Low magnitude loading on the body has become an important issue with the occurrence of injuries that have been attributed to repetitive loading of tissue at magnitudes of forces and motions that are below the known maximum strength of the tissues in question. The general purpose of this thesis was to explore the in vivo low level static and dynamic loading on the spine and determine if these loading parameters could generate injuries when similar low magnitude parameters were tested in vitro. In vivo activities (walking, sitting, standing, and back extensor exercises) were examined to quantify the magnitude of low back joint loads, motions, and muscular activations levels required. In vitro highly repetitive loading was performed at modest flexion/extension moments or angular rotations combined with low magnitude axial compressive forces.

Walking was found to be a highly dynamic/cyclic activity with moderate spine loads and small lumbar spine angular motions. The back extensor exercises produced a range of joint loads from low loads for exercises such as single leg extension to loads exceeding spinal compression limits for contraindicated trunk extension exercises. Sitting and standing both resulted in low magnitude joint forces and muscular activity levels. Standing exhibited very small and static ranges of spine postures whereas sitting resulted in a range of postures from 30% to 80% of the lumbar spines flexion range of motion.

The in vitro highly repetitive testing of porcine cervical spine motion segments at low magnitude compressive loads and modest flexion/extension motions and moments resulted in intervertebral disc herniations. The angular stiffness of specimens increased throughout the testing cycle and increased magnitudes of axial compressive loads resulted in increased probability and severity of disc damage.

This research documents the magnitude of lumbar spine joint forces, spinal motions, and muscular activation levels during common in vivo activities and demonstrates that low level repetitive loading scenarios can result in spinal injuries.