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Balance performance in unstable sitting and trunk muscle response to quick force release were measured in 16 patients with chronic low back pain and 14 matched healthy control subjects.To determine whether patients with low back pain will exhibit poorer postural control, which will be associated with longer average muscle response times.Larger postural sway during standing and delayed trunk muscle response times for patients with low back pain have been reported in several independent studies.Unstable sitting test was accomplished by attaching different sized hemispheres to the bottom of a seat. Subjects performed trials with eyes open and closed while the displacements of the center of pressure were measured with a force plate underneath the seat. Response to a quick force release was recorded from 12 major trunk muscles with surface electromyography. Subjects performed isometric trunk exertions in a semi-seated position when the resisted force was suddenly released with an electromagnet. Average muscle response times and balance performance were correlated using a linear regression analysis.Patients with low back pain demonstrated poorer balance performance than healthy control volunteers, especially at the most difficult levels. Patients also had delayed muscle response times to quick force release. Average muscle onset times together with age and weight correlated significantly with balance performance with closed eyes (R2 = 0.46), but not with eyes opened (R2 = 0.18).Patients with chronic low back pain demonstrated poorer postural control of the lumbar spine and longer trunk muscle response times than healthy control volunteers. Correlation between these two phenomena suggests a common underlying pathology in the lumbar spine.