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Many manual therapists assess and treat spinal stiffness of people with low back pain. The objectives of this study were to investigate: (i) whether spinal stiffness changes after treatment; (ii) the relationship between pre-treatment spinal stiffness and change in stiffness with treatment; (iii) the relationship between spinal stiffness, pain, disability and global perceived effect of treatment; (iv) whether spinal stiffness predicts outcome of treatment or response to treatment in chronic low back pain patients. One hundred and ninety-one subjects with chronic low back pain were randomly allocated to groups that received either spinal manipulative therapy, motor control exercise, or a general exercise program. Spinal stiffness was assessed before and after intervention. All three groups showed a significant decrease in stiffness following treatment (p<0.001). No difference between groups was observed. There was a significant negative correlation between pre-treatment stiffness and change in stiffness (r=-0.61; p<0.001). There was a significant but weak correlation (r=0.18; p=0.02) between change in stiffness and change in global perceived effect of treatment, and a significant but weak correlation between change in stiffness and change in function for subjects in the spinal manipulative therapy group (r=-0.28; p=0.02). No significant association was observed between initial stiffness score and any of the final outcome measures following treatment. Initial stiffness did not predict response to any treatment. In conclusion, spinal stiffness decreases over the course of an episode of treatment, more so in those with the stiffest spines, but the decrease is not dependent on treatment and is not generally related to outcome.