Military load carriage during prolonged marches on lower extremity mechanics: Influence of gender

INTRODUCTION: Stress fractures are the most commonly reported orthopedic injury among military recruits. Whereas females are 2-3 times more likely to incur stress fractures when engaged in athletic activity compared to their male counterparts, among US military recruits, prevalence increases to 4-6 times greater than men. The objective for this study was to identify whether females adopt different gait mechanics than males during simulated prolonged load carriage. Emphasis was on identifying differences in temporal-spatial parameters, ground reaction force magnitudes and profiles, and joint kinematics and kinetics which may predispose females to stress fractures, particularly at the femoral neck (hip.).;METHODS: Twelve inactive and/or sedentary adults aged between 18-29 years with body fat percentages within military protocols for admittance into the service participated. Subjects had no history of orthopedic injury, neurological impairment, metabolic disease, or impaired balance and all underwent gait analysis requiring an 80-minute (4-mile,) 3mph-paced march over a treadmill and level ground while wearing a MOLLE-2 military rucksack loaded with 20kg.;RESULTS: At Mile 1, male stride lengths were significantly greater than that of females (p<0.05.) At Mile 4, female double support and stance times were significantly lower (p<0.05,) their step and stride lengths were statistically shorter (p<0.05,) and they incurred greater step and stride frequencies compared to their male counterparts (p<0.05.) Step times were also shorter for females at Mile 4(p<0.05.) Hip abductor strength was no different between males and females prior to the forced march but was statistically different post-march, with males' strength significantly greater compared to females (p<0.05.) Males demonstrated a significant decrease in strength from pre-march to post-march (p<0.05) while females presented with a similar trend (p=0.05.) Analysis of ground reaction force profiles saw both braking and propulsive impulses significantly higher among females than males at Mile 4 (p<0.05.) In addition, males at both Miles 1 and 4 exhibited greater peak vertical ground reaction forces during weight acceptance (WA) than at toe-off (TO) (p<0.05.) When compared with males, females at Mile 1 demonstrated a trend for greater peak vertical force compared to males during WA (p=0.06.) Despite dissimilar ground reaction magnitudes, overall sagittal and frontal plane joint kinematics (hip, knee and ankle ranges of motion) were statistically similar between genders, with no differences evident. Yet knee ROM magnitudes were found to decrease among males during weight acceptance (from initial contact to peak knee flexion) between Mile 1 and Mile 4 (p<0.05.) Examining internal joint moments however, differences were evident. Males at Mile 1 experienced significantly greater peak knee flexor moments, which occurred at initial contact, compared to females (p<0.05), with similar trends observed at Mile 4 (p=0.06.) Conversely, females demonstrated greater peak knee extensor moments, which occurred at WA, compared to males at Mile 4 (p<0.05.) Ankle moments were also different, with males generating greater peak plantarflexor moments at Mile 4 during terminal stance in preparation for TO (p<0.05.) With respect to frontal plane hip and knee moments, females exhibited significantly greater peak hip abductor moments at Mile 4 (p<0.05) whereas males at Mile 1 incurred significantly higher peak knee abductor moments during WA and at terminal stance (p<0.05.).;CONCLUSIONS: Our findings highlight gender differences in gait mechanics as a result of heavy load carriage which may predispose females to greater risk of fracture. Deficits in hip abductor strength, particularly the gluteus medius, combined with greater hip abductor moments may impair the muscle's ability to neutralize stresses acting on the femoral neck during loading, which consequently increases risk for stress fracture. By increasing cadence, females are subjected to a greater number of loading cycles covering the same distance compared to their male counterparts and, consequently, impact forces which must be transferred up the kinetic chain or absorbed by the musculoskeletal system. (Abstract shortened by UMI.).