Background: Prolonged sitting is recognized as a risk factor for the reporting of low back troubles. Despite the use of exercise balls in replacement of the office chair, little quantitative evidence exists to support this practice and hence motivated this research. Given the potential for several biological effects and mechanisms this study was approached with several layers of instrumentation to quantify differences in muscle activation, spine posture, spine compression and stability while sitting on an exercise ball versus a stable seat surface. Also, differences in the pressure distribution at the seat-user interface were quantified for the different seat surfaces to provide an objective perspective on the mechanism influencing perceived comfort levels.
Methods: Eight male subjects volunteered to sit for 30 min on an exercise ball and on a wooden stool. Muscle activity and spine position were used to model spine load and stability. An additional seven sat on an exercise ball and chair to examine pressure distribution over the contact area.
Findings: There was no difference in muscle activation profiles of each of the 14 muscles between sitting on the stool and ball. Calculated stability and compression values showed sitting on the ball made no difference in mean response values. The contact area of the seat-user interface was greatest on the exercise ball.
Interpretation: The results of this study suggest that prolonged sitting on a dynamic, unstable seat surface does not significantly affect the magnitudes of muscle activation, spine posture, spine loads or overall spine stability. Sitting on a ball appears to spread out the contact area possibly resulting in uncomfortable soft tissue compression perhaps explaining the reported discomfort.