Human foot force reveals different balance control strategies between healthy younger and older adults

Aging can cause the decline of balance ability, which can lead to increased falls and decreased mobility. This work aimed to discern differences in balance control strategies between healthy older and younger adults. Foot force data of 38 older and 65 younger participants (older and younger than 60 years, respectively) were analyzed. To first determine whether the two groups exhibited any differences, this study incorporated the orientation of the foot-ground interaction force in addition to its point of application. Specifically, the frequency-dependence of the “intersection point” of the lines of actions of the foot-ground interaction force were evaluated. Results demonstrated that, like the mean center-of-pressure speed, a traditionally-employed measure, the intersection-point analysis could distinguish between the two participant groups. Then, to further explore age-specific control strategies, simulations of standing balance were conducted. An optimal controller stabilized a double-inverted-pendulum model with torque-actuated ankle and hip joints corrupted with white noise. The experimental data were compared to the simulation results to identify the controller parameters that best described the human data. Older participants showed significantly more use of the ankle than hip compared to younger participants. Best-fit controller gains suggested diminished intrinsic muscle stiffness in older adults, indicative of muscle strength loss, that was likely compensated by increased neural feedback. These results underscore the advantages of the intersection-point analysis to quantify shifts in interjoint control strategies with age, thus highlighting its potential to be used as a balance assessment tool in research and clinical settings.