategies aimed at reducing injury prevalence in the CAF,⁶ and this effort uncovered that females are more likely to be medically released due to MSKi when compared to their male counterparts.⁷ Furthermore, other military bodies, like the British Armed Forces, also face retention challenges, as overuse injuries are the most common cause of medical discharge and the rates of MSKi are found to be higher in women.⁸

With the adoption of gender-free policies and standards, the disparity between rates of MSKi in male and female members has grown even larger, with the likelihood of overuse injuries rising from 4:1 to 7.5:1, for female to male recruits, respectively.⁹ This disparity persists, with a report in 2016 demonstrating female trainees may have a up to six times higher overuse injury rates, depending on location, than their male counterparts.⁸

Our research team conducted a scoping review to synthesize factors that may result in higher risk of MSKi and identify gaps in the current literature.¹⁰ The findings highlighted that being female (vs. male) was associated with a greater risk of MSKi, stress fractures, and general injuries. Furthermore, the scoping review identified that reproductive health factors are implicated and should be taken into account when studying risk of MSKi in female military members. Similarly, a recent report on injuries in the CAF highlighted a need to explore reproductive characteristics as most female CAF members are in their reproductive years.¹¹

Reproductive health factors that have been previously linked with MSKi in female military members are a history of amenorrhea, having no or irregular menses, and late age at menarche.¹⁰ The reasons for the late age of menarche and menstrual irregularities as predictors of increased MSKi risk are not fully understood. It has been proposed that these irregularities might be due to shorter lifetime exposure to endogenous estrogen.¹² The literature is somewhat inconsistent in this regard, as the findings of a recent qualitative systematic review exploring the relationship between late age of menarche, secondary amenorrhea, and MSKi in military populations did not support late age of menarche as a risk factor for MSKi.

Furthermore, there is insufficient scientific evidence for the association between secondary amenorrhea and MSKi.¹³ We have accounted for the results of our scoping review,¹⁰ and other findings from the literature^14
–16 linking reproductive health factors with MSKi in civilian (an individual not in the armed services or police force) or military females. Based on this information, we created an electronic questionnaire to explore the association between seven female reproductive health factors and (i) repetitive strain (RSI) and (ii) acute injuries (described in Table 1). The questionnaire was developed in consultation with an expert panel composed of various military and nonmilitary authorities (e.g., female reproductive health specialists, doctors, physiotherapists, physiologists, and project stakeholders).

Table 1.

Selected Reproductive Health Factors and Reasons for Inclusion in the Study Questionnaire

No.	Reproductive health factor	Examples of previous findings suggesting a relationship with MSKi
1	Age of menarche	Predictors of MSKi, particularly stress fractures in military populations.¹⁰
2	Menstrual cycle periodicity	Predictors of MSKi, particularly stress fractures in military populations.¹⁰
3	Use of hormones for birth control	Female civilians 25–34 years of age, who use oral contraceptives were at an increased risk of ACL injury compared to women who do not use oral contraceptives.¹⁵
4	Given birth while serving in the CAF	Pregnancy and the postpartum period have been associated with a high risk of MSKi in female soldiers.^17,18 Relaxin is an insulin-like hormone produced by the ovaries, which works with estrogen throughout pregnancy to reduce elasticity of the ligaments.^19,20 This leads to joints becoming relaxed, increasing risk of injury.²¹
5	Endometriosis	Among people with endometriosis, the mechanisms that facilitate normal ovulation are impaired,²² with FSH levels being significantly higher in female individuals with stage III/IV endometriosis.²³ FSH has been found to be negatively correlated with bone mineral density,²⁴ which may increase the risk of MSKi.
6	Early menopause	Menopause was associated with a notable decrease in estrogen production by the ovaries. Given that estrogen promotes bone formation and inhibits bone resorption,²⁵ there is a decrease in bone mineral density, leaving female individuals more susceptible to acute fractures.¹⁶
7	Secondary amenorrhea	It was positively related with fracture rates among army recruits¹⁴ and was associated with an increased risk of stress fractures in military populations.¹⁰

ACL, anterior cruciate ligament; CAF, Canadian Armed Forces; FSH, follicle-stimulating hormone; MSKi, musculoskeletal injuries.

Knowing that most female CAF members are in their reproductive years,¹¹ investigating their reproductive health is essential for reducing the burden of MSKi and could inform future mitigation strategies to improve retention rates. To our knowledge, the reproductive profile of female CAF members and its role in MSKi have yet to be characterized. Therefore, this study explores how the reproductive characteristics and related conditions may contribute to the MSKi rates in female CAF members.

Materials and Methods

Study design and sample

This study is a retrospective case–control study. The data presented are a planned subanalysis of a larger study examining sex disparities related to MSKi, reproductive health, recruitment, and retention in the CAF. The inclusion criteria for the larger study were as follows: (i) being a current regular (including reserves), retired, or medically released, member of the CAF, (ii) being between 18 and 65 years of age, (iii) being able to provide informed consent to participate, and (iv) indicated biological sex. For this study, we only included participants who answered “female” for biological sex. Participants were instructed to report MSKi experienced while they were “a serving member.”

The questionnaire, developed explicitly for the CAF, was available electronically through the online cloud-based survey development software SurveyMonkey Inc., (San Mateo, CA, USA). All participants provided their informed consent on the online platform. The survey was released in both English and French between September 2020 and February 2021. This study was approved by the University of Ottawa Research Ethics Board (H-04-19-3442) and all the procedures adopted were in accordance with the Declaration of Helsinki.

Participants were recruited using a variety of methods. The research team utilized social media accounts (e.g., lab website, Facebook, Twitter), and the social networks of the researchers involved, in addition to project stakeholders (e.g., Canadian Forces Morale and Welfare Services, Personnel Support Programs, Defense Fitness), and electronic communications.

The minimum required sample size for the overarching project was estimated accounting for (i) representation of the overall CAF population and (ii) previously reported odds ratios (OR) from studies comparing MSKi rates from females versus males in the military context. A post-hoc sample size calculation was conducted for the present subanalyses aiming to include only female respondents. The estimated number of CAF members (including Regular Forces and Reserves) is 103,873 with 16% of female representation (as of August 2020; n = 16,620).¹ Considering the estimated prevalence of RSI (∼35%) and acute injuries (∼19.8%) reported in female CAF members,¹¹ maximum error of ±5%, and a design effect of 1.5, the minimal sample size for females was n = 508 and n = 355 for RSI and acute injury, respectively.

Variables

Independent variables, covariates, and outcomes selected to assess the impact of reproductive health factors on MSKi are described in Table 2. We describe in this table the original questionnaire categories and the subsequent breakdown used for the analysis.

Table 2.

Independent Variables, Covariates, and Outcomes

No.	Independent variable	Original categories for response	Categories and reference for analysis
1	Age of menarche	(1) 10 years or less (2) 11–15 years (3) 16 years or more (4) I never had a period (5) I don't know	(1) <10 years (2) 11–15 years (reference) (3) ≥16 years
2	Menstrual cycle periodicity	(1) Regular (2) Never regular (3) Irregular for few months (4) Periods stopped while serving (5) Never had a period	(1) Regular (reference) (2) Never regular, irregular for few months, periods stopped while serving and never had a period
3	Use of hormones for birth control	(1) Yes (2) No	(1) Yes (2) No (reference)
4	Given birth while serving in the CAF	(1) Yes (2) No	(1) Yes (2) No (reference)
5	Endometriosis	(1) Yes (2) No	(1) Yes (2) No (reference)
6	Early menopause^a	(1) Yes (2) No	(1) Yes (2) No (reference)
7	Secondary oligomenorrhea/amenorrhea^b	(1) Yes (2) No	(1) Yes (2) No (reference)

No.	Covariate	Original categories for response	Categories and reference for analysis
1	Year of enrollment	Continuous variable (years)	Continuous variable (years)
2	CAF rank	(1) Junior NCM—Pte/OS/AB/Avr (2) Junior NCM—Cpl/LS (3) Junior NCM—MCpl/MS (4) Senior NCM—Sgt/PO2 (5) Senior NCM—WO/PO1 (6) Senior NCM—MWO/CPO2 (7) Senior NCM—CWO/CPO1 (8) Junior Officer—OCdt/NCdt—2Lt/A/SLt (9) Junior Officer—Lt/SLt (10) Junior Officer—Capt/Lt(N) (11) Senior Officer—Maj/LCdr (12) Senior Officer—LCol/Cdr (13) Senior Officer—Col/Capt(N)—General Flag	(1) NCMs (2) Officers (reference)
3	Current BMI classification	(1) Continuous variable (combination of current weight and height (kg/m²))	(1) Underweight and normal weight (reference) (2) Overweight (3) Obese
4	Total physical activity volume	(1) Continuous variable (combination of 5 physical activity categories; minutes/week)	(1) Continuous variable (minutes/week)
5	Parity	(1) Discrete variable (number of biological children)	(1) 0 (reference) (2) 1 (3) 2 (4) 3+
6	Dairy consumption	(1) ≥5 times/week (2) 3–4 times/week (3) ≤2 times/week	(1) ≥5 times/week (2) 3–4 times/week (3) ≤2 times/week (reference)
7	Vitamin D	(1) Never (2) Once/month (3) Once/week (4) Few times/week (5) Daily	(1) Never (reference) (2) Once/month, once/week, few times/week and daily
8	Calcium	(1) Never (2) Once/month (3) Once/week (4) Few times/week (5) Daily	(1) Never (reference) (2) Once/month, once/week, few times/week, daily

No.	Outcomes	Original categories for response	Categories and reference for analysis
1	RSI^c	(1) Yes (2) No	(1) Yes (reference) (2) No
2	Acute injuries^d	(1) Yes (2) No	(1) Yes (reference) (2) No

Defined as menopause before 46 years and not due to hysterectomy.

Defined as cessation of regular periods for 3 months or irregular periods for 6 months.

Defined as injuries to muscles, tendons, or nerves caused by overuse or repeating the same movement (e.g., ruck marching) over an extended period. For example, carpal tunnel syndrome, tennis elbow, plantar fasciitis, or tendonitis.

Defined as serious physical injuries, likely caused by a significant level of exertion or single incident of trauma, which were serious enough to require at least 24 hours off work after it to recover from. For example, a broken bone, a sprain.

BMI, body mass index; NCM, noncommissioned members; RSI, repetitive strain injuries.

Statistical analysis

Data cleaning

The data were made compatible with the analysis, using the recategorization described in Table 2, and performed using the SPSS statistical software, version 23.0 (SPSS Inc., Chicago, IL, USA).

Injury rate data were missing at a level of 19.3% for RSI and 20.4% for acute injuries in the female sample. Considering the missing data rates of ∼20% for the main independent variable and outcomes, we opted for the listwise deletion method for dealing with missing data. Although other missing data methods (e.g., multiple imputation) can reduce the standard error (SE) estimates, the work by Dong and Peng²⁶ found that the improvements when multiple imputations were compared to listwise deletion were minimal (e.g., SE: 0.091 vs. 0.086; 0.046 vs. 0.044) for a missing data rate of ∼20%. More meaningful changes in the SE can be seen when the missing data rate is ∼60%.²⁶ In addition, even accounting for the missing data, our sample is adequately powered for the main analysis.

For categorical variables with <20 observations per category, we opted to combine categories when possible (e.g., Variable: Menstrual cycle periodicity; Recategorization: [1] Regular; [2] Never regular, irregular for few months, periods stopped while serving and never had a period). This is a common procedure when categorical variables are involved to improve the interpretation of results.

Main analyses

Descriptive statistics included number and percentage for categorical variables and mean and standard deviation for continuous variables. The descriptive statistics were used to identify the sample characteristics.

We initially tested the bivariate associations between the independent variables selected and the MSKi outcomes (Table 2). The bivariate associations for 2 × 2 tables were analyzed using a chi-square test or Fisher's exact test and the bivariate associations for greater than 2 × 2 tables (e.g. _, 2 × 3) were analyzed using a chi-square test or the Likelihood ratio when the assumption of having <20% of the cells, with an expected count of <5, was not met. For the associations of reproductive health factors and MSKi outcomes in which p-values were ≥0.05, the logistic regressions were not performed.

When a p < 0.05 was found in the bivariate association between independent variables and outcomes, logistic regression analyses were performed. The logistic regressions were adjusted for covariates described in Table 2. After performing the regressions, a summary of the main findings was described in a tabular format. Adjusted odds ratios (aOR), 95% confidence intervals (CI), and p-values are described for each association tested.

Results

A total of 2,253 questionnaires were accessed on SurveyMonkey, and 2001 participants provided their consent and completed the questionnaire. Forty-nine consenting participants did not report their biological sex. We were able to determine the biological sex of two who did not respond to the question by analyzing their responses (e.g., responded to questions related to female reproductive health), thus including them in the analysis. In addition, only one (0.05%) and five (0.26%) participants responded “intersex” or “preferred not to answer,” respectively, to the biological sex question and were excluded from the analysis. A final exclusion was a civilian participant, leaving us with a total of n = 1,947, whose biological sex was known. Finally, from those, n = 855 were females and included in this study. Figure 1 illustrates the study participant flow.

FIG. 1.

Study participant inclusion flowchart.

Demographic characteristics, reproductive health, and injury rates of the available sample are reported in Table 3.

Table 3.

Participant Demographics, Reproductive Health Characteristics, and Injury Rates

Variable	Females
Variable	N	(%)
Demographics
Age (years), mean ± SD	845	38.8 ± 9.7
Year of enrollment, mean ± SD	815	2005 ± 9.9
Military branch
Canadian Army	384	45.8
Royal Canadian Navy	97	11.6
Royal Canadian Air Force	357	42.6
CAF status
Regular Force	648	75.9
Reserve Force Class A, B, C	117	13.7
Retired	40	4.7
Medically released	49	5.7
Rank
NCM	482	57.2
Officers	360	42.8
Location
Eastern provinces	156	18.2
Quebec	202	23.6
Ontario	359	42.0
Western provinces, territories, and outside Canada	138	16.1
Gender
Man	5	0.6
Woman	846	99.4
Marital status
Single and other^a	182	21.4
Married and common law	573	67.3
Widowed, separated and divorced	97	11.4
Education
Less than high school and high school	183	21.6
Trade certificate and College/CEGEP	217	25.6
University and bachelor's degree	299	35.3
Post-graduate and professional degree	148	17.5
Having children while serving^b
Yes	377	44.4
No	472	55.6
Parity
0	474	56.9
1	112	13.4
2	189	22.7
3+	58	7.0
Current BMI classification
Underweight and normal	334	41.8
Overweight	270	33.8
Obese	195	24.4
Total physical activity (minutes/week), mean ± SD	625	548.3 ± 376.6
Consumption of dairy products
≥5 times/week	246	41.8
3–4 times/week	171	29.1
≤ 2 times/week	171	29.1
Vitamin D
Never	278	52.5
1 time/month, 1 time/week, few times/week and daily	252	47.5
Calcium
Never	373	73.4
1 time/month, 1 time/week, few times/week and daily	135	26.6
Reproductive health
Age of menarche, years
<10	49	7.6
11–15	554	86.2
≥16	40	6.2
Menstrual cycle periodicity
Regular	381	58.1
Never regular, irregular for few months, periods stopped while serving, and never had a period	275	41.9
Use of hormones for birth control
Yes	500	76.1
No	157	23.9
Given birth while serving in the CAF
Yes	270	41.1
No	387	58.9
Endometriosis
Yes	57	8.7
No	600	91.3
Early menopause
Yes	31	4.7
No	626	95.3
Secondary amenorrhea
Yes	83	12.6
No	574	87.4
Injury rates
RSI
Yes	526	76.2
No	164	23.8
Acute injuries
Yes	418	61.4
No	263	38.6

Refers to individuals living alone.

Includes adoption and/or partner giving birth.

CEGEP, general and professional teaching college; SD, standard deviation.

The bivariate associations between each reproductive health factor and the MSKi outcomes can be seen in Table 4.

Table 4.

Bivariate Associations Between Reproductive Health Factors and Repetitive Strain Injuries/Acute Injuries in Female Canadian Armed Forces Members

Category	Total No.	% RSI	Chi-square p-value
Age of menarche, years
≤10	49	71.4
11–15	554	77.3	0.157
≥16	40	65.0
Total No.	603
Menstrual cycle periodicity
Yes, regular	381	72.7	0.017
Never regular, irregular for a few months, never had a period, and periods stopped while serving	275	80.7
Total No.	656
Use of hormones for birth control
No	157	70.1	0.048
Yes	500	77.8
Total No.	657
Given birth while serving in the CAF
No	387	71.8	0.002
Yes	270	82.2
Total No.	657
Endometriosis
No	600	75.7	0.394
Yes	57	80.7
Total No.	657
Early menopause
No	626	75.7	0.299
Yes	31	83.9
Total No.	657
Secondary amenorrhea
No	574	74.9	0.060
Yes	83	84.3
Total No.	657
Age of menarche, years
11–15	554	61.9
≤10	49	71.4	0.184
≥16	40	52.5
Total No.	603
Menstrual cycle periodicity
Yes, regular	381	58.5	0.072
Never regular, irregular for a few months, never had a period, and periods stopped while serving	275	65.5
Total No.	656
Use of hormones for birth control
No	157	61.8	0.931
Yes	500	61.4
Total No.	657
Given birth while serving in the CAF
No	387	60.5	0.517
Yes	270	63.0
Total No.	657
Endometriosis
No	600	59.8	0.005
Yes	57	78.9
Total No.	657
Early menopause
No	626	60.5	0.025
Yes	31	80.6
Total No.	657
Secondary amenorrhea
No	574	61.1	0.636
Yes	83	63.9
Total No.	657

The three reproductive health factors associated with RSI and the two associated with acute injuries were moved to the logistic regression analysis adjusting for potential covariates. Regarding RSI, the only reproductive factor that remained as a predictor after the adjustments was menstrual cycle periodicity. The association was adjusted for the year of enrollment, rank, parity, current body mass index classification, total physical activity, consumption of dairy products, use of calcium as a nutritional supplement, and use of Vitamin D as a nutritional supplement.

Female members with menstrual cycles reported as never regular, irregular for a few months, who never had a period, and whose periods stopped while serving presented a greater likelihood of RSI were compared to the ones who had regular menstrual cycles (aOR: 1.898, 95% CI: 1.138–3.166). The associations between use of hormones for birth control and giving birth while serving in the CAF with RSI were lost after adjustments (Table 5).

Table 5.

Binary Logistic Regression for Reproductive Health Variables Versus

Outcome: RSI	aOR	95% CI	p
(1) Menstrual cycle periodicity (Ref: Yes, regular)
Never regular, irregular for a few months, never had a period, and periods stopped while serving	1.898	1.138; 3.166	0.014^a
(2) Use of hormones for birth control (Ref: No)
Yes	1.490	0.843; 2.632	0.170
(3) Given birth while serving in the CAF (Ref: No)
Yes	0.930	0.180; 4.807	0.931

Outcome: acute injuries	aOR	95% CI	p
(1) Endometriosis (Ref: No)
Yes	2.426	1.030; 5.709	0.042^a
(2) Early menopause (Ref: No)
Yes	1.219	0.302; 4.928	0.781

RSI and acute injuries after adjusting for covariates in female CAF members.

RSI—N = 406 (1); N = 406 (2); N = 406 (3). All analyses were adjusted by Year of enrollment, Rank, Parity (except for model 3), Current BMI classification, Total physical activity, Consumption of dairy products, Use of Calcium as nutritional supplement, and Use of Vitamin D as nutritional supplement.

Acute injuries—N = 406 (1); N = 406 (2). All analyses were adjusted by Year of enrollment, Rank, Parity, Current BMI classification, Total physical activity, Consumption of dairy products, Use of Calcium as nutritional supplement, and Use of Vitamin D as nutritional supplement.

p-value is statistically significant.

aOR, adjusted odds ratio; CI, confidence interval.

The analysis related to acute injuries demonstrated that those who have endometriosis presented a higher likelihood of reporting acute injuries compared to the ones who did not have endometriosis (aOR: 2.426, 95% CI: 1.030; 5.709). This association was adjusted for the same covariates as outlined above. The association between early menopause with acute injuries was lost after adjustments (Table 5).

Discussion

This study aimed to explore how the female reproductive characteristics and related conditions/factors may contribute to MSKi in CAF members. Our main findings were that (i) female CAF members with irregular menstrual cycles or absent periods had a higher likelihood of experiencing RSI compared to those who reported regular menstrual cycles and (ii) having endometriosis was associated with greater rates of acute injuries. To achieve an overall goal of 25% female representation in the CAF, understanding why they are at a greater risk to MSKi is important and necessary. Exploring female-specific reproductive health factors associated with MSKi can aid in guiding future research and inform changes in practices and policies to improve the experience of female CAF members.

Based on our previously published scoping review¹⁰ illustrating that having no or irregular menses, and late age menarche were predictors of MSKi in female military members, we explored these reproductive health factors as predictors of RSI and acute injury in the CAF. The lack of association between late age of menarche, secondary oligomenorrhea/amenorrhea, and MSKi supports the findings from the systematic review by Sammito et al,¹³ who also found no association between late age of menarche, secondary oligomenorrhea/amenorrhea and MSKi in female military members.

We did, however, find a positive relationship between irregular menstruation and MSKi, suggesting that irregular menstruation may increase the likelihood of experiencing RSI compared to having a regular menstrual cycle. The association with menstrual cycle periodicity and RSI could be due to the link between relaxin and menstruation.²⁷ A potential explanation is the shorter lifetime exposure to estrogen, the primary female reproductive hormone, when a menstrual cycle is not regular,^12,28 which can increase the risk of MSKi, and lower relaxin levels. The physiological changes resulting from short exposure to estrogen can lead to stiffer tendons,²⁰ causing muscle strain.²⁹ Estrogen exposure is not, however, the only concern.

Low energy availability is also associated with irregular menstruation in female athlete and non-athlete populations.³⁰ This relationship exists as part of the female athletic triad and can be extended to female military members.³¹ It is possible that lower physical performance and higher incidence of MSKi are present in female members due to the female athletic triad.³² Since energy deficiency is often an underlying cause of menstrual irregularity in physically active women, the association between experiencing RSI and reporting irregular menstruation shown in this study may be related to low energy availability. This relationship needs to be further explored in the female military population.

This study found an association between endometriosis and MSKi in female CAF members. Reporting endometriosis, a disease characterized by the presence of functional endometrial glands and stroma outside the uterine cavity,³³ was associated with greater rates of acute injuries. With endometriosis, normal ovulation is impaired, and follicle-stimulating hormone (FSH) levels are significantly higher in individuals with later stages of endometriosis.^22,23 Furthermore, FSH has been found to be negatively correlated with bone mineral density (BMD) in females who have experienced menopause.²⁴ The lower BMD could predispose one to acute injuries. An alternative explanation is that the use of gonadotropin-releasing hormone (GnRH) agonists, a well-known treatment for endometriosis, could have impacted the bone health of female CAF members living with endometriosis.³⁴ Further exploration of reproductive health markers and bone loss among female military members who experience endometriosis is required.

Together, the findings from the scientific literature and our work reinforce the need to investigate reproductive characteristics in female military personnel. These future investigations can be accomplished by using prospective longitudinal research designs that address the cause and effect of these reproductive health characteristics, and changes over time, to establish appropriate strategies to mitigate the risk of MSKi. Moving in this direction is necessary for the CAF, as most female members are in their reproductive years³⁵ and can be exposed to different factors (e.g., irregular menstruation, endometriosis) impacting MSKi rates, recruitment, and retention within the military.

This study has important strengths, including incorporating several female reproductive health aspects into the analysis to test their associations with MSKi. In addition, we had a large sample size providing adequate power to address our primary outcomes. Although the sample included in the regression for RSI is lower than n = 508, we could test the bivariate association with a sample n > 508 for all independent variables of interest.

In terms of limitations, there were some factors known to influence MSKi, which we were unable to take into consideration (e.g., load carriage, PA intensity). Our recruitment did not account for strategies that would allow us to select participants randomly; the survey was subject to participant self-selection. The research team utilized convenience, purposive, and “snowball” sampling, as well as assistance from project stakeholders (e.g., Canadian Forces Morale and Welfare Services, Personnel Support Programs, Defense Fitness, Defense Women's Advisory Organization social media page, DND CAF Connection website page and Banner, Facebook posts, Maple Leaf and Defense News) to recruit survey participants. After meeting with our stakeholders, these strategies were deemed the best option for reaching as many members as possible, particularly the female members.

Furthermore, we accounted for the possible impact of study design/recruitment within our sample size calculations. These data were self-reported and subject to potential recall bias of the participants. Social desirability could have influenced responses; however, given the anonymous nature of this survey, this effect should be minimal. Finally, given our study design (i.e., retrospective case–control study), we are unable to establish causation with these data.

Conclusion

Female CAF members with irregular or absent menstrual cycles had a higher likelihood of experiencing RSI than those who reported regular menstrual cycles. Reporting endometriosis was associated with greater rates of acute injuries. Our findings highlight female reproductive characteristics as potential contributors to MSKi in the CAF. Therefore, female members who are affected by these predictors may need to be monitored for MSKi by clinical practitioners and provided with effective preventative strategies. Further research should investigate methods to mitigate MSKi risks associated with female reproductive health characteristics and create policies to address these barriers in the CAF to improve recruitment and retention of female members.

Footnotes

Acknowledgments

The authors acknowledge the assistance of Canadian Forces Morale and Welfare Services, Personnel Support Programs, and Defense Fitness in the recruitment. They would also like to thank the members of the Adamo Team who contributed to the IDEaS project, and all who participated in the study.

Authors' Contributions

All authors confirm their authorship. J.L.P. and D.F.d.S. contributed to the writing and analysis of the article with direction from all authors. J.L.P., D.F.d.S., K.S., and K.B.A. conceived the study and were in charge of direction and planning. All authors discussed the results, reviewed the article, and approved the final version.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

Canadian Department of National Defense (DND), Innovation for Defense Excellence and Security (IDEaS) Program. Contract Number W7714-207286/001/SV1.

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