Using a stair horizontal-vertical illusion to increase foot clearance over an inconsistently taller stair-riser

Timmion K. Skervin, Neil M. Thomas, Andrew J. Schofield, Mark A. Hollands, Constantinos N. Maganaris, Thomas D. O’Brien, Vasilios Baltzopoulos, Richard J. Foster*, Kei Masani (Editor)

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


Introduction: Stair falls can be caused by inconsistent stair dimensions. During ascent, inconsistently taller stair risers lead to reduced foot clearances as the inconsistency goes unnoticed. A stair horizontal-vertical illusion increases perceived riser heights and foot clearance and could offset reduced foot clearances over inconsistently taller risers, though this might impact other stair safety measures.

Method: Twelve participants (age: 22 (3) years) ascended a seven-step staircase under three conditions: i) all steps consistent in riser height (consistent), ii) a 1cm increase in step 5 riser height (inconsistent) and iii) a 1cm increase in step 5 riser height, superimposed with a stair horizontal-vertical illusion (illusion). Vertical foot clearance, foot overhang, and margins of stability were assessed over step 4, 5 and 6. Perceived riser height due to the illusion was determined through a computer perception test. A One-Way Repeated Measures ANOVA compared biomechanical variables between conditions. A One Sample t test compared perceived riser height to the true height.

Results: Over the inconsistent step 5, foot clearance reduced by 0.8cm compared to consistent. Illusion increased foot clearance by 1.1cm and decreased foot overhang by 4% compared to inconsistent. On step 4 the illusion led to more anterior instability compared to inconsistent. Illusion and inconsistent led to more mediolateral stability compared to consistent. The illusion increased perceived riser height by 12%.

Discussion: Foot clearance reductions over inconsistently taller risers can be offset by a stair horizontal-vertical illusion. Additional benefits included a safer foot overhang and unaffected stability over the inconsistent riser. Changes to step 4 stability might have resulted from leaning forward to look at the step 5 illusion. The stair horizontal-vertical illusion could be a practical solution for inconsistently taller stair risers, where a rebuild is usually the only solution.
Original languageEnglish
Article numbere0257159
JournalPLoS ONE
Issue number9
Publication statusPublished - 14 Sept 2021

Bibliographical note

Copyright: © 2021 Skervin et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.

Funding: TKS was awarded a PhD scholarship,
funded by Liverpool John Moores University (UK).


  • Vision
  • Elderly
  • Falls
  • Psychophysics
  • Sensory perception
  • Kinematics
  • Toes
  • Young adults


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