Biomechanics of the hip and pelvis and predictors of injury during military load carriage

Abstract

Musculoskeletal injuries are among the most common and costly in the British military. It is well reported that female soldiers have significantly higher rates of injury than males, particularly in the hip and pelvis. In the UK, female military trainees have been found to be up to 48 times more likely to obtain a pelvic musculoskeletal injury than male recruits. In 2016 the ban on females serving in Ground Close Combat (GCC) roles was lifted in the UK. These roles are characterised by their extremely high physical requirements and hence higher rates of injury. Male GCC trainees have a seven-fold increased risk of hip and pelvic fractures compared with standard male entrants. It is expected that as more females begin training for GCC roles, their already significantly increased risk of injury will rise further. Marching with body borne loads is an integral aspect of military training and has been suggested as a leading cause of musculoskeletal injury. The effects of carrying military loads on spatiotemporal and kinematic gait parameters are well reported in male soldiers. However, the few studies directly comparing males and females present conflicting findings and, as a result, the biomechanical factors involved in hip and pelvic injuries are still unclear.

The gait of 38 subjects walking and 19 subjects running both unloaded and with body borne military-relevant loads of 20kg and 40kg was recorded. Volunteers were either current or ex-military personnel, or physically fit civilians with load carriage experience. Gait data of the trunk and lower limb were captured using a 10 camera optical motion tracking system, force plate and surface EMG electrodes. Data was processed through an inverse dynamics based musculoskeletal model, Freebody, to obtain joint angles, torques, muscle forces and contact forces. Differences according to sex and carried load were examined using repeated measures ANOVA. Data obtained from these studies was used to develop static finite element models of the hemi-pelvis for an average male and female subject, and to simulate loading during walking with and without carried loads at three points in the gait cycle; weight acceptance, midstance, and push off.

It was found that males and females walked with similar gait parameters when carrying no load and 20kg. However when 40kg was carried, females displayed significantly shorter stride length and spent longer in double support than males, possibly due to fatigue or in an effort to reduce internal forces in the hip. When running, females were observed to have significantly shorter stride length when carrying load. Females were also found to generate a significantly greater proportion of propulsive torque at the hip compared with males, suggesting that they may be inherently more hip dominant. Factors associated with increased hip dominance included faster speed and female sex. During both walking and running, females' hip joint contact force was angled significantly more anteriorly and superiorly than males. Results of the finite element analysis revealed that peak von Mises stresses in the pubic rami occur during toe push off. Stress in the female pelvis model was found to be higher across all load conditions. Increased adductor muscle force, pelvic obliquity and medial hip joint contact force were found to be significant predictors of greater von Mises stresses in the rami. Analysis of EMG signals from four lower limb muscles revealed that females were operating at a greater subjective workload and energetic cost than males when carrying load. Regression analyses confirmed that female sex was a significant predictor of increased hip joint contact force over the gait cycle.

The work in this thesis supports the theory that males and females adapt their gait differently in response to body born load carriage, and that females are more likely to be injured because they are working at closer to the maximum threshold where they can no longer maintain normal gait function. This, combined with inherent biomechanical differences and other intrinsic and extrinsic factors, may explain female soldiers' significantly higher rates of hip and pelvic injuries. Increased emphasis on strength training of the quadriceps and core musculature together with a more progressive approach to load carriage may improve clinical outcomes.