riccio lee & martin 1993

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Studies in Perception and Action II: Posters Presented at the VIIth International Conference on Event Perception and Action, August

8-13, 1993, University of British Columbia, Vancouver BC, Canada. Contributors: S. Stavros Valenti - editor, John B. Pittenger -editor, International Conference on Event Perception and Action - orgname. Publisher: Lawrence Erlbaum Associates. Place of

Publication: Hillsdale, NJ. Publication Year: 1993. Page Number: 306.

Task Constraints on Postural Control

Gary Riccioa, Dongwoo Lee

a, and Eric Martin

b

a. University of Illinois at Urbana-Champaign, IL, U.S.A.

b. Systems Research Laboratories, Inc., Dayton, OH, U.S.A.

The purpose of this investigation was to examine task constraints on the nonrigid movements of

the body during bipedal stance. Nonrigid movements are revealed by patterns of movement that

vary across body segments. The central assumption is that such nonrigid movements should be

both observable and controllable because they have specific affordances for perception and action

(Riccio, 1993, in press; Riccio & Stoffregen, 1988, 1991). Movement of the head relative to an

object of visual regard, for example, can have a greater influence on visual perception than does

movement of some other body segment. In addition, head movement can have a greater influence

on a task that requires high-acuity vision than on a task that simply requires the maintenance of

balance.

Method

Procedure. Observability and controllability of nonrigid movements is necessitated by task

constraints on such movement, and it should be revealed in an interaction between task and body

segment with respect to various movement parameters. This hypothesis was tested by comparing

patterns of movement at various body locations (analogous to modal analysis in mechanical

engineering) during a variety of tasks. The participant stood on a moveable platform while (a)

doing nothing, (b) tapping lightly at constant intervals on a keyboard, or (c) reading text that was

at a fixed location and not attached to the body or the moveable platform. Performance on the

tapping and reading tasks was not measured. Accelerometers were attached to the head, hip,

ankle, and the platform. The accelerometers were sensitive to tilt and acceleration in the sagittal

plane (e.g., anterior-posterior "sway") and, thus, they provided an estimate of the variation in

gravitoinertial force on the corresponding body segments. The variation in postural configuration

(accelerator outputs) was sampled 100 times per s. These postural movement patterns were

measured in trials that were 2.5 s in duration (250 samples per trial). The platform moved 5 cm

forward or backward at constant velocity for 0.4 s (from 0.5 to 0.9 s into each trial). There were

twelve trials for each combination of task (nothing, tapping, or reading) and direction of platform

movement (forward or backward). Order of the six conditions was randomized.

Analysis. Trials were arbitrarily divided into five equal time intervals (each 0.5 s). The mean

accelerometer output, at each location for the first 0.5 s in each trial, was subtracted from each of

the 250 samples in the corresponding trial (this minimized differences across trials that were due

initial tilt of the accelerometers). Histograms of this postural variation were constructed for each

task, direction, body segment, and time interval from the twelve trials and 50 samples for each

interval (600 samples per histogram). The (task x segment x direction x interval) matrix ofhistograms is presented in Figure 1a. The associated distributions were quantitatively summarized

by the mean, standard deviation, skewness, and kurtosis of the accelerometer outputs for each

time interval (each set of 50 samples) in each trial. Box plots for these parameters provided

summaries across each set of twelve trials per condition. The (task x segment x direction x

interval) matrix of box plots for the standard deviation are presented in Figure 1b.


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Figure 1. (a) Each panel in the matrix is a histogram for the accelerometer outputs over an

interval of 0.5 s. (b) Each panel in the matrix includes box plots for the three tasks. The ordinate

for the box plots in each cell is the standard deviation of the accelerometer outputs, and it

provides a measure of the (horizontal) spread in the corresponding histogram.


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Results and discussion

Mean change in postural configuration. Since the velocity at the end an interval was generally

close to the velocity at the beginning of an interval (i.e., near zero), the mean accelerometer

output primarily reflects the mean tilt in each interval (relative to the initial tilt). There was

residual forward tilt of the body segments after backward movement of the platform and residual

backward tilt after forward movement of the platform. The spread in the box plots (not presented)indicated that the greatest variation across trials, within each condition, occurred at the head

during and after platform movement. This suggests that adaptation in postural control strategies

has its greatest effect at the head where the affordances of postural perturbations are most

important.

Standard deviation of postural variation. Standard deviation of the accelerometer outputs

provides the best estimate of the overall amount of movement during each interval. Figure 1b

reveals a main effect of segment that primarily reflects perturbation at the ankle due to platform

movement during Interval 2. Figure 1b also reveals a clear task x segment interaction that is due

to differences in movement of the head across tasks. More specifically, this effect suggests an

effortful backward tilt of the head to compensate for forward translation, and an effortful forward

tilt of the head to compensate for backward translation. This is because force due to tilt in onedirection adds to force due to acceleration in the "opposite" direction (tilt and acceleration affect

accelerometers in the same way and, thus, cannot be differentiated in this experiment). Such

compensation should be most important in the reading task since stability of the eyes has

affordances for performance in the task. Compensation should be least important for simple

stance (i.e., no other task); note that tilt and acceleration in the same direction (i.e., falling over)

would result in minimal variation in accelerometer output.

Higher-order moments of postural variation. Effects on skewness and kurtosis of the

accelerometer outputs are not as striking as the effects on the mean and standard deviation. These

higher-order moments of movement distributions indicate the degree of departure from smooth

continuous control because Gaussian distributions are characteristic of linear control. Skewness

can arise when movements that are large in extent but short in duration (or few in number ornumber of samples) in one direction compensate for movements that are small in extent but long

in duration (or many in number or number of samples) in the other direction. Kurtosis can arise

when there are a relatively large number of moderate or large movement samples in both

directions. Such signs of ballistic (intermittent) control are apparent only as platykurtosis at the

ankle during Interval 2 (during platform movement) and to a greater degree during Interval 3

(after platform movement) and as platykurtosis together with skewness, in the direction of

platform movement, at the ankle during Interval 3.

Implications for future research. This exploratory experiment provides a powerful

demonstration that postural control cannot be understood without considering affordances at the

outset (see also, Riccio, 1993, in press; Riccio & Stoffregen, 1988, 1991). Postural movements

can vary significantly across tasks even when there are no variations in mechanical orneurophysiological constraints on movement. In addition, the data on higher-order moments

suggests that, in some cases, standard engineering techniques can be used to model these task

constraints as viscoelastic stabilization of various body segments by the surroundings with which

the postural system is visually or manually coupled. It should be noted, however, that linear

models of postural control are not generally appropriate since task constraints and balance

constraints often interact (Riccio, in press).


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References

Riccio G. E. ( 1993). Multimodal perception and multicriterion control of nested systems: Self

motion in real and virtual environments (UIUC-BI-HPP-93-02). Urbana, IL: Beckman Institute

for Advanced Science and Technology.

Riccio G. E. (in press). "Information in movement variability about the qualitative dynamics ofposture and orientation". In K. M. Newell & D. M. Corcos (Eds.), Variability and Motor Control.

Champaign, IL: Human Kinetics.

Riccio G. E., & Stoffregen T. A. ( 1988). "Affordances as constraints on the control of stance".

Human Movement Science, 7, 265-300.

Riccio G. E., & Stoffregen T. A. ( 1991). "An ecological theory of motion sickness and postural

instability". Ecological Psychology, 3, 195-240.

riccio lee & martin 1993

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