To find the optimal height for sagittal motion axis for trunk strength test in chronic low back trouble.
Cross-sectional study.
The strength of trunk muscles of low back pain patients is decreased. The measured strength depends on the height of the sagittal motion axis but the differences between patients and controls are not known.
114 (67 female) patients with chronic low back trouble are classified according to Quebec Task Force, 50 (31 female) patients with rheumatic disorder, but without low back trouble, and 33 (22 female) healthy controls, no appreciable physical differences but clear differences in Oswestry score. Isometric trunk extension-flexion test with different heights for the pelvic fulcrum.
Force decreased in extension, increased in flexion, and torque increased both in flexion and extension in every group (P<0.001) as the fulcrum was moved caudally. The male controls were stronger than patients with low back trouble (P<0.01). The female controls were stronger only if the fulcrum was set at the hip joint level (P<0.05). There were no differences between patients with rheumatic disorder and low back trouble, except in extension if the fulcrum was at the hip joint level (P<0.02).
Trunk muscle weakness is a common sign of different rheumatic disorders. Proper setting of sagittal motion axis and concomitant measurement of trunk and hip extensor or flexor muscles increases the specificity of the strength test for low back trouble.
The rotation axis in trunk extension-flexion strength test should be set at the level of the hip joint.