Current treatments for low back pain have small effects. A research priority is to identify patient characteristics associated with larger effects for specific interventions.

The aim of this study was to identify simple clinical characteristics of patients with chronic low back pain who would benefit more from either motor control exercises or graded activity.

This study was a secondary analysis of the results of a randomized controlled trial.

One hundred seventy-two patients with chronic low back pain were enrolled in the trial, which was conducted in Australian physical therapy clinics. The treatment consisted of 12 initial exercise sessions over an 8-week period and booster sessions at 4 and 10 months following randomization. The putative effect modifiers (psychosocial features, physical activity level, walking tolerance, and self-reported signs of clinical instability) were measured at baseline. Measures of pain and function (both measured on a 0–10 scale) were taken at baseline and at 2, 6, and 12 months by a blinded assessor.

Self-reported clinical instability was a statistically significant and clinically important modifier of treatment response for 12-month function (interaction: 2.72; 95% confidence interval=1.39 to 4.06). Participants with high scores on the clinical instability questionnaire (≥9) did 0.76 points better with motor control exercises, whereas those who had low scores (<9) did 1.93 points better with graded activity. Most other effect modifiers investigated did not appear to be useful in identifying preferential response to exercise type.

The psychometric properties of the instability questionnaire have not been fully tested.

A simple 15-item questionnaire of features considered indicative of clinical instability can identify patients who respond best to either motor control exercises or graded activity.