Objective
To evaluate the correlation of the cross-sectional areas (CSA) of paraspinal muscles (back extensors and psoas muscles) and full range-of-motion isometric lumbar extension strength in the individuals with low back pain.

Method
Twenty four subjects (14 men and 10 women) with low back pain completed a maximum isometric lumbar extension strength test at seven angles through a 72^o range of motion (0, 12, 24, 36, 48, 60, 72 degrees of lumbar flexion). CSA of back extensors and psoas muscles were measured from standardized transaxial view by CT scanner.

Results
CSA of lumbar extensor and psoas were correlated with isometric lumbar extension strength from full flexion to extension in the low back pain patients. The greater the lumbar flexion angle, the greater the coefficient of determination (R²). The correlation coefficients of psoas muscles were greater than those of lumbar extensors.

Conclusion
Both back extensors and psoas muscles do their important role during isometric lumbar extension. Isometric lumbar extension strength of full lumbar flexion is well correlated with CSA of paraspinal muscles.