Effect of compliant flooring on impact force during falls on the hip.
Compliant flooring represents a promising but understudied strategy for reducing impact force and hip fracture risk due to falls in high-risk environments such as nursing homes, hospitals, gymnasiums, and senior centers. We conducted "pelvis release experiments" with young women (n=15) to determine whether floor stiffness influences peak hip impact force during safe, low-height falls. During the trials, we used a pelvic sling and electromagnet to lift and instantly release the participant from a height of 5 cm above a force plate, which measured the force applied to the hip region during impact. Trials were conducted for rigid floor conditions and with layers of ethylene vinyl acetate foam rubber overlying the floor that we regarded as firm (1.5-cm thick; stiffness=263 kN/m), semifirm (4.5-cm thick; stiffness=95 kN/m), semisoft (7.5-cm thick; stiffness=67 kN/m), and soft (10.5-cm thick; stiffness=59 kN/m). When compared to the rigid condition, peak hip impact force averaged 8% lower in the firm condition and 15% lower in the semifirm condition. Peak forces were not significantly different between the semifirm, semisoft, and soft floor conditions, indicating that a 4.5 cm-thick foam mat provides nearly the same force attenuation as a 10.5 cm-thick mat. These results support the need for laboratory experiments to measure the effect of floor stiffness on postural stability and for clinical trials to determine the effect of compliant flooring on hip fracture incidence in high-risk environments.