Age-related changes in balance control system: initiation of stepping.

The balance control system of a group of healthy and fit, young and elderly subjects was studied during the initiation of stepping in one of three directions: forward, sideways, and backwards in response to a light cue. The performance of these movements requires shifting support from two to one foot, moving the centre of mass outside the initial base of support and creating a new support configuration. By recording and analysing the vertical ground reaction force beneath the subject's stepping foot, we were able to examine the two phases prior to limb lift-off for stepping: reaction time and weight transfer time. Both reaction time and weight transfer time increased with age: The elderly subjects had a proportionately larger increase in weight transfer time compared to the reaction time. The peak force generated showed an age by stepping direction effect: the elderly had a significantly lower peak force for the forwards stepping compared to the younger subjects. The larger increase in weight transfer results in a slower stepping response. Since a stepping task is often recruited to avoid a fall, the increase in response execution time can have undesirable consequences.