Cadaveric lumbar intervertebral joints were subjected to physiologic loads to simulate flexion. The resistance to bending was measured first with the joint intact and again after cutting in turn through the supraspinous/interspinous ligaments, the ligamentum flavum, and the capsular ligaments of the apophyseal joints. In this way the part played by each of these structures, and the intervertebral disc, in resisting and limiting flexion was determined. The results show that at the limit of flexion the supraspinous and interspinous ligaments sprain first and that the capsular ligaments and the intervertebral disc offer considerably more resistance than the ligamentum flavum and the supraspinous/interspinous ligaments. The intervertebral joint was found, on average, to be sufficiently stiff to balance about half the bending moment exerted by the trunk in full flexion.