Effects of sitting posture and seat backrest on the biodynamic response of the human body and the prediction of spinal forces during vertical whole-body vibration

Abstract

Biodynamic models have been developed to predict the dynamic spinal forces induced by whole-body vibration but the effects of sitting posture and backrest conditions on these forces are unclear. The main objectives of the research reported in thesis were to advance understanding of: (i) how sitting posture and contact with a backrest affect the biodynamic responses of the human body, and (ii) the effects of sitting posture and backrests on spinal forces during exposure to vertical whole-body vibration.

Experimental measurements found that the apparent mass of the body and transmissibilities to the spine (to the pelvis, L5, L3, and T5) are affected by the presence of a vertical backrest or an inclined backrest (inclined by 10º, 20º, and 30º). An inclined backrest induced a broad peak, or even two peaks, around 4 to 8 Hz in the vertical apparent mass at the seat pan, probably because the backrest separated the body modes contributing to the principal resonance around 5 Hz evident when sitting with no backrest. Sitting with either vertical or inclined backrests increased vertical motions of the pelvis and the spine.

Leaning forward in an ‘anterior leaning’ sitting posture increased the frequency of the principal resonance in the vertical apparent mass at the seat, possibly due to increased tension in back muscles. Leaning forward in ‘anterior leaning’ or ‘kyphotic leaning’ postures induced a resonance around 2.5 Hz in the vertical apparent mass at the seat pan, due to excitation of body modes at frequencies less than 5 Hz associated with fore-and-aft motions of the pelvis and the spine. Changing sitting posture changes muscle activity. Tensing muscles in the lower body (including the lower lumbar spine, pelvis, and thighs), or tensing muscles in the whole body (the lower and upper torso), produces similar increases in the frequency of the principal resonance in the vertical apparent mass at the seat around 5 Hz. This suggests tensing muscles in the lower body causes a greater increase in the frequency of the principal resonance than tensing muscles in upper body. Biodynamic models of the seated human body that included forces from muscles were developed to fit the measured responses of the body (apparent mass and transmissibilities) in the various sitting conditions (normal and leaning forward, vertical and inclined backrests). The spinal forces in the vertical and fore-and-aft directions at the L5/S1 intervertebral disc were estimated from the sum of the predicted static and dynamic forces in both directions. In each sitting condition, a linear model was used to predict the frequency-dependent transfer function between the vertical seat acceleration and the dynamic forces in the spine. For the sitting conditions studied in this research, the contributions from the muscles to static spinal forces were comparable to the forces from gravity of the body mass supported on the intervertebral disc. Dynamic muscle forces were predicted to contribute significant dynamic spinal forces in the vertical and fore-and-aft directions during vertical whole-body vibration.

Varying the sitting conditions varied the spinal forces predicted by the models, both with and without exposure to vibration. Transfer functions between vertical seat acceleration and dynamic spinal forces showed one or two resonances around 4 to 8 Hz. The resonance frequency in the transfer function between vertical seat acceleration and dynamic vertical spinal force increased with increasing inclination of a backrest, similar to the effect of backrest inclination on the vertical apparent mass at the seat pan. Compared to a normal sitting posture, sitting with 20º-inclined backrest increased the predicted static and dynamic spinal forces in the spine in the vertical and fore-and-aft directions, due to increased forces at the backrest and increased motion of the spine in both directions. Forward leaning sitting postures increased the fore-and-aft motions of the spine and increased the fore-and-aft dynamic spinal forces predicted by the model.

It is concluded that sitting posture and contact with vertical or inclined backrests alter the biodynamic responses of the seated human body. The changes arise from several mechanisms including the backrest supporting some of the body mass, changes in static muscle activity, changes in dynamic muscle activity, and changes in the modes of vibration in the body. These mechanisms are also responsible for predicted changes in the forces in the spine during vertical whole-body vibration.

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