Disability reduction following a lumbar stabilization exercise program for low back pain: large vs. small improvement subgroup analyses of physical and psychological variables.

BACKGROUND: Little is known about why patients with low back pain (LBP) respond differently to treatment, and more specifically, to a lumbar stabilization exercise program. As a first step toward answering this question, the present study evaluates how subgroups of patients who demonstrate large and small clinical improvements differ in terms of physical and psychological changes during treatment.

METHODS: Participants (n = 110) performed the exercise program (clinical sessions and home exercises) over eight weeks, with 100 retained at six-month follow-up. Physical measures (lumbar segmental instability, motor control impairments, range of motion, trunk muscle endurance and physical performance tests) were collected twice (baseline, end of treatment), while psychological measures (fear-avoidance beliefs, pain catastrophizing, psychological distress, illness perceptions, outcome expectations) were collected at four time points (baseline, mid-treatment, end of treatment, follow-up). The participants were divided into three subgroups (large, moderate and small clinical improvements) based on the change of perceived disability scores. ANOVA for repeated measure compared well-contrasted subgroups (large vs. small improvement) at different times to test for SUBGROUP × TIME interactions.

RESULTS: Statistically significant interactions were observed for several physical and psychological measures. In all these interactions, the large- and small-improvement subgroups were equivalent at baseline, but the large-improvement subgroup showed more improvements over time compared to the small-improvement subgroup. For psychological measures only (fear-avoidance beliefs, pain catastrophizing, illness perceptions), between-group differences reached moderate to strong effect sizes, at the end of treatment and follow-up.

CONCLUSIONS: The large-improvement subgroup showed more improvement than the small-improvement subgroup with regard to physical factors typically targeted by this specific exercise program as well as for psychological factors that are known to influence clinical outcomes.