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It has been shown that prolonged sitting and maintaining rigid postures can induce and exacerbate back pain. Postural fixity can have painful and debilitating implications including spinal shrinkage, increased intervertebral disc pressure and edema in the legs. Incorporating movement within a chair?s seat in the form of continuous passive movement (CPM) has been shown to assuage these physiological effects. However, apart from physiological studies, measuring effects of task performance, back pain, personal comfort, motion sickness, and the extent of torso movement are sparse. Experiment 1 measured the affect of CPM built within a chair?s seat pan on these factors by comparing static and CPM conditions among 36 participants, half of whom had pre-existing back pain. Participants performed 3 work related tasks separated by 2 relaxation tasks, during two one-hour sitting conditions: one static and one CPM. The seat pan was set at a constant speed setting #4 throughout the CPM condition. Torso and seat pan movement were recorded using nano-accelerometers; a general comfort rating scale (GCRS) and a body map instrument were used to measure body discomfort over 6 collection times throughout the testing session; speed and error rate were used to measure typing, writing, and mousing performance; and a post-test questionnaire measured psychological and personal comfort, including measures of motion sickness.

Results showed that CPM did not statistically significantly affect participant performance or back pain (p>.05). There were statistically significant differences between personal comfort measures relating to postural stability (p<.000), postural instability (p<.003), limitations in writing and typing (p<.001), feelings of nausea (p<.023), and dizziness (p<.024) between the static and CPM conditions. Feelings were worse in the CPM condition for all variables. There were no statistically significant differences in questionnaire responses between those with and without back pain (p>.05) for either condition. Results further showed that participant?s torsos moved more while sitting in the CPM condition [F(3.334, 106.687)=17.285, p=.000] and while performing active versus passive tasks [F(1, 32)=26.531, p=.000]. Looking at open-ended comments specific to the chair?s CPM, there were more comments in disfavor (n=28) than in favor of the motion (n=14), and, when asked to reflect on the sitting experience, participants commented more negatively (n=46) than positively (n=28). Overall, people without pre-existing back pain commented more negatively on the chair?s motion (n=25) than those with back pain (n=18).

The first experiment maintained a constant speed setting #4 within the seat pan, restricting participants from manipulating the speed controls according to their personal preferences. A second experiment complemented this experiment by giving participants control over the speed setting to test preferences for specific speeds of chair seat pan movement according to particular task demands. This second experiment tested 12 participants with pre-existing back pain. Participants experienced similar protocol as the first experiment, except participants were allowed to manipulate the chair?s speed and the nano-accelerometers were not used.

Results showed that people preferred having no motion during performance tasks (writing, typing, and using the mouse) and a moderate to low speed for passive, relaxation tasks (movies). Subjective responses were similar to Experiment 1. Overall, there were more negative (n=20) than positive comments (n=11) when asked to reflect on the sitting experience. These comments were almost exclusively related to the movement of the chair?s seat pan.

Results for Experiments 1 and 2 suggest that CPM may be best applied intermittently, allowing users to vary the speed settings according to task demands and comfort level. This has broad application to work places where traditional work breaks, such as walking, stretching, and standing, may be difficult to execute, or for individuals who are sedentary for long durations, such as truck drivers or wheelchair bound individuals.

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continuous passive movement; back pain; chair design; torso movement; task performance; subjective comfort; nano accelerometers; seat pan movement speed preference


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