Understanding falls in osteoporosis: the viscoelastic modeling perspective.

Unsteadiness and increased postural sway predispose individuals with osteoporosis to falling. We examined this relationship from a biomechanical perspective using a viscoelastic model. Specifically, we investigated the center-of-pressure (COP) in quiet stance in 37 postmenopausal women (aged 42-79 years), diagnosed with osteopenia or osteoporosis and who engaged in a range of activity levels. Subjects stood on a force platform for two 20s intervals: one with eyes open (EO) and the other with eyes closed (EC). The COP signals were used to compute traditional measures of sway: standard deviation, range, and mean velocity. Based on the difference between the COP and its smoothed representation, we used the "COP minus center-of-mass" signal to compute selected properties of postural mechanics (stiffness and viscosity) and dynamics (frequency and damping) of stance. Sway variability and incidence of falls were not correlated even though our subjects displayed higher sway than age-matched subjects from other studies. Bone density was correlated with frequency in the mediolateral (ML) plane with EO (r=0.50, p<0.002). Irrespective of age, the decrease in postural frequency and increase in postural viscosity in the ML plane were determinants of imbalance and accounted for 18% and 8% of the variance, respectively. With EC, an additional 10% of the variance was contributed by the ML range. The increased ML viscosity was the sole predictor of falls accounting for 20% of the variance of the tendency to fall. Thus, ML frequency plays a fundamental role in preserving balance in that a low value is an important predictor of falls. Finally, although being physical activity may reduce sway, physical activity fails to improve poor postural strategies. To conclude, increases in viscosity and range elucidate further the unique strategy of ML body sway in individuals with osteopenia or osteoporosis and provide insight into how posture may be maintained by those who fear falling.