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Body movements during perceptual tasks can be considered as exploratory activity that facilitate perception. In the present study we tested whether the complexity of postural sway is related to perception of affordances. Effort-to-compress (ETC), a novel measure of complexity, was shown to be related to perception as compared to gross measures of body sway (mean magnitude and variability). Specifically, complexity was related to perceptual responses in a behavioral task (judge "standonableness" of sloped terrain), but not when numerical angle judgments of slope were solicited. Furthermore, ETC was extreme at the action boundary of standonableness whereas magnitude and variability of body sway were not. This provides further evidence that the purpose of perception is to guide meaningful behavior (perceive affordances) via active exploration, and not to estimate abstract numerical quantities such as slope angles of ramps. We concluded that moving the body in ways that produces complex exploratory activity is necessary to perceive affordances. [ABSTRACT FROM AUTHOR]