Perceptual and Motor Skillr, 1991, 72, 539-543.

O Perceptual and Motor

Skills 1991

VARIABLES AFFECTING PERCEPTUAL BOUNDARIES I N BIPEDAL STAIR CLIMBING ' HARRY J. MEEUWSEN Texas Woman's University Summary.-The present study examined the effects of hip joint mobility and relative leg strength on perceived and attained action boundaries in bipedal stair climbing. If action boundaries are directly perceived and action is intimately linked to perception, one should be able to quantify these boundaries (Warren, 1984) and identify what variables affect these perceived boundaries. In the present experiment the perceived and attained absolute and relative (riser height divided by leg length) action boundaries were significantly affected by hip joint flexibility. Relative leg strength also affected absolute and relative perceived action boundaries. These results clearly show that factors i n addition to body size and body proportions may affect the perception of affordances and their boundaries.

Recent studies have provided empirical support for Gibson's theory of direct visual perception (1972, 1979). According to Gibson's theory, an actor directly perceives what actions certain environmental features afford. Researchers (e.g., Lee, 1980; Lee, Lishman, & Thomson, 1982; Warren, 1984; Mark, 1987; Mark & Vogele, 1987) have claimed empirical support for such a view by showing that action boundaries of persons of very different body size and proportions are governed by an invariant relationship between the size of the environmental object and the size of the observer. Warren (1984) suggested that perceived affordances may not only be described by geometric variables such as body dimensions and object size, but also by kinetic variables such as mass, force, friction, elasticity, and work. These dynamics, he argued, are determined by invariant proportions of some aspect of the actor's body scale and the environment. Several studies have yielded support for the presence of such an invariant proportion for different tasks, e.g., sitting (Mark & Vogele, 1987) and catching (Todd, 1981). Warren (1984) referred to the invariant body-scaled information as "pi": the dimensionless ratio of body size to object size. As such, pi reflects action category boundaries. For example, in bipedal stair climbing the critical riser height that may be climbed successfully is determined by the relationship between the riser height and the actor's leg length (Warren, 1984) or eyeheight (Mark, 1987; Mark & Vogele, 1987). Warren (1983) suggested that critical riser height (Rmax, measured in centimeters) in bipedal stair climbing may also be affected by variables such

'Send correspondence to Harry J. Meeuwsen, Texas Woman's Universiry, Department of Kinesiology, Denton, TX 76204.

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H. J. MEEUWSEN

as sex and joint stiffness. If these variables do affect Rmax, one may expect differences in pi for individuals of similar body size and body proportions but with different values for these variables. Mark (1987) suggested that there is a need to address specific types of experiences responsible for producing changes in action boundaries-such as change in body size, loss of mobility, footwear, and injury-to understand more fully how anatomical and physiological features affect perception of action boundaries. The present study examined the effects of flexibility and leg strength on young adults' perception of affordances in two experiments. Also of interest was confirmation of the proposed direct link between perception and action in a bipedal stair-climbing task. Method Particzpants.-In this experiment the effect of hip-joint mobility on perceived and attained maximum riser height in a bipedal stair-climbing task was examined using two groups of ten young adult females (average age 22.6 yr.). Woinen were chosen because there would be a larger range of hip flexibility between subjects. Apparatus and Procedure The height of 11 stairs ranged from 15 cm to 82.5 cm in increments of 7.5 cm. On the front edge of each of the black stairs was a 2-cm wide strip of white tape to distinguish the edge of the stairs. After an informed consent statement was read and signed, the experimenter measured the participants' standing height, weight, sitting height, and active hip flexibility. Sitting height was subtracted from standing height to obtain leg length (Warren, 1984). For the measurement of hip flexibility, each participant was asked to stand erect, hold the edge of a table for balance, and lift the knee as high as possible. The axis of a goniometer was lined up with the greater trochanter, while the stationary arm was aligned with the midline of the trunk and the movable arm was aligned along the thigh with the lateral condyle of the femur. The angle between the upper body and the thigh was measured three times for each leg (a smaller angle indicated a greater range of motion in degrees). Based on these measurements the women were classified in either a flexible (n = 10) or less flexible group (n = 10; see Table 1 below). Leg-strength measurements were obtained on a CYBEX I1 isokinetic strength measurement apparatus set at 30'1s (damping was set at two) that did not correct for the effect of gravity. The participants ~erformedfive trials using maximum effort in knee extension and flexion. Relative strength was calculated by averaging the maximum flexion and extension torques and then dividing the average by the participant's body weight. This measure

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PERCEPTUAL BOUNDARIES IN STAIR CLIMBING

was used to classify the men in Exp. 2 in strong (n = 10) and less strong groups (n = 10; see Table 2 below). Instructions about how to step upon the stairs were standardized for each experiment. Subjects were directed to (a) walk up to the stairs, (b) put one foot on the stair, (c) step up on the stair without putting their hands on any surface or limb or requesting help, and (d) finish with the body in a balanced erect stance. Next, the participants were led to another room where the stairs had been placed in random order in a semicircle. The participants viewed the stairs from a distance of 5 m. They were asked to judge which set of stairs they thought were the highest possible stairs they could climb. The experimenter recorded their judgments as the perceived maximum riser height (PRmax). H e then asked each participant to walk to the selected stair and step upon it. When the subject indicated that the srair was too high or when she was not able to step up on it, the experimenter asked the participant to try the next lower stair until the climbable riser height was found. If a participant was able to step up on the selected stairs, the experimenter asked whether she could step up on the next higher set. This continued until the attained maximum riser height (ARmax) was obtained. Safety of the participants was ensured by the close presence of the experimenter. Results

Group characteristics.-Table 1 displays the average physical characteristics of the women. A one-way analysis of variance yielded significant effects for flexibility (F,,,, = 99.37, p < .OOOl). Flexibility and relative leg strength did not correlate significantly (r = -.347). TABLE 1

Measure

Standing height (cm) Leg length (cm) Body weight (kg) Relative leg strength (proportion of body weight) Hip flexibility (degrees)' Perceived Rmax (cm) Attained Rmax (cm) Pi-PRmax (proportion of leg length) Pi-ARmax (proportion of leg length)

Less Flexible (n = 10) M SD

Flexible (n = 10) M

SD

167.66 5.73 80.06 3.84 66.54 19.40 .60 .07 69.35 4.46 64.77 8.58 70.87 5.76 .81 .10 .88 .06

163.12 77.34 58.18 .65 43.55 76.96 82.30 1.00 1.07

7.61 4.52 15.70 0.04 4.18 5.74 9.64 .11 .15

"Hip flexibility is the angle between the trunk and the upper leg: a smaller angle indicates greater hip flexibility.

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H. J. MEEUWSEN

Perceived and attained maximum riser heights.-Flexibhty significantly affected PRmax (F,,,, = 8.4, p = .01), ARmax (F,,,, = 6.38, p = .02), pi-PRmax (F,,,, = 11.54, p = .003), and pi-ARrnax (F,,,, = 10.52, p = .005). Dependent t tests showed that within the flexible and less flexible groups pi-ARmax was significantly larger than pi-PRmax (t, = 2.3, p = .05, and t, = 2.5, P = .03, respectively).

EXPERIMENT 2 Participants In this experiment the effect of relative leg strength on perceived and attained maximum riser height was examined using two groups of ten young men (average age 25.4 yr.). The apparatus and procedures were identical to those used in Exp. 1. Men were selected because they show a larger range of relative leg strength. Results Group characteristics.-Table 2 displays the average physical characteristics of the male participants. A one-way analysis of variance gave significant effects for relative strength (F,,,, = 61.02, p < .0001). Flexibility and relative leg strength did not correlate significantly (r = ,032). TABLE 2

PHYSICAL CHARACTERISTICS OF LESS STRONG AND STRONG MALE ADULTPARTICIPANTS IN EXPERIMENT 2 Less Strong (n = 10)

Measure

A4

SD

Strong (n= 10)

M

SD

Standing height (cm) Leg length (cm) Body weight (kg) Relative leg strength (proportion of body weight) Hip flexibility (degrees)' Perceived Rmax (cm) Attained Rmax (cm) Pi-PRmax (proportion of leg length) Pi-ARmax of leg length) .95 .08 .97 .08 'Hip flexibility is the angle between the trunk and the upper leg: a smaller angle indicates greater hip flexibility. -

~

Perceived and attained maximum riser heights.-Relative leg strength significantly affected PRmax (F,,,, = 8.1, p = .01), and pi-PRmax (F,,,,= 5.77, p = .O3) but not pi-ARmax. Dependent t tests showed that within the less strong group pi-ARmax was significantly larger than pi-PRmax (t, = 4.86, p = .001).

PERCEPTUAL BOUNDARIES IN STAIR CLIMBING

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D~scussro~ The results of Exp. 1 show that flexibility affects the perceived and attained maximum action boundaries. In addtion, the dependent t test showed that absolute and relative attained maximum riser heights were significantly larger than the absolute and relative perceived maximum riser heights. Exp. 2 showed that relative strength affected absolute and relative perceived action boundaries but not the attained action boundary. This may be because strength is not as crucial to the physical success in this task as is flexibility. The t tests suggest that less strong men were conservative in perceiving their maximum action boundaries, whereas stronger men did not show this discrepancy. Present results support and extend previous findings by Warren (1984), Mark (1987), and Mark and Vogele (1987) by showing that body-scaled relationships with the environment are affected by hip-joint mobility and relative leg strength. As Warren (1983) and Mark (1987) indicated, perception may be affected by more than anthropometric variables. The results further suggest that women tend to be more conservative in the perception of action boundaries regardless of task-related physical abilities, while only men with less physical ability seemed to be conservative in their perceived boundaries. Further investigation of the effects of sex .and the role of perceived physical ability in the perception of action boundaries may clarify these issues. REFERENCES (1972) A theory of direct visual perception. In J. R. Royce & W. W. Rozeboom (E s ), The psychology of knowing. New York: Gordon & Breach. GIBSON,J. . (1979) The ecological approach to visual perception. Boston, MA: Houghton Mifdm. LEE, D. N. (1980) The optic flow field: the foundation of vision. Philosophical Transactions of the Royal Society of London B, 290, 169-179. LEE, D. N., LISHMAN,J. R., & THOMSON, J. A. (1982) Regulation of gait in long jumping. Journal of Experimental Psychology: Human Perception and Performance, 8, 448-459. MARK, L. S. (1987) Eyeheight-scaled information about affordances: a scudy of sitting and stair climbing. Journal of Experimental Prychology: Human Perception and Performance, 13, 361-370. MARK,L. S., & VOGELE,D. (1987) A biodynamic basis for perceived categories of action: a study of sitting and stair climbing. Journal ofMotor Behavior, 19, 367-384. TODD,J. T. (1981) Visual information about moving objects. Journal of Experimental Psychology: Human Perception and Performance, 7, 795-810. WARREN,W. H. (1983) A biodynamic basis for perception and action in bipedal climbing. Dissertation Abstracts International, 43, 4183-B. (University Microfilms 83-09263) WARREN, W. H. (1984) Perceiving affordances: visual guidance of stair climbing. Journal of Experimental Psychology: Human Perception and Performance, 10, 683-703.

J.d.

Accepted March 12, 1991.

Variables affecting perceptual boundaries in bipedal stair climbing.

The present study examined the effects of hip joint mobility and relative leg strength on perceived and attained action boundaries in bipedal stair cl...
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