Background In case of people suffering from chronic low back pain, specific movements of the hip, pelvis, and trunk are associated with pain. Comparing range of motion measurements for multiple planes and from different segments and lines in reference to those of healthy individuals seems interesting but present interpretations challenge in relation to important number of variables and correlation with clinical data. Methods The proposed index is based on using principal component analysis to quantify differences in trunk mobility between patients with chronic low back pain and a control group. Kinematic data were recorded for the cervical and thoracic vertebrae, the lumbar spine, and the pelvic and scapular belts during repeated trials (hip flexion and extension, hip bending, and trunk twists). Angular motion values were calculated. Principal component analysis was used to convert 10 discrete variables (kinematical data) extracted from control data into 10 independent variables. Findings The proposed index comprises the sum of the variables. Initial demonstration of its clinical utility and statistical tests of this index validity were revealed. It establishes correlations between the psychosocial impact of chronic low back pain, trunk mobility (as summarized by the index) and the positive effects of functional restoration program. Interpretation This index let to assess the absolute potential benefits of rehabilitation in term of kinematic motion. Functional restoration program promotes the physical functioning of patients by increasing their range of motion. This index uses kinematic motion to assess the potential benefits of such rehabilitation program.