1. The Apparatus Seat height was manipulated via a motorized medical table. Participants held the floor pedal and could adjust the height of the table with considerable precision. Weights equivalent to 50% and 100% of the estimated weight of the shank-plus-foot (cf. Dempster, 1959) were attached at the ankle (Fig. 3). Sit-to-Stand (STS) movements (Fig. 1), described with kinetic and kinematic variables, are used to evaluate the functional mobility of the elderly, stroke patients and those with Parkinson’s disease (Janssen, Bussmann, & Stam, 2002). Previous research has focused largely on the action characteristics of this task as a proxy measure of lower limb strength. Studies that addressed perception (e.g. Lord, Murray, Chapman, Munro, & Tiedemann, 2002) focused only on the perceptual components of the person (i.e., proprioception) and not on the task itself. At best, existing research can explain only 35% of the variance in STS movement. Perhaps what is needed is a more detailed investigation into the perceptual aspects of the task itself. Influences of Lower-Limb Inertia on Judging “Sit-to-Stand” Julia J. Carroll 1, Marisa Mancini 2, Robert W. Isenhower 1, and Claudia Carello 1 1 Center for the Ecological Study of Perception and Action, University of Connecticut 2 Federal University of Minas Gerais, Brazil The Experiment: Select a Chair Height to Optimize STS We evaluated the influence of ankle weights on affordance perception. Additionally, since standing up from a low seat is a different task from stepping down from a high seat, the standard manipulation of presentation order— ascending or descending—is potentially interesting. Acknowledgments. This research was supported by grants from the National Science Foundation (SBR 00-04097), the University of Connecticut (to establish the Co-laboratory of Rehabilitation Research), and a CAPES Award from the Brazilian Ministry of Education. References Dempster, W. T. (1959). Space requirements of the seated operator. Wright Patterson Air Base, WADC-TR, 55, 159. Janssen, W. G. M., Bussmann, H. B. J., & Stam, H. J. (2002). Determinants of the sit-to-stand movement: a review. Physical Therapy, 82, 9, 866-879. Kuželički, J., Žefran, M., Burger, H., & Bajd, T. (2005). Synthesis of standing-up trajectories using dynamic optimization. Gait and Posture, 21, 1-11. Lord, S. R., Murray, S. M., Chapman, K., Munro, B., & Tiedemann, A. (2002). Sit-to-Stand performance depends on sensation, speed, balance, and psychological status in addition to strength in older people. Journal of Gerontology: MEDICAL SCIENCES, 57A, 8, M539-M543. Mark, L. S. (1987). Eyeheight-scaled information about affordances: A study of sitting and stairclimbing. Journal of Experimental Psychology: Human Perception and Performance, 13, 361-370. Pagano, C. P., & Turvey, M. T. (1998). Eigenvectors of the inertia tensor and perceiving the orientations of limbs and objects. Journal of Applied Biomechanics, 14, 331-359. Schenkman, M., Riley, P. O., & Pieper, C. (1996). Sit to stand from progressively lower seat heights—alterations in angular velocity. Clinical Biomechanics, 11, 3, 153-158. Turvey, M. T. (1996). Dynamic Touch. American Psychologist, 51, 1134-1152. The Task: Judgment of the Sit-to-Stand Affordance The seat was started at either the top or the bottom of its range. Participants were instructed to move the seat up or down “to the position at which you would feel the most comfortable going from sitting to standing onto the black floor.” Participants were allowed to swing their legs if they wished (as long as they avoided hitting anything). Each condition was repeated three times. Trials were randomized. The Experimenter measured the response (Fig. 4) once the participant was satisfied with his or her adjustment. Results The larger weight did, indeed, lead to a higher preferred seat than the smaller weight but this effect was asymmetrical (Fig. 3a). An enhanced contrast effect (descending judgments > ascending judgments) characterized both the no weight and asymmetrically weighted conditions; this difference was reduced when the right leg was weighted (Fig. 3b). It also diminished during the course of the experiment for weighted but not unweighted conditions (Fig. 3c and d). Affordances of Sit-to-Stand (STS) Previous research has shown the optimal height of a chair for STS (in both young and elderly patients) to be 1.15 times the combined length of the shank and foot. Moreover, chair height has been identified as a strong predictor of movement strategies and efficiency of the task (Schenkman, Riley, & Pieper, 1996). Following attempts to link the perceptual boundary of "sit-on-able" to seat height (Mark, 1987), we examined seat height as a measure of the perceptual boundary of "stand-from-able." Conclusions and Possible Applications The affordance of the STS task can be modulated, both by an asymmetrical inertial loading of the legs and by raising or lowering the seat. Moreover, the preferred Seat/Shank ratios were less than what is recommended as optimal for STS. These results have functional relevance in two domains. Current prosthetic limb designs are optimized for walking, rendering them useless during the STS movement. This requires compensatory asymmetrical behavior (Ku ž eli č ki, Žefran, Burger, & Bajd, 2005). Designers of prosthetic limbs should attend to asymmetries in both perception and action of STS. Those elderly who have difficulty on STS movements may also have perceptual deficits. Consequently, practice with weights (which reduced enhanced contrast) may benefit this population in stabilizing perceptual judgments in STS task performance. The low Seat/Shank ratios enhance the importance of lower limb strength, which would also benefit from such training. The Participants Six females and four males (between the ages of 22 and 40ish) volunteered to participate. Their average height was 172.7 cm (SD = 10.0), with an average shank length of 51.0 cm (SD = 4.4) and an average weight of 73.2 kg (SD = 16.1). All had normal or corrected-to-normal vision. Limb Dynamics Length perception by dynamic touch (see Turvey, 1996, for a review) is modulated by an object’s mass distribution (Fig. 2a). Mass distribution also affects the perceived orientation (Fig. 2b) of the forearms (Pagano & Turvey, 1998). The present research applies this framework to the lower limbs in the context of affordances. Will the addition of weight at the ankle alter perceptual judgments regarding “standable from a seat” with consequences for actual STS performance (Fig. 2c)? The Experiment Symmetrical Condition. Stand-from-able was judged without ankle-weights. Asymmetrical Condition. Stand-from-able was judged with no weight on one leg and 50% or 100% of the weight of the shank-plus-foot (hereafter referred to simply as the shank) on the other leg. Figure 2. (a) Rods are perceived as longer when the mass is distributed farther from the hand. (b) The arms feel aligned when the symmetry axes are aligned. (c) The addition of an ankle weight changes the mass distribution of the leg. If that increases the perceived length of the leg, the seat height that will feel appropriate for achieving sit-to-stand should be affected. abc Figure 1. Standing from a seated position involves moving the center of mass from centered over the chair to centered over the base of support. Figure 3. A stiff felt “skirt” occluded participants’ legs from view. A U-shaped false floor was built to allow the legs to go below floor level; thus, seat heights shorter than the shank were possible. Participants wore goggles to prevent them from seeing too much of the surrounding environment. A pillow under the knees ensured a 90˚ angle between thigh and shank. Figure 4. Each participant’s response was first measured as the distance from the actual floor (not the false floor) to the participant’s left heel. These were converted into ratios of seat height to shank length. LeftRight Weighted Leg Seat /Shank 1.1 1.9.8 50% 100% a Figure 3. (a) The Weighted Leg x Weight interaction (p =.03) revealed that 100% on the right leg resulted in the highest seat height. (b) There was an overall preference for a higher seat in the descending series than the ascending series. In the asymmetric condition, this preference was less for weighting of the right leg than weighting of the left leg (p <.02). (c) Enhanced contrast was reduced over trials with weights (p <.003), but (d) the difference persisted without weights (F < 1). Ascending Descending LeftRight Weighted Leg Seat /Shank 1.1 1.9.8 No Weight b Unweighted d Ascending Descending One Three Two Trial Number Seat /Shank 1.1 1.9.8 Weighted Ascending Descending c One Three Trial Number Two Seat /Shank 1.1 1.9.8