Stiffness Control of Balance during Quiet Standing

38 39 Distractions affect postural control, but this mechanism is not well understood. Diversion of 40 resources during cognitive stress may lead to decreased motor drive and postural muscle tone. 41 This may appear as decreased postural stiffness, and increased postural sway amplitude. We 42 hypothesized that dual tasking leads to decreased stiffness and increased sway amplitude. 43 Postural sway (center of pressure; COP) data were used from 724 participants aged 77.9±5.3 44 years, a representative sample of community-dwelling older adults, the MOBILIZE Boston 45 Study cohort. Subjects stood barefoot with eyes open for 30 seconds per trial on a force plate. 46 Five trials were performed each with and without a serial subtractions-by-3 task. Sway data 47 were fit to a damped oscillator inverted pendulum model. Amplitudes (COP and center of mass), 48 mechanical stiffness, and damping of the sway behavior were determined. Sway amplitudes and 49 damping increased with the dual task (P<0.001); stiffness decreased only mediolaterally 50 (P<0.001). Those with difficulty doing the dual task exhibited larger sway and less damping 51 mediolaterally (P≤0.001), and an increased stiffness with dual task anteroposteriorly (interaction 52 P=0.004). Dual task could still independently explain increases in sway (P<0.001) after 53 accounting for stiffness changes. Thus the hypothesis was supported only in mediolateral sway. 54 The simple model helped to explain the dual task related increase of sway only mediolaterally. It 55 also elucidated the differential influence of cognitive function on the mechanics of 56 anteroposterior and mediolateral sway behaviors. Dual task may divert the resources necessary 57 for mediolateral postural control, thus leading to falls. 58