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Fall, Steps & Lights
Marc Green
Parts of Steps
In talking about steps, there is a standard terminology. Each step consists of two basic parts, the "tread," the flat horizontal surface, and the "riser," the vertical surface. The horizontal area before the first tread and after the last tread is called the "landing." Stairs often have a nosing, which extends out from the tread edge. The tread depth minus the nosing is the "going." Lastly, the rise divided by the tread depth gives the "pitch," which can be converted into degrees by taking the inverse tangent.
Safe stair descent may be described as a controlled fall in which a walker must perform two tasks. First, the walker must accurately move the foot so that it clears the upper tread edge and lands squarely on the next, lower tread. Second, the walker must maintain balance while standing on one leg and shifting weight forward and downward.
The starting point for understanding stair descent is the problem of maintaining balance. The behavior of staying in upright posture, even when standing quietly, is complex. Most people take posture maintenance for granted because it is performed automatically, requiring little or no conscious thought. In fact, it is a complex operation that requires integration of body movement with several perceptual abilities.
The body's weight is distributed around an imaginary point called the "center of gravity" (CoG). Balance occurs when the center of gravity is located within a "base of support," usually the two feet. When standing quietly, the body maintains a "static balance" but when walking or using a stairway the balance is "dynamic" because the CoG is moving relative to the base of support.
Maintaining balance requires active effort. Even when standing quietly, the body makes small movements causing sway that must be quickly corrected to prevent tipping over. The amount of sway depends on several factors, including the breadth of the center of support (feet together is a less stable stance than feet apart) and age (sway is greater for the old than for the young.) Usually, people make the corrections quickly and automatically, unaware that compensatory movements are constantly taking place.
The corrections needed for balance require two pieces of information: 1) the gravitational axis and 2) the location of the body in space. Three sense modalities, vestibular, visual and somatosensory, provide the information required for postural maintenance. The vestibular system detects changes of head position with respect to gravity. It is centered in the inner ears and consists primarily of three semicircular canals, filled with fluid, that are positioned at right angles. When the head moves, the liquid shifts in the canals, signaling a change in position with respect to gravity.
The visual modality detects changes in the visual scene that occur during sway. Just as bumping a camera changes the information imaged on the film, sway changes the image that falls on the eye and reveals change in position.
The somatosensory modality consists of tactile and proprioceptive perception. Tactile sense ("sense of touch") creates perception of felt position. It signals properties of the surface that the feet are contacting, slope, roughness, etc. Proprioception is the ability to sense the motion and position of the body through sensory organs located inside the body muscles, tendons and joints. It is possible, for example, to know the position of a leg, even with the eyes closed. Vision and proprioception work together to sense and to correct deviations from balance.
Once a person begins walking, the center of gravity changes with respect to the base of support. People walk with a "gait," a systematic pattern of movements that maintains balance despite the constant relative motion of the center of gravity and base of support. People use different gaits, depending on the surface that they are ambulating.
Walking down steps
Templer (1992) described the typical stair descent gait. As shown in Figure 2, the gait consists of several distinct stages. When a person descends from one step to the next (or from the landing to the first step), he/she performs a standard series of actions (See Figure - from Muybridge, 1955):
- Swing the lead foot over the edge of the tread and above the next lower tread.
- Lift the heel of the rear foot off the higher tread (or landing if it is the first step.). The body's weight is then supported on the "balls" of the rear foot.
- Simultaneously, the rear leg begins to bend and the knee and the hip swing forward. This commits the body to a forward, controlled fall.
- The toe of the forward foot points downward in order to absorb the shock of the fall on the ball of the foot when it reaches the tread.
- Heel of the forward foot gradually falls to the tread to cushion impact.
- The body's weight is completely transferred to the front foot.
The Causes of Falls on Steps: Missteps
One common type of fall occurs when the walker missteps. This can occur by an overstep - s/he swings the forward foot too far out. The front foot lands on the riser's edge rather than on the tread. The foot is already angled downward in order to absorb the shock, so when weight is shifted, it slides forward causing the walker to lose balance and to fall. If the walker oversteps greatly, the front foot lands two treads down instead of one. The controlled fall then becomes an uncontrolled fall, and the walker to lose balance.
Falls also occur when the walker understeps. In this case, the stride is not sufficiently long and the heel catches on the edge of the tread. Again, the walker is pitched off balance and falls forward.
Stairs and Lighting
In order to avoid overstepping, the walker must properly perceive the location of the next tread. This requires detection of the edge that separates one tread from the next or the tread from the landing (if it is the first or last step.)
The visual ability to perform this discrimination is "contrast sensitivity," which requires the sensing the "brightness" (or color) difference between the treads (or tread and landing.) The visual system uses the contrast to see the edge and to help estimate the depth between the treads. There are three factors that determine ability to perceive contrast: physical contrast, illumination, and adaptation state.
As shown in the Figure above (from Templer, 1992), the physical contrast between step treads is typically very small because they are generally painted the same color. (Treads having a textured covering or pattern are even harder to discriminate.) This means that safe descent of stairs (and to a lesser extent ascent) requires good lighting, especially at night. Moreover, the difficulty of discriminating treads is greater when there is a change in adaptation state, such as occurs when someone is moving outdoors to indoors or vice versa.
The Causes of Falls on Steps: Failure to Notice
Falls sometimes occur simply because the walker fails to detect the stairs. A well-constructed and easily detectable stair should have the following characteristics:
- Uniformity. There should be little or no variation between the dimensions of one step to the next.
- High elevation. Low elevation stairways may be difficult to detect. The NFPA Life Safety Code, for example, once banned the use of stairways when elevation was under 21 inches. They now allow such low elevations, if absolutely unavoidable, as long as their noticeability is enhanced.
- A Minimum of three risers. Some codes/guidelines (e g. NBC, ASTM 1637; Templer, 1992) do not specifically talk about elevation but rather warn against using stairways that have fewer than three risers. The idea is the same - low elevations are less noticeable. If unavoidable, stairways with 2 risers should have additional cues to loudly announce their presence.
- No distractions. It takes attention to both notice a stairway and to descend safely. Stairs should not open into areas that are likely to draw the walker's eye away from the stairs.
- Enhanced noticeability for low vision walkers. Elderly people and others with low vision will have more difficulty detecting and descending stairways. If it is reasonable to expect that many sight-impaired viewers will use the stairway, then care should be taken to create especially conspicuous cues.
- warning signs
- increased illumination
- high contrast tread color
- high contrast strips at the tread edge
- extended tread depths
- highly salient handrail
- change in floor texture to draw attention downward
When these ideal design characteristics are not possible, then several methods are available to enhance stairway and tread visibility, including:
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Copyright © 2024 Marc Green, Ph. D.
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