The effects of Müller-Lyer illusion on toe clearance height in stair ascent

The appearance of a stair can impact the safe negotiation of stair climbing. Increasing toe clearance height is a common strategy for reducing the risk of tripping when stepping onto a raised surface.

Can the Müller-Lyer illusion cause people to increase their toe clearance height when walking over a stair?

Indoors, 15 healthy individuals (9 women and 6 men) walked up a 3-step staircase with each step 60 cm wide, 18 cm tall, and 30.5 cm deep. Outdoors, 253 people walked up a 2-step staircase with each step 6 m wide, 15 cm tall, and 38 cm deep. Four visual conditions were displayed at both the indoor and outdoor locations: fins out, fins in, line, and control.

While the fins out stairs were perceived taller than all the other displayed stair conditions, the toe clearance height was not different between display conditions when tested indoors. However, toe clearance height was greater in the fins out condition when compared to all other conditions for the outdoor stairs location.

In the natural environment when walkers were oblivious to themselves being filmed and not having any knowledge of the task demand, the Müller-Lyer illusion significantly impacted their toe clearance height.