Does postural chain muscular stiffness reduce postural steadiness in a sitting posture?

This study investigated the effect of postural chain muscular stiffening on postural steadiness when it is rhythmically perturbed by respiration. It consisted of an analysis of centre of pressure (CP) displacements when constant sub-maximum pushing efforts were performed in a sitting posture. Muscular stiffness, assessed by surface electromyography (iEMG), was imposed at two controlled levels, using two intensities of pushing effort (20% and 40% of the maximum voluntary contraction: 20MVC and 40MVC). Lumbo-pelvic mobility was varied using two different support areas at the seat contact (100% and 30% of the ischio-femoral length: 100BP and 30BP). Respiratory disturbance to posture was varied using two respiratory rate conditions (quiet breathing (QB), which is the spontaneous rate, and fast breathing (FB) at a rate imposed by a metronome).The results demonstrated that an increased push effort was associated to a higher iEMG level, and induced greater mean deviation (X¯p) and sway path (SP) of antero-posterior CP displacements. It was concluded that postural muscle stiffness reduces postural steadiness. It was suggested that it could be related to a weaker compensation of respiratory disturbance to body posture.