Postural control of trunk during unstable sitting

A method for quantifying postural control of the lumbar spine during unstab le sitting was developed. The unstable seat apparatus was equipped with leg and foot supports to isolate the control of the lumbar spine and trunk fro m the adjustments in the lower body joints. Polyester resin hemispheres wit h decreasing diameters were attached to the bottom of the seat to achieve i ncreasing levels of task difficulty. The seat was placed on a force plate a t the edge of a table and the participating subjects were instructed to mai ntain their balance while sitting on the seat. Coordinates of center of pre ssure (CoP) were recorded and quantified with summary statistics and random walk analysis. The CoP movement increased significantly with increased sea t instability (task difficulty) (p < 0.01). Stabilogram plots of the CoP mo vement revealed short and long-term regions consistent with the hypothesis that the two regions reflect open and closed-loop postural control mechanis ms. Repeatability of the CoP parameters was excellent for the summary stati stics and the short-term random walk coefficients (0.77 < R < 0.96). It was fair for the long-term diffusion coefficients (0.56 < R < 0.57) and poor f or the long-term scaling exponents (0.14 < R < 0.40). Summary statistics of the CoP movement were positively correlated with body weight (0.69 < R < 0 .73) and the T9 to L4/L5 distance (0.43 < R < 0.54) of the subjects. This m ethod can be applied to study the deficits in postural control of the lumba r spine in low-back pain population. (C) 2000 Elsevier Science Ltd. All rig hts reserved.