Relative stability improves with experience in a dynamic standing task

This study tested the hypothesis that subjects improve their relative stabi lity as they learn a dynamic pulling task. Healthy adult subjects practiced making brief horizontal pulls (<300 ms) on a handle to a range of target f orces ranging from 20 to 80% of their estimated maximum for 5 days. They we re instructed to always keep their feet flat and begin and end their motion in an upright posture. In order to do this, subjects had to develop the ap propriate body momentum prior to the pull and then recover their balance fo llowing the pull. We analyzed relative stability during balance recovery, u sing two measures: spatial safety margin (minimum distance of the center of pressure, COP, to the edges of the feet) and temporal safety margin (minim um extrapolated time for the COP to reach the edges of the feet). We hypoth esized that: (1) spatial and temporal safety margins would be uncorrelated; (2) safety-margin means would increase with practice; and (3) safety-margi n standard deviations would decrease with practice. Two experiments were co nducted: one where subjects practiced three force targets and positioned th eir initial COP in a small window, and one where subjects practiced two for ce targets with no initial COP constraint. Results showed that spatial and temporal safety margins were correlated but shared less than 6% variance, i ndicating that they reflected different aspects of control. Safety-margin a verages in creased with practice and standard deviations decreased with pra ctice, indicating that the stability of balance control in the execution of this task became more robust. We suggest that the nervous system could use safety margins in both feedback and feedforward control of balance.