Spinal shrinkage is commonly used as a measure of spinal loading. Spinal shrinkage has been shown to be affected by design features of chair backrests. A freely reclining (dynamic) backrest, passive movement of the lumbar spine via means of a cyclically inflated bladder and passive side-to-side rotation of the spine through a very small angle about its vertical axis at a low frequency via motion of the seat pan have all been shown to produce decreased spinal shrinkage.In the current study we investigated the effect of two independent variables (pivoting backrest and activity level) on spinal shrinkage. The first variable was a pivoting backrest design with two use conditions, locked vs. unlocked. A locked backrest could recline but not pivot while an unlocked backrest could do both. Activity level consisted of an active condition during which the participant typed for 25 minutes and then moused for 25 minutes, which was interspersed every five minutes with a maximum forward reach to touch targets located at about 30, 60, 120, 150 180 degrees relative to the participant's centerline. During the passive activity condition, the participant only sat and typed.It was hypothesized that the pivoting backrest would flex and extend the lumbar spine and/or stabilize the pelvis during the subjects' normal movement patterns and that it would have an effect similar to the previous spinal motion studies, which had found decreased spinal shrinkage as an effect of passive spinal motion. It was further hypothesized that the more active conditions would work synergistically with the pivoting backrest to retard spinal shrinkage.Five individuals participated in the study, three males and two females.Spinal shrinkage was statistically significantly less when the backrest pivot action was unlocked (t = 0.044) and nearly so during the active condition (t = 0.059). The pivoting backrest condition was also significantly preferred to the locked condition.
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