Role of movement stability in reducing slip-related balance loss and falls among older adults

Falling is a major cause of injury and death among older adults. Improving stability of the body center of mass (COM) state (i.e., its velocity and position) may reduce fall incidence. We used a combination of experimental and mathematical models to verify the hypothesis that an improvement in mathematically predicted dynamic stability at perturbation onset correlated with a subsequent reduction in balance loss. Slips were induced during a sit-to-stand task in 41 older adults, who were exposed to a block of slip trials followed by a block of nonslip trials and a reslip trial. COM stability was quantified as the shortest distance between its state measured at seat-off (slip onset) and the mathematically predicted feasible stability region boundary. With repeated exposure to slips, older adults exponentially reduced their incidence of falls and backward balance loss, which significantly related to an increase in predicted stability. With exposure to slip and nonslip conditions, subjects began to select "optimal" movements that allowed a balance loss to be avoided under both conditions. These results can be fully accounted for when we assume that an internal model representing the COM stability limits guides the feedforward control during adaptation to a risk of slipping.