Control of body centre of mass momentum during sit-to-stand among young and elderly adults

The purpose of this study was to identify hypothesized age-related changes in the momentum of the body centre of mass across different speeds of ascent during sit-to-stand. The peak horizontal and vertical momenta were measured among 16 young and 16 older adults with the aid of a motion analysis system. The results revealed a between-group similarity in the horizontal location of the centre of mass at the time of takeoff, and the magnitude and timing of the peak horizontal momentum of the centre of mass. Together with a between-group difference in the timing of the peak vertical momentum of the centre of mass, an age-associated decline in the magnitude of the peak vertical momentum of the centre of mass was most pronounced at rapid speed of ascent, and accounted for the major difference in performance between age groups. Whereas the magnitude of the peak vertical momentum of the centre of mass varied continuously across speeds, its horizontal counterpart did not increase when speed of ascent increased from natural to fast for both groups. Such contrasting results in the peak horizontal versus vertical momentum of the centre of mass observed across different speeds of ascent and age groups suggested that this horizontal parameter may normally be tightly regulated to maintain upright stance at the completion of the sit-to-stand transfer. The constraints on the projection of the centre of mass with respect to the base of support and the horizontal momentum of the centre of mass serve as two necessary conditions for maintaining upright stance at the termination of dynamic weight transfers.