The International Code Counci (ICC), International Building Code (IBC), and certain walkway safety standards require changes of elevation to be less than 0.5 inches in height unless that change of elevation, depending on height, is beveled or sloped. The standard allows for smaller changes of elevation to exist since they are not considered hazardous for the general population. Consistent with this, there are engineering and architectural tolerances associated with the flatness and fairness of constructed walking surfaces. The safety question proposed is; When is a walking surface change in elevation that is considered to be de minimis a hazard to certain elderly or walking deficient pedestrians?
Where does this dimensional tolerance come from and why choose 0.5-inch as the standard? Human walking is bipedal and is characterized by continuous, successive alternating periods of a leg stance while the other leg swings forward, then striking the ground with the heel. The stance phase has several divisions that can be simplified as heel strike, the heel strikes the ground; mid-stance, the full weight of the body is supported as the other leg swings forward; and push off, where the standing leg pushes off the ground to start the next swing phase. A pedestrian can trip when the swinging leg and foot are impeded from transitioning forward by, for example, a swinging foot contacting a change of elevation. This can happen at push off when the foot is about to lift from the ground and pushed forward, or during the swing process at the lowest point of clearance with the ground.
The smallest distance from the toe to the ground can be known as Minimum Toe Clearance (MTC) or Minimum Foot Clearance. MTC research evaluates the distance at which the toe of the shoe travels above the walking surace. MTC research is a major consideration for the standard of the 0.5-inch change of elevation, because the average MTC for pedestrians is approximately 0.6 inches. The 0.5-inch standard is a value where the average pedestrian is expected to clear a walking surface protrusion, likely with a factor of safety. However, the MTC value of 0.6 inches is for the average population which includes a mix of young, elderly, healthy and any other group of pedestrians. MTC varies with a wide spread of biomechanical parameters of leg motion and depends on the individual muscle control and/or strength during walking. Khandoker et. al. stated, “It has been well documented in literature that aging contributes to altered control mechanism of human locomotor, balance, which in turn can influence gait parameters.” R. Begg et. al. performed a study to analyze MTC between a young population of people and the elderly. The mean MTC for the young group was 0.61 inches with a standard deviation of 0.24 inches; for the elderly group the mean MTC was 0.58 with a standard deviation of 0.30 inches. If we analyze these numbers at glance, it will indicate that both means are above the 0.5-inch standard; however, when we further investigate this statistic we realize that there is a significant group of elderly that may find 0.5-inch standard to be a problem.
The standard deviation of the elderly can be observed to be significantly higher than the young group. This means that the elderly population has a greater variance in their ability to clear a 0.5 inch obstacle height. In the elderly group 31% had an MTC of less than 0.05 inches and 81% had MTC of less than 0.5 inches. When we analyze these values, there were small groups of elderly people that decreased the MTC by a significant amount. Even if you analyze the specific numbers for the young group, 76% had an MTC less than 0.5 inches. Falling after a trip event is also different for the younger population and the elderly, since the ability to recover is affected by muscle control and other reaction parameters, but this is a topic for another blog. The sample size of the R. Begg et. al. study was small; that is why the percentages are so high, but it allows for a discussion on what walkway surface locations/uses should have an altered tolerance standard for walkway safety. For example, walkway surfaces where a large population of pedestrians have locomotor problems, and elderly people walk on a regular basis should be reconsidered for the 0.5-inch standard. For example, hospitals, nursing care facilities/homes for the elderly, rehabilitation centers, etc. are locations where changes of elevations of less than 0.5 inches may pose a trip hazard for the expected elderly population with ambulatory issues that walk along those walking surfaces.
Falls by the elderly are a major public health concern, due to their association with morbidity and mortality rates as well as the financial cost. It is important to understand that certain locations used by a population of elderly people with walking deficiencies may all these pedestrians to come in contact with differences in elevation of less than 0.5 inches.
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Author: Andrés J. Calderón, Ph.D