Task, Environmental structure, and Illumination Influences on Posture
Cedrick T. Bonnet1, Claudia Carello1, Jeffrey M. Kinsella-Shaw1,2, Deborah Bubela1,2, and M. T. Turvey1
1CESPA 2Department
of Physical Therapy, University of Connecticut, Storrs, CT, U.S.A.
INTRODUCTION
The Problem:
Old fall more than young
RESULTS—POSTURE
The Context
Task
Organism
Relationships between postural fluctuation and
incidence of fall have been found (Campbell,
Borrie, & Spears, 1989).
Environment
Statistical Analyses of Posture. A variety of linear dependent measures were evaluated in 2
(environment)  2 (task)  2 organism) ANOVAs: standard deviation of fluctuation in the anteriorposterior direction (SDAP), standard deviation of fluctuation in the medial-lateral direction (SDML),
path length of the COP excursion, and ellipse area traced by the COP. Nonlinear measures included
detrended fluctuation analysis DFAAP and DFAML and, from the recurrence quantification analysis,
the percentage of recurrent points (%RECAP and %RECML) and the longest string of recurrent points
(MAXAP and MAXML)
Table 2. Statistical Evaluation of Posture Measures in Experiment 1.
Postural activity can be understood in the context
of three sources of constraint: organism, task, and
environment (e.g., Slobounov & Newell, 1994).
Age
Two Experiments
Task
Visual Search
(with a text target)
Visual Fixation
(with a blank target)
vs.
Experiment 1 (E1):
grating array vs.
blank background
Task
Young
vs. Old
Organism
Environment
3 lux
440 lux
Experiment 2 (E2):
low vs. high
level of illumination
Will the goal-directedness of the supra-postural visual search task interact with the influences of passive
visual factors and age (cf. Riccio & Stoffregen, 1988)?
SDAP
SDML
Path
length
Old
0.44*
(0.14)
0.32*
(0.21)
40.39*
(12.98)
1.69*
(1.33)
1.51
(0.10)
1.46
(0.13)
3.02
(3.03)
3.96
(4.40)
830.60
(566.55)
909.21
(664.26)
Young
0.33*
(0.15)
0.18*
(0.10)
31.48*
(6.66)
0.78*
(0.74)
1.51
(0.08)
1.36
(0.13)
3.28
(2.49)
2.90
(2.83)
898.49
(581.28)
549.29
(530.85)
Old
0.42*
(0.17)
0.21*
(0.12)
36.02
(8.42)
1.12
(1.02)
1.51
(0.09)
1.39*
(0.14)
3.03
(2.17)
2.57*
(2.61)
854.56
(508.80)
616.98*
(671.83)
Young
0.36*
(0.13)
0.29*
(0.22)
35.85
(13.51)
1.35
(1.30)
1.50
(0.09)
1.43*
(0.13)
3.27
(3.27)
4.30*
(4.41)
874.53
(634.13)
841.53*
(558.65)
0.45
(0.16)
0.25+
(0.14)
39.38*
(8.78)
1.34
(1.17)
1.51
(0.10)
1.43+
(0.14)
2.35
(1.61)
2.80
(2.64)
711.5
(466.7)
795.6+
(764.9)
Young
0.40+
(0.17)
0.17
(0.10)
32.66*+
(6.65)
0.90
(0.82)
1.52+
(0.07)
1.34
(0.14)
3.71
(2.45)
2.34
(2.62)
997.7
(518.1)
438.3+
(520.4)
Old
0.44*
(0.11)
0.40*+
(0.25)
41.40*
(16.28)
2.05*
(1.42)
1.52
(0.09)
1.49*+
(0.11)
3.70
(3.90)
5.13
(5.42)
949.7
(639.1)
1022.8+
(538.3)
Young
0.28*+
(0.11)
0.19*
(0.10)
30.29*+
(6.61)
0.66*
(0.65)
1.49+
(0.09)
1.37*
(0.12)
2.85
(2.51)
3.46
(2.98)
799.3
(633.6)
660.2+
(528.6)
Blank Old
Text
Ellipse
area DFAAP DFAML
%
RECAP
%
RECML
MAXAP
MAXML
Table 2 shows means (and standard deviations) from E1. It also shows the results of the planned ttests, either independent t-tests (between young and old) for both blank target and text target
conditions, or the paired t-tests (between blank target and text target conditions) for both old and
young adults. (* indicates a significant difference between age groups; + indicates a significant
difference between both tasks for a particular group (p < .05).
METHOD
letter
search
task
(c) E2: 2 (Illumination)  2 (Task)
letter
search
task
letter
search
task
letter
search
task
0.6
(a
)
Old
Young
0.5
Young
0.4
SD (ML) (cm)
SD (AP) (cm)
Old
0.5
0.5
0.4
0.3
0.2
6.0
(b
)
0.4
0.3
0.3
0.2
0.2
Young
0.0
blank
text
blank
4.0
3.0
2.0
Task
Task
A text or blank target (visual angle ≈ 13  14o) was mounted on the front of the grating or white
board. For the visual search task, participants reported how many times a specified letter appeared;
for the visual fixation task, participants simply fixated the smaller blank card (cf. Stoffregen,
Pagulayan, Bardy, & Hettinger, 2000). A trial lasted 35 s; faster readers should get further in the
passage (and, consequently, a higher absolute letter score).
400
0.0
0
blank
blank
text
text
Task
Task
Postural fluctuation, E2. ANOVAs revealed significant main effects of age, task, and illumination, p
< .05, as well as interactions of task with age (Figure 3 a and b) and task with illumination interaction
(Figure 3c), p < .05. For the main effect of illumination, SDAP and MAXML were lower with high
illumination than low illumination. The influence of task depended on measure: The search task
decreased SDAP and COP path length but increased DFAML.
3.0
(a)
Old
1000
900
Young
2.5
(b)
Old
Young
(c)
high illu
1.56
Low illu
1.54
800
2.0
1.5
1.0
1.52
700
DFA (AP)
Environment
E1: Background. Three rows of nine rods (92 cm high) comprised the grating array (Figure 1b, left);
white poster board provided a blank background (Figure 1b, right).
E2: Illumination. High illumination (440 lux) was at a level comparable to normal room lighting
(Figure 1b, left); low illumination (3 lux) approximated the functional limit of recognition vision
(Figure 1c, right).
600
200
Task
MAXLINE (AP))
Organism
12 younger adults (college students) and 12 older adults (65+ years of age) were enclosed by three
white sheet walls (Figure 1 a).
800
Figure 2. The search task reduced SDAP for younger adults (a) but increased SDMLfor older adults
(b). Two nonlinear measures, %REC (c) and MAXLINE (d), paralleled each other: an increase for
older adults and a decrease for younger adults.
Ellipse area (cm)
Figure 1. Participants stood barefoot with left and right feet on separate force platforms (a). In E1, a
text or blank target was mounted on the front of a dowel array or a large white board (b). In E2, the
text or blank target was mounted on the front of a large white board in bright or dim illumination.
1000
1.0
text
(d
)
Young
1200
0.1
0.0
Old
1400
5.0
0.1
0.1
(c
)
Old
MAXLINE (AP))
(b) E1: 2 (Background)  2 (Task)
% RECUR (AP)
(a) Posture for E1 and E2
Postural fluctuation, E1. ANOVAs revealed significant main effects of age and task as well as their
interaction, p < .05. Generally, older adults fluctuated more than younger adults. The task effect
depended on age, with the particular Task  Age interaction differing for different measures (Figure
2).
600
500
400
300
1.50
1.48
1.46
1.44
200
0.5
0.0
blank
text
Task
100
1.42
0
1.40
blank
text
Task
blank
text
Task
Figure 3. For younger adults, the search task reduced ellipse area (a) and MAXAP (b). The search
task decreased DFAAP under high illumination only.
SUMMARY AND CONCLUSION
RESULTS—VISUAL SEARCH
Both speed (number of letters found in a 35 s trial) and accuracy (proportion of letters found given
how much a participant read) were evaluated (Table 1). Although the number of letters counted was
significantly higher for young people, t(22) > -2.33, p < .05, indicating faster reading, the mean
proportion correct did not differ for the two age groups in either experiment, t(22) < 1.
Table 1. Visual Search Task Mean (with SD) for Reading Speed and Accuracy
Overall, older adults fluctuated more than younger adults. The structure of the environment
surrounding the target did not influence postural fluctuation (E1) for either age group. The influence
of level of illumination (E2) was age-specific: Older adults fluctuated more when the level of
illumination was low (replicating Kinsella-Shaw, Harrison, Colon-Semenza, & Turvey, 2006).
E1
speed
accuracy
18.30 (3.33)
34.38% (22%)
21.92 (4.21)
32.29% (33%)
The supra-postural visual search task was the most important constraint in reducing postural
fluctuation in younger adults (Prado, Duarte, & Stoffregen, 2007; Stoffregen et al., 2000). Given that
this was not the case for older adults, it suggests that they may engage in different postural behaviors
than younger adults.
E2
speed
accuracy
11.74 (2.52)
22.92% (23.13%)
14.70 (2.64)
22.92% (23.74%)
This research was supported by a grant from the Provost of the University of Connecticut to the Collaboratory of Rehabilitation Research
Old Adults
Young Adults
REFERENCES
Campbell, A. J., Borrie, M. J., & Spears, G. F. (1989). Risk factors for falls in a community-based
prospective study of people 70 years and older. Journal of Gerontology, 44, M112-M117.
Kinsella-Shaw, J. M., Harrison, S. J., Colon-Semenza, C., & Turvey, M. T. (2006). Effects of visual
environment on quiet standing by young and old adults. Journal of Motor Behavior, 38, 251-264.
Prado, J. M., Duarte, M., & Stoffregen, T. A. (2006). Postural sway during dual tasks in young and
elderly adults.
Riccio, G. E., & Stoffregen, T. A. (1988). Affordances as constraints on the control of stance. Human
Movement Science, 7, 265-300.
Slobounov, S. M. & Newell, K. M. (1994). Postural dynamics as a function of skill level and task
constraints. Gait and Posture, 2, 85-93.
Stoffregen, T. A., Pagulayan, R. J., Bardy, B. G., & Hettinger, L. J. (2000). Modulating postural
control to facilitate visual performance. Human Movement Science, 19, 203-230.