The relationship between cross sectional area and strength of back muscles in patients with chronic low back pain

Objective: To evaluate the relationship between the cross sectional area (CSA) and isokinetic strength of the back muscles in patients with chronic low back pain. Method: Data of twenty-eight middle-aged patients with chronic back pain were analyzed retrospectively. CSAs of both paraspinal muscles and the disc at the L4-L5 level were measured in MRI axial images and the relative CSAs (rCSA: CSA ratio of muscle and disc) were calculated. Th e degree of paraspinal muscle atrophy was rated qualitatively. Isokinetic strengths (peak torque, peak torque per body weight) of back flexor and extensor were measured with the isokinetic testing machine. Multiple regression analysis with backward elimination was used to evaluate relations between isokinetic strength and various factors, such as CSA or rCSA and clinical characteristics in all patients. Th e same analysis was repeated in the female patients. Results: In analysis with CSA and clinical characteristics, body mass index (BMI) and CSA were significant influencing factors in the peak torque of the back flexor muscles. CSA was a significant influencing factor in the peak torque of total back muscles. In analysis with rCSA and clinical characteristics, BMI was signifi cant in infl uencing the peak torque of the back flexors. In female patients, rCSA was a signifi cant infl uencing factor in the peak torque per body weight of the back flexors, and age and BMI were infl uencing factors in the peak torque of back fl exors and total back muscles. Conclusion: In middle-aged patients with chronic low back pain, CSA and rCSA were infl uencing factors in the strength of total back muscles and back fl exors. Also, gender and BMI were influencing factors.