Dimensional analysis and ground reaction forces for stair climbing: effects of age and task difficulty

Altered perception–action capability is often associated with falls and diminished self-efficacy in olderpeople. This study evaluated and compared perception–action capability in stair-climbing performanceof 18 healthy volunteers assigned to two age groups (mean age, 26.3 ! 4.3 years and 66.4 ! 4.7 years,respectively). The experimental set-up included 14 stairs (50 cm wide, 60 cm deep, riser height 35–90 cm)positioned at the edge of a force platform. The task was to climb the stair with the greatest riser heightsubjects thought they could climb without outside support or use of hands. Dimensional and dynamic datawere collected and analyzed to reveal the invariant relationships that sustain action preparation andexecution. All subjects chose the same proportion between stair height and distance covered before mountingthe stair, as expressed by the invariant angle (a). While the geometric invariant relationship was picked up asa visual guide prior to action, there was a dynamic invariance in the forces applied during actual execution. Toestablish whether the invariance still held in extreme cases, two perturbed conditions were introduced inwhich stair distances were changed, forcing subjects to execute a foot-strike, either very far from or near tothe stair, before climbing it, so as to reveal any significant adaptations the climber would undertake to avoidslips or falls. Older and younger subjects applied appropriate visual and motor guidance by scaling their motorcapabilities to the environmental dimensions.