The funny problem with the stairs at Brooklyn’s 36th Street subway. Why we trip..
At Brooklyn’s 36th Street subway stop, one of the steps is slightly higher than the others. Stairs have a standardized Rise and Run and when this is altered, specifically the Rise, funny things happen. Filmmaker Dean Peterson set up his camera to capture the stumbles and the video can be seen here http://vimeo.com/44807536 and above on our blog.
The dangerous step, it turns out (which has since this video been repaired), is apparently a half-inch higher than the others. Stairway design guidelines vary within a small range. Guidelines call for risers to be a minimum of 6 inches and a maximum of 7 ¾ inches. The allowable variance between steps is 3/8 of an inch depending on the source you seek.
(The general rule (in the US) is 7-11 (a 7 inch rise and 11 inch run). More exactly, 7 ¾ rise and an 11 ½ inch run, although some people will use a run of as little as 9 inches.)
This is a perfect example of how sensitive and predictive the human body is with all of its amazing joint position sense receptors. But there is more to it than a simple step height differential. Read on.
There are multiple demands that stairs place on the neuro-musculoskeletal system. There are needs for input from the somatosensory, visual, and vestibular systems at various stages in the task. Some of these collaborating systems deteriorate with the aging process making the failure of stair negotiation a legitimate risk for the elderly or those that are handicapped in one of the 3 primary systems noted above. Studies (see references below) strongly link impairments in the visual system to safe stair negotiations. The Buckley study found that the mediolateral balance during stepping up and down stairs (single limb support stability) was significantly reduced (especially stepping down) by blurred vision highlighting the critical necessity of vision in stepping dynamics. Hence, one must be aware of people traffic on steps, if a stairway is dense with traffic the ability to gain the visual cues of the successive stairs is paramount. The next time you are in a school or subway stairwell notice the undercurrent of your discomfort may be from the inability to see enough steps in front of you. Letting the person ahead of you clear some distance is a must, especially if you are vision impaired, elderly, wearing dark tinted glasses or are without the ability to use other cues such as railings (ie. adding a tactile feedback system to satisfy the impairment of visual cues).
There are other user created impairments that we may be unaware of consciously. In the Miyasike-daSilva study it was determined that as participants approached and walked stairs, gaze was within 4 steps ahead of their location indicating that individuals often rely on spatial cues from prior experience or from other visual cues to obtain the necessary information from the environment. Thus, one must be careful carrying something such as a baby, groceries or laundry basket in front of you thus impairing the lower visual field. We have all carried something up or especially downstairs and either thought we were on the last step or found we had one more to go and found ourselves either stumbling forward or hyperextending our knee as we lurch down onto the unexpected step. In the video you will see a great example of this forward catch as one of the ladies is carrying a baby in front of her, luckily she makes the correct saving motor choice. Being able to plan/control landing mechanics are significantly different when the visual system is locked out or impaired from stepping tasks. Timmis found that the contribution of information from the lower visual field of gaze in controlling the landing strategy occurs predominantly prior to or during movement initiation of the foot and limb and that ‘online’ or immediate vision is used only in the latter portion of the descent phase to fine tune the step landing. Buckley found that under visual impairments subjects used the cautious strategy of keeping their weight back on the trail limb longer making weight transfer noncommittal affording the time necessary for the lead limb to fish around for the next step.
There is so much involved in negotiating stairs and steps, even level ground walking. There are many cues we have learned to subconsciously glean information from. Sadly, when we begin to age and lose proprioceptive or visual information things begin to fall apart. The system is so sensitive and intuitive. This is why when someone changes the ground level, or the height of a step as in this video, the system fails even the best of us who have all of our faculties about us. And, we learn more about gravity at that moment than we wish to learn.
Shawn and Ivo, The Gait Guys
7. PLoS One.2012;7(9):e44722. Epub 2012 Sep 6.Does it really matter where you look when walking on stairs? Insights from a dual-task study. http://www.ncbi.nlm.nih.gov/pubmed/22970297Miyasike-Dasilva V,McIlroy WE.PMID:22970297[PubMed - in process] PMCID:PMC3435292 Free PMC Article