Military body armour functions to protect the vital thoracoabdominal organs of soldiers from ballistic, fragmentation and stab threats (Choi et al., 2016; Laing and Jaffrey, 2019). Body armour, however, can introduce integration issues between the human user and the system, as well as present a mass burden (Knapik et al., 2004). These integration issues are amplified if the body armour system is ill-fitting. Soldiers wear body armour during operational deployment and training or field-based exercises for 5.2–6.4 ± 4.7 h per day (Coltman et al., 2020). It is therefore vital that a body armour system (comprising front and rear hard and soft ballistic plates encased in a carrier) interfaces appropriately with a soldier's torso (i.e., fits correctly) and integrates well with other elements of a soldier's combat ensemble (Furnell et al., 2017). Proper fit will maximise protection and maintain coverage requirements over this long duration of wear (Laing and Jaffrey, 2019). The fit of body armour also has important implications for job and combat performance. That is, poor-fitting body armour can impede a soldier's ability to run efficiently, to shoulder and fire a rifle, to manoeuvre in and out of a vehicle and to complete other mission essential tasks (Mitchell et al., 2010; Choi et al., 2016, 2018; Coltman et al., 2020, 2021a). Consequently, designing and sizing body armour to properly fit all soldiers is vitally important for military organisations to achieve correctly.

Previous research has revealed that most female soldiers are dissatisfied with the fit of their current body armour systems (Epstein et al., 2013; Toma et al., 2016; Coltman et al., 2020, 2021a; Davis et al., 2020). Specifically, questionnaire data related to the fit of body armour was completed by 147 female Australian Defence Force (ADF) soldiers in both combat and non-combat roles. Of the women surveyed, 68, 56, and 12%, respectively, found the body armour to be ill-fitting, too large, and too small (Coltman et al., 2020). Similarly, researchers investigating the fit and function of body armour in the United Kingdom found that female soldiers reported numerous instances of discomfort, particularly at the hip (Davis et al., 2020). In the same study 29–59% of participants experienced task interference during a range of basic job roles (Davis et al., 2020). Another group found that integration between a soldier's bra and body armour was a source of substantial discomfort (Malbon et al., 2020). The notion of sex-specific discomfort with body armour was supported by findings from an Australian study, which revealed that limited adjustability of the system, insufficient space for breasts and oversized length and width were all common problems reported by female soldiers (Coltman et al., 2021a). Female soldiers now comprise 10.4–17.5% of Defence Force populations in Australia, Canada, New Zealand, United Kingdom and United States of America (Defence People Group., 2017; Ministry of Defence., 2018; Mark, 2019; Service Women's Action Network., 2019; Government of Canada., 2020). Ensuring that body armour is appropriately sized and designed to enable female users to undertake their occupational roles is essential to fostering women's participation and inclusion in Defence.

A primary source of poor body armour fit for female soldiers is how it is sized and designed (Epstein et al., 2013; Toma et al., 2016; Coltman et al., 2020, 2021a; Malbon et al., 2020). Although several anthropometric surveys have been conducted on global soldier populations to inform the design and sizing of protective equipment, military body armour is commonly issued to soldiers in a limited, unisex sizing range (Todd, 2007). Moreover, the dimensions and specifications of body armour systems are traditionally based upon male anthropometric data (Todd, 2007; Epstein et al., 2013; Toma et al., 2016). This is likely a function of body armour primarily being designed for combat soldiers. Until 2011, women in the ADF were ineligible to enlist in combat coded roles (Wadham et al., 2018) and therefore only men served in combat. Furthermore, women comprise only a small percentage of the soldier population in anthropometric surveys and there is a lack of breast specific measurements included in such surveys to better inform body armour designs (Edwards et al., 2014). Given that women have different torso shapes and more developed breast tissue compared to men (Edwards et al., 2014; Gordon et al., 2014), designs that have not specifically accounted for female anthropometry are unlikely to properly fit female soldiers. Poor personal protective equipment (PPE) fit has also been observed among female workers in a range of historically male-dominated occupations such as firefighting and construction (Barker et al., 2012; Onyebeke et al., 2016). Evidence also highlights the sex disparity in PPE design affecting occupational task performance (Park and Hahn, 2014). No previous research, however, has specifically investigated which breast and torso characteristics contribute to poor fit of body armour systems for female soldiers. Such information is required to understand the association between anthropometric torso dimensions and equipment fit in body armour design and sizing. This exploratory study, therefore, aimed to: (i) provide normative data characterising the torso and breast size and shape of female soldiers to inform evidence-based design modifications to current-issue body armour, and (ii) determine which breast characteristics and torso anthropometric measurements contributed most to poor body armour fit reported by female soldiers. It was hypothesised that female soldiers would display a wide range of breast and torso characteristics and that these characteristics would differ between soldiers who reported that their body armour was too small, too large or a good fit.

This is the first published study to provide normative data on the torso and breast size and shape of female soldiers. These soldiers displayed a wide range of anthropometric characteristics that must be considered when sizing and designing body armour. Sixteen of the twenty-two measurements assessed were significantly associated with the soldiers' ratings of body armour fit, suggesting a link between breast and torso characteristics and the overall perceived fit of body armour systems by female soldiers. The implications of these findings in terms of the design and sizing of current-issue body armour systems are discussed below.

Given that body armour systems are torso borne, the size and shape of the torso is a key consideration when designing body armour systems to closely interface with the human body. Among study participants, torso characteristic data were similar to all comparable measurements previously reported by the AWAS for ADF female soldiers (Edwards et al., 2014). Large variations, however, were observed in individual measurements, evident in the large standard deviations (Table 4). This large variability highlights the range of shapes and sizes that body armour systems should be designed to accommodate, as well as sex-related differences in torso dimensions compared to male AWAS data (Edwards et al., 2014). For example, female soldiers in the present study had substantially narrower chests (27.8 vs. 30.5 cm AWAS; Chest Breadth) and smaller waists (74.7 vs. 92.4 cm AWAS; Waist Circumference) but larger hips (102.8 vs. 99.7 cm AWAS; Hip Circumference) when compared to male AWAS data (Edwards et al., 2014). Given the limited, unisex sizing range of current issue armour, it is unsurprising that many females have reported to be wearing ill-fitting body armour (Coltman et al., 2020).

Consistent with our hypothesis, several torso measurements were significantly associated with a poor fit. Participants who perceived their body armour to be too large (n = 53) were more likely to be shorter in Stature and Suprasternale Height (Table 5) compared to those who perceived their body armour to be a good fit (n = 30), suggesting the length of the in-service body armour system may be too long for many users. Body armour is designed to be positioned superiorly at the level of the suprasternal notch. As armour length increases relative to overall height and height of the suprasternal notch, it is increasingly likely to interfere with trunk mobility and task performance and, therefore, perceptions of armour fit (Molloy et al., 2020; Coltman et al., 2021a, 2022). Conversely, participants who reported their body armour to be too small (n = 14) had a significantly greater Mass, chest circumference (OBCC and UBCC), Chest Depth, Chest Breadth, Waist Circumference, and Neck Circumference compared to those who perceived their body armour to be too large (n = 54). This either suggests that the body armour system is not wide enough to accommodate large torso circumferences or that the mechanism to tighten the system does not have sufficient adjustment capacity. Designers of body armour for women should pay particular attention to ensure adjustment points on the front and rear carriers, including the shoulder straps and cummerbund, can cater for the variability in chest curvature of women's torsos due to their additional breast tissue. Ultimately, military organisations should consider developing body armour systems that are better suited to the range of female soldier torso sizes, both large and small (Wen and Shih, 2020). Such systems are likely to improve perceptions of fit and have important implications for task performance and efficiency in the field (Mitchell et al., 2010; Choi et al., 2019; Coltman et al., 2021a). Issuance procedures (e.g., sizing and allocation of systems) and education (e.g., training on system use) should also be updated to accompany the introduction of any new equipment because previous research has identified that these factors contribute to reports of dissatisfaction among female body armour users (Coltman et al., 2021a).

Clearly, the breasts of female soldiers are substantially different to male soldiers, and this disparity can affect body armour fit. The mean Breast Volume reported in this study for female soldiers (365 ml) was consistent with mean Breast Volume for women from an athletic population (401.7 ml; Brisbine et al., 2020a), but smaller than that previously reported for the general female population (653 ml left and 647 ml right breast; Coltman et al., 2017a). This smaller average breast size of the female soldiers is likely to be a function of the younger age (mean: 25.6 years) and lower body mass index (mean: 25.4 kg/m²) of the study participants compared to the general population (Edwards et al., 2014; Coltman et al., 2017a). There was, however, a wide range of breast sizes (see Table 3). This diversity in breast size has important implications for armour design. For example, female soldiers with large breasts are likely to benefit from body armour systems designed with space to accommodate additional breast tissue, although such space would not be needed for soldiers with small breasts. Given the similarity of average breast size between participants who rated their armour as a good fit (344.5 mL; n = 14; 14%) and too large (340.5 mL; n = 30; 31%) compared to those who rated their armour as too small (503.0 mL; n = 53; 55%; Table 6), it would appear that female soldiers with a breast size of medium or large (formally classed as 350 mL and above; Coltman et al., 2017b) are most affected by sizing of body armour systems around the chest. This represents a substantial portion of the female soldier population, thus highlighting the importance of using anthropometric data to inform future armour design.

In addition to breast size, breast shape characteristics, such as Breast Surface Area and Anterior Breast Projection, will influence the overall fit of a torso borne body armour system. Given the range of breast projection distances documented in the current study (23–71 mm), women with breasts that protrude further anteriorly from the chest wall are likely to experience greater difficulty achieving conformity between a body armour system and their torso (Coltman et al., 2021b). The resultant compression and deformation of the breast when wearing body armour, which is exaggerated in women with large breasts (McGhee and Steele, 2020), is likely to have negative functional and protection impacts (e.g., difficulty breathing, limited ROM, exposed lateral breast tissue, and compromised positioning of vital torso protection). Consequently, our findings corroborate previous reports that occupational body armour does not adequately accommodate the full range of female breast characteristics (Coltman et al., 2020, 2021a; Malbon et al., 2020; Niemczyk et al., 2020). It was therefore unsurprising that Breast Volume, Breast Surface Area and Anterior Breast Projection were each associated with the perception of poor body armour fit (Table 6). All three breast measures were significantly larger in female soldiers who perceived their body armour to be too small compared to those who perceived it to be too large, with Breast Surface Area additionally differing between those who perceived their body armour to be too small and a good fit. As previous research has demonstrated the capacity of body armour to cause mild restrictive ventilatory impairment in male soldiers (Armstrong and Gay, 2016; Armstrong et al., 2019), reports of difficulty breathing due to excessive compression of the breasts and chest in the present study are likely magnified as breast size increases. Further research, however, is required to confirm this notion. Similarly, experiences of body armour being too loose around the waist to accommodate additional tissue in the upper chest may be further associated with breast size and shape variations amongst female soldiers (Coltman et al., 2021a). In both instances, design modifications to body armour have the potential to reduce breathing restrictions and, in turn, minimise discomfort and impairments to performance (Armstrong and Gay, 2016). Body armour system weight and load carriage have previously been associated with expiratory flow limitations (Armstrong et al., 2019). Furthermore, excessive weight of body armour has been reported as the third-most disliked characteristic of current-issue body armour by female soldiers (Coltman et al., 2022). Therefore, a lighter body armour system, one with smaller sizing dimensions, is likely to afford similar benefits to respiratory function.

Female soldiers in the present study also displayed a wide variety of breast positions relative to their torso. This was illustrated by a wide range of Sternal Notch to Nipple Distances (95.8–228.3 mm) and breast position measures, whereby the superior border of the breast was found to sit just 3 cm below the sternal notch for some soldiers and more than 5 cm lower for others. Body armour should be positioned at the level of the sternal notch to ensure sufficient coverage of the vital thoracoabdominal organs (Laing and Jaffrey, 2019). Knowledge of breast position is therefore crucial when designing body armour for female soldiers. Importantly, female soldiers whose breasts were situated lower on their torso were more likely to perceive their body armour system fit as too small. Coupled with other breast size and shape measures, breast position data indicate where potential plate shape changes or additional adjustability features need to be incorporated into a body armour system to better accommodate female breasts. Police body armour manufacturers have explored thermal forming and darting to provide a female-specific shape to soft ballistic plates (Basich, 2007). There is similar support in the literature for a contoured plate based on three-dimensional torso scans or anthropometry of female wearers (Boussu and Bruniaux, 2012; Cichocka et al., 2014; Mahbub et al., 2014; Abtew et al., 2018). The concept of a formed plate also warrants consideration for military body armour applications. The benefits of contouring rigid military hard ballistic plates, however, are not fully understood, and it is unknown how mobility and ballistic performance may be influenced by any changes. Female soldiers have previously reported that body armour is either too tight over the breasts to fit correctly around the waist or too loose around the waist to fit correctly over the breasts (Coltman et al., 2021a). An average difference of 15 cm was reported between mean chest circumference (OBCC; 89.7 cm) and mean Waist Circumference (74.7 cm) in this study, suggesting that incorporating multiple adjustments points on body armour systems is warranted. Any form changes, however, must be evaluated against protection implications because protection remains the primary function of military body armour systems (Laing and Jaffrey, 2019).

Fit and function issues associated with body armour are further exacerbated by wearing a bra because a bra can change breast shape and limit breast deformability under a hard plate. Moreover, bras function to lift and support the breasts to reduce pain and discomfort associated with breast movement (Scurr et al., 2014; Brisbine et al., 2020b), whereas body armour places downwards pressure on the breasts, effectively acting against the support provided by a bra (Niemczyk et al., 2020). Therein lies an additional challenge in designing body armour suitable for female soldiers with large breasts. That is, large breasts require more upwards support from a bra (thus limiting deformability) yet are more susceptible to the downward compression from body armour that may act to counteract this support. Bras can also be a major source of discomfort for female wearers of body armour because of the way body armour affects the fit and form of otherwise well-fitting, supportive bras (Niemczyk et al., 2017, 2020; Burbage et al., 2021). Furthermore, body armour can limit the type of bra a female soldier can wear. Compression-style sports bras, which are commonly wire-free and function to reduce anterior protrusion of the breast, are the most commonly worn type of bra among Australian (Coltman et al., 2021b) and British female soldiers (Burbage et al., 2021). Compression-style bras, however, do not provide sufficient breast support for women with medium-large breast sizes (McGhee et al., 2008), who comprised 44% (n = 43) of the study population. Researchers examining police body armour have suggested that designing a supportive bra to specifically integrate with body armour may be an effective short-term solution (Malbon et al., 2020; Niemczyk et al., 2020). Such a bra should be a wire-free design that reduces anterior breast protrusion while distributing breast tissue to improve the fit of body armour over the breasts and alleviate discomfort associated with current bra-armour incompatibility (Malbon et al., 2020; Coltman et al., 2021b).

The findings of the current study must be considered in light of their limitations. Although this study was the first to provide normative data on female soldiers' torso and breast characteristics, no objective measures of body armour fit or performance were collected. Therefore, breast characteristics could only be associated with the fit of body armour reported by the female soldiers. Participant perceptions have been used in other research evaluating protective equipment (Park and Hahn, 2014) and therefore are deemed to be a valuable metric for assessing overall fit in the present study. Future research, however, is recommended to incorporate both subjective and objective measures related to body armour fit, including static, dynamic, occupation-specific and cognitive fit (Stirling et al., 2020). Additionally, the sizes of the body armour systems issued to the participants were not recorded; TBAS Tier 2 is only available in one size, but TBAS Tier 3 is available in four sizes (S-XL), which may account for some differences in participant ratings of subjective fit. Participants were not asked to bring their body armour system to the test session and most participants were unaware of the size of the body armour that they had been issued, preventing collection of these data. As this study focused exclusively on the anthropometry, design, and sizing requirements of female soldiers, it is also recommended that future research similarly profile the male torso to assess potential fit and form issues experienced by male soldiers and to better target design and sizing improvements to the entire soldier population. Fundamentally, any design changes must be considered against known protection requirements, including vital thoracoabdominal organ protection.

Normative data characterising the torso and breasts of 97 female soldiers highlight the variation in anthropometric dimensions that body armour systems must cater for, as well as the implications of the varied torso and breast sizes and shapes for perceived body armour fit. Future body armour systems should cater for female soldiers' physical diversity by developing an expanded sizing range and female-specific design features to improve perceptions of fit. Improved perceptions of fit will, in turn, reduce movement interference, discomfort, and barriers to performance in the field. Any modifications to body armour should be informed by anthropometric data representing female soldiers and aim to ensure that vital thoracoabdominal organ protection recommendations are maintained.

The datasets presented in this article are not readily available because participants of this study did not agree for their data to be shared publicly. Requests to access the datasets should be directed to Celeste.Coltman@Canberra.edu.au.

The studies involving human participants were reviewed and approved by Defence Science and Technology Group Low Risk Ethics Panel, Department of Defence, Australia. The participants provided their written informed consent to participate in this study.

CC was the primary investigator. She was responsible for the study design, recruiting the participants collecting, analysing and interpreting the data, co-writing the first full draft of the manuscript, and approving the final version of the manuscript. BB was responsible for analysing and interpreting the data, co-writing the first full draft of the manuscript with the primary investigator, and approving the final version of the manuscript. RM was responsible for helping to develop the study design, recruiting the participants, assisting to interpret the data, providing feedback on versions of the manuscript, and approving the final version of the manuscript. JS was responsible for helping to develop the study design, assisting to interpret the data, providing substantial feedback on versions of the manuscript and approving the final version of the manuscript. All authors contributed to the article and approved the submitted version.

This study was funded by the Defence Science and Technology Group, Department of Defence, Australia under a Defence Science Partnering Deed (reference number 7974) as part of an ongoing effort to improve the fit, form, and function of equipment, to accommodate the diverse body shapes of Australian soldiers.

RM was an employee of Defence Science and Technology Group, Department of Defence, Australia. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.