Seated human response to simple and complex impacts: Paraspinal muscle activity

Prolonged exposure to seated impacts has been associated with low back disorders, although the underlying mechanism is not well understood. In this pilot study, 12 healthy right-handed male subjects were exposed to low amplitude simple (from one direction) and complex (from two directions) seated impacts under two suspension conditions (enabled or disabled) and two sitting postures (supported or unsupported). A simple impact consisted of a single strike in either a side-to-side or a vertical-upwards direction with peak acceleration of about 4.6 m/s2 and 100 ms in duration. Complex impacts consisted of two simple impacts applied in sequence with 100 ms delay in between. Myoelectric activity of the left and the right erector spinae muscles at L3 (LES and RES) were recorded during impacts. It was found that the muscle response timing was dictated by the direction of the first impact during complex impacts, while the peak muscle response amplitude varied in accordance with the second impact. The combined muscle activity was slightly higher during complex impacts. It was also found that enabling the seat suspension resulted in a significantly lower muscle response in LES but not RES. In contrast, enabling the seat suspension tended to cause a delay in muscle response timing in RES but not LES. The combined muscle activity was significantly lower when the seat suspension was enabled. Sitting posture did not affect muscle response to impacts. These results suggest that the control system may not be able to protect the lumbar spine from seated impacts. Furthermore, muscle activity during impacts may significantly increase spinal joint load. Therefore, back injury is more likely to occur in work environments containing mechanical shocks. Better shock isolation is warranted.