An invariant guiding stair descent by young and old adults

In this study the author compared the perception-action capability of young and old adults (respectively, mean age 22 and 62 years old) when descending stairs and examined the relevant task constraints that guide the action. It was found that old adults selected and descended stairs that were significantly lower than young adults and showed less hip joint flexibility. However, the performance parameter, defined as the ratio between the height of the stair and the distance taken by the stepping foot to the top edge of the stair, was invariant for both groups. Thus, despite different ability levels, young and old adults are constrained by the same perception-action invariant for guiding the act of stair descent.