Effects of hip abductor muscle forces and knee boundary conditions on femoral neck stresses during simulated falls. 
Through experiments that simulated sideways falls with a mechanical hip impact simulator, we demonstrated the protective effect of hip abductor muscle forces in reducing peak stresses at the femoral neck and the corresponding risk for hip fracture.  Over 90% of hip fractures are due to falls, and an improved understanding the factors that separate injurious and non-injurious falls (via their influence on the peak stress generated at the femoral neck) may lead to improved risk assessment and prevention strategies.  The purpose of this study was to measure the effect of muscle forces spanning the hip, and knee boundary conditions, on peak forces and estimated stresses at the femoral neck during simulated falls with a mechanical system.  We simulated hip abductor muscle forces and knee boundary conditions with a mechanical hip impact simulator and measured forces and stresses at the femoral neck during sideways falls.  Peak compressive and tensile stresses, shear force, bending moment, and axial force are each associated with hip abductor muscle forces and knee boundary conditions (p < 0.0005).  When muscle force increased from 400 to 1,200 N, peak compressive and tensile stresses decreased 24 and 56%, respectively.  These effects were similar to the magnitude of decline in fracture strength associated with osteoporosis and arose from the tension-band effect of the muscle in reducing the bending moment by 37%.  Furthermore, peak compressive and tensile stresses averaged 40 and 51% lower, respectively, in the free knee than fixed knee condition.  Contraction of the hip abductor muscles at the moment of impact during a fall, and landing with the knee free of constraints, substantially reduced peak compressive and tensile stresses at the femoral neck and risk for femoral fracture in a sideways fall.