Does body stability depend on postural chain mobility or stability area?

The purpose of this study was to examine whether postural stability depends only on the support base perimeter, that is the stability area, when body balance is perturbed by respiration. To this end, seven normal subjects were asked to breathe quietly, breathe deeply and to hold their breath (apnoea). They were asked to maintain a standing posture (Sta), and two sitting postures differing by the ischio femoral contact with the seat (Sit100 and Sit30). In other words, these three postures differed not only by the stability area, but also by pelvis mobility. The thoracic perimeter, displacement of the centre of pressure (CP) and iliac crest acceleration (Ah), taken as an index of pelvis mobility, of seven normal subjects were recorded. The results showed that the sway path (SP) was longer in seated subjects than in standing ones, and in Sit100 than in Sit30. The distance between the CP extreme positions (Delta Xp) varied in the opposite direction to SP. Iliac crests and thoracic displacements were shown to be in phase in Sit condition, and did not display any particular pattern in Sta. It was concluded that postural steadiness depends on the postural chain mobility in addition to stability area. As pelvis and lumbar column mobility are related, it is proposed that both contribute to postural chain mobility, owing to respiratory perturbation being compensated.