The aim of this study was to determine the relative risks and patterns of injuries, including SPI, associated with contemporary military load carriage in female soldiers when compared to male soldiers. As a preface to the discussion of the study’s findings, it is important to reiterate that the study was conducted in the period that immediately preceded the removal of gender restrictions in Australian Army combat arms trades. During the study period female soldiers in the Australian Army were not permitted to serve in combat arms corps of service (like infantry, engineers, artillery and armoured corps) [8]; corps known to carry heavier absolute loads [5]. The removal of this gender restriction [8], means that female soldiers may be exposed to heavier absolute loads should they serve in these trades. As such, the timing of this study is important as it means that the rates and patterns of load carriage injuries observed in the full-time female soldiers included in this study arose from carrying loads that would usually have weighed substantially less than the loads typically worn by soldiers in the combat arms trades both then and now [5]. Higher rates and levels of severity of injuries could be expected in female soldiers when heavier loads are carried. However, it should also be noted that many of the comparator male soldiers included in this study may not have been serving in combat arms roles, and so would also not usually have carried the substantially heavier loads typical of those roles. Though this limitation must be noted, the gender-based comparisons of injury rates and patterns conducted in the current study are likely to retain validity, albeit representing a conservative estimate of contemporary load carriage injury incidence rates in female soldiers.
The results of this study indicate that overall levels of load carriage injury risk between genders were not discernibly different in this period. However, our best estimate is that female soldiers had twice the level of risk experienced by male soldiers of suffering serious personal injuries and of suffering foot injuries, from carrying loads. The back was the leading body site of reported load carriage injuries in both male and female soldiers, and of reported serious injuries, in both female and male soldiers. Aside from the foot, the other most common injury sites in both female and male soldiers were the ankle, knee and ‘neck and shoulder’ complex, though the risks of these injury types were similar when female and male soldiers were compared. Systemic illness associated with heat stress was also reported to have occurred in numerous soldiers during the study period, mostly in male soldiers.
The results of the current study, identifying similar overall rates of load carriage injuries between genders, differ to results from studies comparing general injury rates in female and male army recruits. It is important to note that no comparative research could be found in a comprehensive search of the literature to have specifically investigated differences in load carriage injury rates between female and male soldiers. O'Connor [21] found that female army recruits had an overall higher incidence of injuries during recruit training. Similarly, in a study of serving soldiers, Strowbridge [22] found that female soldiers were more likely to sustain an injury than their male counterparts. They found that female soldiers, making up 5% of the unit’s population, sustained 8% of injuries over a three-year period. Interestingly, although female soldiers sustained significantly more injuries during military training, physical training and work than male soldiers, they sustained similar rates of injury to male soldiers when playing sport. This may reflect a greater capacity during sport to self-pace, in contrast to military and physical training and work, where workloads are often fixed.
One potential explanation for the differences in results between our study and others mentioned above is differences between the genders in the levels of exposure to the causal stimulus, load carriage. For example, in the study reported by Strowbridge [22] all injuries sustained were included, rather than just those from a specific task like load carriage. Any potential gender differences in injury rates reported in the current study may have been minimized by the limited participation of female soldiers in load carriage tasks relative to the levels of participation of male soldiers in load carriage tasks. As noted earlier, the current study was conducted prior to the integration of female soldiers into Arms Corps. Research within this population at that time has identified that male soldiers were exposed to operational load carriage tasks more often than female soldiers [5]. In addition, the nature of the tasks conducted and the loads carried varied. Female soldiers were less likely to conduct combat foot patrol tasks (requiring load carriage) and more likely to conduct administration tasks. As well, the absolute loads carried by male soldiers on operations were significantly heavier than loads typically carried by female soldiers [5]. Essentially, a lower rate of exposure to load carriage tasks and lighter absolute loads may have contributed to the lack of significant differences in overall injury rates between the genders in the current study even though the activities thought to cause these load carriage injuries were similar.
However, while absolute loads were higher in male personnel there were no statistically significant differences in relative loads between the two genders when the carrier’s body weight was accounted for (even though female soldiers carried slightly lighter relative loads of 43% of body weight when compared to male soldiers at 47% [5]). This suggests that while female soldiers were carrying lighter absolute loads, when the loads were considered in relation to their lighter body weight, the relative loads between genders were similar. This may raise concern in the future. As aforementioned, the removal of gender restrictions means that female soldiers may now be serving in corps carrying heavier load weights. This in turn may mean that while female soldiers, in general, would carry similar absolute loads to male soldiers, their relative loads would increase.
Despite similarities in overall load carriage injury rates observed in the current study, female soldiers were observed to be at a higher risk of suffering a serious personal injury related to load carriage. Three potential explanations include: carriage by some female soldiers in some instances of heavier total absolute or relative loads; a lower maximal aerobic capacity; and poor equipment fit. As noted above, previous research within this population suggests that the relative external loads typically carried by female (43% body weight) and male soldiers (47% body weight) may have been similar [5] However, the potentially higher levels of body fat mass in female soldiers may have further increased the total loads carried and the resultant lean muscle mass loading [23, 24]. In the study by Scott and Ramabhai [23], female soldiers who were carrying similar relative external loads (37% body weight) were found to work harder than the male soldiers. This higher work effort was considered to be due to greater body fat mass in the female participants who carried approximately 8 kg more body fat than their male counterparts. Thus, the female participants carried a mean load of 24 kg (37% body weight) plus 17 kg (fat mass), equating to a total passive load of 41 kg, whereas the male participants carried a mean load of 27 kg (37% of body weight) plus nine kilograms (fat mass), equating to a total passive load of 36 kg.
Absolute VO2 max has a strong correlation with load carriage task performance [25]. As noted earlier, female soldiers typically have a lower absolute maximal aerobic capacity than male soldiers [12]. These lower levels of fitness mean that female soldiers would typically have to work at a higher percentage of VO2 max than male soldiers to perform a given load carriage task [9]. In addition, lower levels of aerobic capacity are associated with an increased risk of injury [26]. As such, female soldiers may be more susceptible to severe load carriage injuries due to the higher level of metabolic stress they endure when performing a load carriage task.
Finally, the fit of load carriage equipment may have contributed to the heightened rate of serious load carriage injuries observed in female soldiers in the current study. Harper et al. [10] noted that female soldiers reported more problems with equipment fit than male soldiers, most notably shoulder straps and the stability of the rucksack. The authors [10] also noted that the effects of this poorer equipment fit meant female soldiers reported greater levels of pain, soreness and discomfort in the back region when carrying heavier loads and were hypothesized to contribute to the gender differences in pack marching injury rates between genders. In combination, a heavier load, lower general aerobic capacity and poorer equipment fit may have contributed to a greater incidence rate of serious load carriage injuries in female soldiers in the current study by creating additional stress on overloaded musculoskeletal structures and physiological systems.
In two previous studies [3, 27] investigating load carriage injuries over a single event, both with male only infantry populations, the body sites of injuries sustained during and immediately following the event were similar to the sites of injuries sustained by male participants in this study. In the study by Knapik et al. [3], back injuries, foot injuries and sprains to the foot, knee and ankle were the leading types of musculoskeletal injuries sustained. Conversely Reynolds et al. [27] reported the foot, back and knee to be the most common sites of musculoskeletal injuries during a load carriage event. Interestingly, neither of these two comparative studies reported any shoulder and neck complaints, injuries which were prominent in our study. A potential reason may be that the neck and shoulder complaints are more chronic in nature and occur over a more prolonged period as opposed to a single load carriage event as reported in these two studies. In addition, the mean Australian soldier loads reportedly carried over the period may have been a contributing factor. Where the infantry populations in the studies Knapik, et al. [3] and Reynolds, et al. [27] carried loads of 46 kg and 47 kg respectively, the Australian Army Infantry soldiers carried a mean general load of 49 kg and a mean operational load of over 60 kg over a two year (2009 and 2010) period [5]. This heavier load may have caused a greater compression force over the brachial plexus and surrounding musculoskeletal structures, leading to a higher incidence of neck and shoulder injuries. Potential difference in backpack design, shoulder strap width for example, may likewise have influenced the number of neck and shoulder injuries reported in male soldiers in this study.
The lower limbs in particular have been found to be the leading site of injury to military populations not on combat operations [28–30]. In some instances, injuries to the lower limbs have represented over 80% of reported injuries in a military population [30]. However, when serving in a combat theatre, the lower back typically presents as the leading site of injury [31, 32]. Potential reasons for this change in injury site may be the wearing of body armour and load carriage. Wearing body armour has been found to increase the physical demands of performing a given task [33] and as such it is not unexpected that it is associated with causing lower back injuries in military populations [34]. Likewise, load carriage has been found to change the shape of the spine [35, 36] and create cyclic stresses to the Vertebrae [37].
The finding in this study of a significantly greater frequency of foot injuries related to load carriage in female soldiers when compared to their male counterparts may be indicative of differential biomechanical loading of the musculo-skeletal system. Female soldiers have been found to have lower strength in ankle dorsiflexion, inversion and eversion than male soldiers from the same population [12]. Therefore, the fatigue resilience and ability to bear load afforded by the strength of these muscles which support the foot may be reduced. Greater frequency of foot injuries may also indicate an equipment fit issue, and this warrants further investigation. Ill-fitting boots have been noted as an issue for female soldiers [38]. Relative boot stiffness and the numbers of available boot width sizing in smaller boot sizes typically required by women are thought to contribute to problems with the boot-foot interface [38].
