Individuals maintain balance as long as the footprint of the body’s Center Of Gravity (COG) remains within the body’s base of support (see figure below). Since the footprint of the COG is very close to the rear edge of the base of support (and relatively far away from the front and side edges), imbalance normally occurs when the COG sways beyond the rear edge of the base of support. People regain balance by moving a leg backward, thus increasing the base of support. This backward step is sometimes also called a “compensation step” or “extra step”.
Imbalance is sensed by the vestibular, the visual and the proprioceptive biosystems. Degraded sensing systems of older people lead to delayed detection of imbalance, which means that the imbalance is detected when sway of the COG is already too large. Reflexes in the older people cause the backward step to be taken too slowly.
In short, elderly people perform the backward step too late, too slowly and often inaccurately, all of which leads to a fall.