The effects of body mass index and sex on impact force and effective pelvic stiffness during simulated lateral falls

Background: The incidence of hip fractures is highest for underweight females with low body mass index (BMI). However, it is unknown how these factors influence impact dynamics during in-vivo lateral hip impacts. We used a pelvis release paradigm to compare: (1) absolute and normalized forces applied to the femur-pelvis system across sex and BMI groups; (2) the force-prediction accuracy of vibration-based versus force-deflection-based estimates of effective pelvic stiffness; and (3) effective pelvic stiffness between BMI and sex groups. Methods: Twenty-eight persons participated (7 low-BMI females, 7 low-BMI males, 7 high-BMI females, 7 high-BMI males, with BMI criteria of <22.5 and >28 for low- and high-BMI groups respectively). The participant's pelvis was released from heights of 0 to 5 cm. A force plate measured impact loads, while a motion capture system measured pelvic deflection. Findings: Peak impact forces were 22.6% higher, while normalized peak forces were 31.2% lower, for high- compared to low-BMI participants. Accuracy of peak force predictions improved by 25% for the force-deflection versus the vibration-based stiffness estimation method. Effective pelvic stiffness was greater for males than females, but no significant differences were observed between BMI groups. Interpretation: This study adds to clinical understanding of the effects of sex and BMI on impact dynamics during falls on the hip, and raises questions about the biomechanical mechanisms underlying the protective role of high BMI on hip fracture risk. Understanding the relationship between impact mechanics and faller characteristics should lead to more effective prevention of hip fractures. (C) 2013 Elsevier Ltd. All rights reserved.