Postural control of trunk during unstable sitting.

A method for quantifying postural control of the lumbar spine during unstable sitting was developed. The unstable seat apparatus was equipped with leg and foot supports to isolate the control of the lumbar spine and trunk from the adjustments in the lower body joints. Polyester resin hemispheres with decreasing diameters were attached to the bottom of the seat to achieve increasing levels of task difficulty. The seat was placed on a force plate at the edge of a table and the participating subjects were instructed to maintain their balance while sitting on the seat. Coordinates of center of pressure (CoP) were recorded and quantified with summary statistics and random walk analysis. The CoP movement increased significantly with increased seat instability (task difficulty) (p<0.01). Stabilogram plots of the CoP movement revealed short and long-term regions consistent with the hypothesis that the two regions reflect open and closed-loop postural control mechanisms. Repeatability of the CoP parameters was excellent for the summary statistics and the short-term random walk coefficients (0.77