Academic journal article Perception and Psychophysics

Why Walkers Slip: Shine Is Not a Reliable Cue for Slippery Ground

Academic journal article Perception and Psychophysics

Why Walkers Slip: Shine Is Not a Reliable Cue for Slippery Ground

Article excerpt

In a series of four studies, we investigated the visual cues that walkers use to predict slippery ground surfaces and tested whether visual information is reliable for specifying low-friction conditions. In Study 1, 91% of participants surveyed responded that they would use shine to identify upcoming slippery ground. Studies 2-4 confirmed participants' reliance on shine to predict slip. Participants viewed ground surfaces varying in gloss, paint color, and viewing distance under indoor and outdoor lighting conditions. Shine and slip ratings and functional walking judgments were related to surface gloss level and to surface coefficient of friction (COF). However, judgments were strongly affected by surface color, viewing distance, and lighting conditions-extraneous factors that do not change the surface COF. Results suggest that, although walkers rely on shine to predict slippery ground, shine is not a reliable visual cue for friction. Poor visual information for friction may underlie the high prevalence of friction-related slips and falls.

Visual information plays a crucial role in the prospective control of locomotion. It provides observers with the requisite perceptual information to plan their movements adaptively. For example, walkers must lift their feet to avoid tripping over the curb and veer to avoid slipping on a slick patch of ice. A variety of visual depth cues-texture gradients, linear perspective, occlusion, motion parallax, binocular disparity, convergence, and so on-prompt walkers to lift their legs to clear the curb. But what visual information alerts walkers to beware of low-friction conditions? Are there visual "friction" cues that allow for prospective control as walkers approach a slippery ground surface? In four studies, we examined the cues that walkers use to predict slippery ground surfaces and asked whether visual information is reliable for specifying friction conditions.

Prospective Control of Walking Under Low-Friction Conditions

Friction presents an interesting problem for understanding the prospective control of locomotion for several reasons. First, friction is everywhere: It is an emergent, resistive force that is created when two surfaces come into contact. In locomotion, friction is necessary for balance and forward propulsion. Friction keeps one foot planted firmly to the ground while the other foot swings forward, and friction stops the swinging foot from sliding when it recontacts the floor.

Second, friction is of practical importance for safety during everyday walking. Generally, walkers fare poorly under low-friction conditions. Falls are the leading cause of unintentional, nonfatal injuries in the home and workplace in the United States across all age groups (National Center for Injury Prevention and Control, 2002). Almost half of all falls result from friction-related slips (Courtney, Sorock, Manning, Collins, & Holbein-Jenny, 2001), a fifth of all emergency-room visits in the United States are precipitated by slip-and-fall injuries (National Safety Council, 1998), and almost two thirds of workplace injuries are due to slipping, tripping, or stumbling (Courtney et al., 2001).

Third, friction poses a special dilemma for prospective control because walkers must obtain visual information for low friction prior to landing on the target surface (Adolph & Berger, 2006; Adolph, Eppler, Marin, Weise, & Clearfield, 2000; Patla, 1991, 1997). Given sufficient warning, walkers have multiple coping strategies for avoiding low-friction surfaces or for increasing friction at the foot-floor interface. If alerted one to two steps ahead of time (approximately 500-1,120 msec), walkers can avoid stepping onto a slippery patch of ground by shortening the step prior to the surface, lengthening the step over the surface, or widening the step around the surface (Patla, 1991; Patla, Robinson, Samways, & Armstrong, 1989). They can veer slightly (0°0 -30°) to the side opposite their last footfall and stop short in front of the target surface (Patla, 1997; Patla, Prentice, Robinson, & Neufeld, 1991). …

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