Surface stiffness affects impact force during a fall on the outstretched hand.

Falls on the outstretched hand are among the most common causes of traumatic bone fracture. However, little is known regarding the biomechanical factors that affect the risk for injury during these events. In the present study, we explored how upper-extremity impact forces during forward falls are affected by modification of surface stiffness, an intervention applicable to high-risk environments such as nursing homes, playgrounds, and gymnasiums. Results from both experimental and linear biomechanical models suggest that during a fall onto an infinitely stiff surface, hand contact force is governed by a high-frequency transient (having an associated peak force Fmax1), followed by a low-frequency oscillation (having an associated lower magnitude peak force Fmax2). Practical decreases in surface stiffness attenuate Fmax1 but not Fmax2 or the magnitude of force transmitted to the shoulder. Model simulations reveal that this arises from the compliant surface's ability to decrease the velocity across the wrist damping elements at the moment of impact (which governs Fmax1) but inability to substantially reduce the peak deflection of the shoulder spring (which governs Fmax2). Comparison between model predictions and previous data on fracture force suggests that feasible compliant surface designs may prevent wrist injuries during falls from standing height or lower, because Fmax1 will be attenuated and Fmax2 will remain below injurious levels. However, such surfaces cannot prevent Fmax2 from exceeding injurious levels during falls from greater heights and therefore likely provide little protection against upper-extremity injuries in these cases.