This investigation of the effects of trunk exercises performed on an unstable surface on trunk muscle activation in stroke
patients showed that the activation of the external and internal oblique muscles, excluding the erector spinae, significantly
increased, compared to that in the control group. When the arm is lifted in the sitting position, the moment of the trunk acts
backward to maintain stability as the center of mass moves forward22), and postural sway occurs because of this shift in the
center of mass and the reaction force23). In addition, to maintain trunk stability during weight-shifting in order to counteract
the change in the center of mass, a properly coordinated action of muscles regulated by the nervous system is needed14).
When trunk exercise was performed on a balance ball in this study, trunk muscle activation might have been further promoted
because of the reaction force acting against the shaking of the surface.