For understanding of the mechanical causes of low-back pain, knowledge of the biomechanics of the various spinal elements is essential. In this in vitro biomechanical study, in situ behavior of spinal ligaments of the L3-4 and L4-5 functional spinal units during physiologic activities was studied in a three-stage procedure. First, 72 load-displacement curves were obtained to determine the three-dimensional flexibility characteristics of the spinal units. Second, three-dimensional morphometric measurements were made of all the spinal ligament attachment points. Finally, a mathematical model was constructed to combine the flexibility and morphometric data and compute the ligament length changes and strains as functions of various spinal movements. In flexion movement, the interspinous and supra-spinous ligaments were found to be subjected to the highest strains, followed by the capsular ligaments and the ligamentum flavum. During extension, it is the anterior longitudinal ligament that has the maximum strain. In lateral bending, the contralateral transverse ligaments carried the highest strains, while the interspinous and supraspinous ligaments were relatively unstrained. In rotation, the capsular ligaments were by far the most strained ligaments.