Low back pain syndrome (LBPS), is one of the most prevalent medical problems in western societies. The estimated incidence rate is as high as 75--80% of all adults. Biomechanical factors influence the pathogenesis of LBPS, yet little is known about the mechanical function of the spine (lumbar or otherwise) in vivo.; A compression device made of non-metallic materials was constructed and calibrated to apply specified loads on the spine. Torso musculature was relatively inactive during pilot loading experiments, and the total spine compression force was estimated to be equal to the external force.; Subjects' lumbar spines shortened 3.9 +/- 1.2 mm during ten minutes of compression (1.0 x bodyweight). Segment compression, bending, and lumber rotation contributed to shortening, but bending was the dominant influence in five of eight subjects. Lumbar rotation dominated the shortening in two subjects and shared equal influence with compression in one subject. Those three subjects had distinctly different shaped spines compared to the other five (and compared to each other). Pure compression was insubstantial in most cases, but five individual segments compressed more than 1.0 mm. Changes opposite to the passive direction in all three components of shortening suggested the presence of an active component(s) in the overall response.; Flexion and posterior translation of the disc at the lumbosacral joint, and posterior sacral translation described the most common response pattern of the lumbosacral joint. Still, only four subjects underwent that exact aggregate response pattern. There were changes opposite to each of those trends, and the subjects did not group according the initial spine shape as they did with regard to the shortening mechanism. Posterior sacral rotation and lumbosacral flexion represented changes requiring muscle activity. Gluteal muscles were probably recruited to control spine posture and help support the load, but their activity was not measured. Thus, the spine did not act as a passive continuum beam, but rather as a driven mechanical system.; The present results characterize deformation response patterns of the lumbar spine during loading, and they serve as a reference point for further investigation at different scales (e.g. the thoracolumbar spine or the intervertebral disc).
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