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Education Article

Human Factors and Falls

Human locomotion, walking and using stairs, is a relatively complex activity taken for granted by all except the disabled. Photographic studies show that there is a constant threat of falls during walking and stair movement, counteracted only by controlled shifts in body weight and exact placement of the feet. Considering the precise coordination involved, it is remarkable that the pedestrian fall is a relatively rare occurrence. However, virtually everyone is likely to have a serious, injury-causing fall during a lifetime, with a large portion of injuries resulting in permanent disability, and some in death.

 

A general knowledge of the cycle of movement in walking and on stairs is useful in understanding the causes of falls, and for developing possible countermeasures. Also, human body measurements help establish desirable dimensions for stair treads and risers, ramps, and handrails. Factors such as reaction times, balance, postural sway, and visual perception can also be involved in the falling accident.

Walking
Human Walking Cycle

Figure 1: The walking cycle is begun by leaning forward and swinging the leading foot into a heel strike. At about the same time the rear foot begins a rolling push off and is swung forward for a new heel strike and repeat of the cycle.

The walking cycle is begun by leaning forward and swinging the leading foot into a heel strike. At about the same time the rear foot begins a rolling push-off and is swung forward for a new heel strike and repeat of the cycle (see Figure 1). Both the heel strike and the push-off are the points in the walking cycle when a person is likely to slip.

 

Slip resistance, as determined by the frictional force of shoe materials against the walking surface, is important in preventing falls. The stability of both the heel strike and the push-off is dependent upon sufficient opposing surface friction. Measurements of the horizontal component of foot force at the heel strike have shown that it is about 15 percent of body weight, and 20 percent for the push-off. This corresponds to the minimum walking surface coefficient of friction – the resistive force necessary to maintain the stability of the heel strike and push-off in the walking cycle – of .5 COF.

Slip & Trip

Figure 2: Slip at heel strike & Trip after heel strike

Tripping would likely occur when the leg is swung forward and there is insufficient ground clearance for the foot. Minimum ground clearances of the toe when the foot is swung forward were observed to average 0.6 in. (14 mm) and range between 3/8 and 1-1/2 in. (10 and 38 mm) in one controlled study (see Figure 2).