The purpose of the present experiment was to compare in normal human subjects the differential effects on postural stability of introducing somatosensory noise via compliant and/or sway-referenced support surfaces during quiet standing. The use of foam surfaces (two thicknesses: thin (0.95cm) and thick (7.62cm)) and sway-referenced support allowed comparison between two different types of destabilizing factors that increased ankle/foot somatosensory noise. Under some conditions neck extensions were used to increase sensory noise by deviating the vestibular system from its optimal orientation for balance control. The impact of these conditions on postural control was assessed through objective measures of instability. Thick foam and sway-referenced support conditions generated comparable instability in subjects, as measured by equilibrium score and minimum time-to-contact. However, simultaneous application of the conditions resulted in greater instability, suggesting a higher level of generated sensory noise and thus, different receptor types affected during each manipulation. Indeed, sway-referenced support generated greater anterior-posterior center-of-mass (COM) sway, while thick foam generated greater medio-lateral COM sway and velocity. Neck extension had minimal effect on postural stability until combined with simultaneous thick foam and sway-referenced support. Thin foam never generated enough sensory noise to affect postural stability even with noise added by sway-reference support or neck extension. These results provide an interesting window into the central integration of redundant sensory information and indicate the postural impact of sensory inputs is not solely based on their existence, but also their level of noise

Forth, Katharine E.

Paloski, William H.

Taylor, Laura C.

NASA Technical Reports Server

1The effect of sensory noise created by compliant and sway-referenced 
support surfaces on postural stability.
Katharine E. Forth1, Laura C. Taylor2, and William H. Paloski3
1Universities Space Research Association, Houston, TX, USA
2Wyle Laboratories, Houston, TX, USA
3Neurosciences Laboratory, NASA Johnson Space Center, Houston, TX, USA
Corresponding author:
William H. Paloski, Ph.D.
Human Adaptation & Countermeasures Office
Mail Code SK
NASA Johnson Space Center
Houston, TX  77058
william.h.paloski@nasa.gov
phone: (281) 244-5315
fax: (281) 244-5734
https://ntrs.nasa.gov/search.jsp?R=20060026427 2019-08-29T22:06:46+00:00Z
2Abstract
The purpose of the present experiment was to compare in normal human subjects the 
differential effects on postural stability of introducing somatosensory noise via compliant 
and/or sway-referenced support surfaces during quiet standing.  The use of foam surfaces 
(two thicknesses: thin (0.95cm) and thick (7.62cm)) and sway-referenced support allowed 
comparison between two different types of destabilizing factors that increased ankle/foot 
somatosensory noise.  Under some conditions neck extensions were used to increase 
sensory noise by deviating the vestibular system from its optimal orientation for balance 
control.  The impact of these conditions on postural control was assessed through 
objective measures of instability.  
Thick foam and sway-referenced support conditions generated comparable instability in 
subjects, as measured by equilibrium score and minimum time-to-contact.  However, 
simultaneous application of the conditions resulted in greater instability, suggesting a 
higher level of generated sensory noise and thus, different receptor types affected during 
each manipulation.  Indeed, sway-referenced support generated greater anterior-posterior 
center-of-mass (COM) sway, while thick foam generated greater medio-lateral COM sway 
and velocity.  Neck extension had minimal effect on postural stability until combined with 
simultaneous thick foam and sway-referenced support.  Thin foam never generated enough 
sensory noise to affect postural stability even with noise added by sway-reference support 
or neck extension.  These results provide an interesting window into the central 
integration of redundant sensory information and indicate the postural impact of sensory 
3inputs is not solely based on their existence, but also their level of noise.  
Keywords: posture, foam, neck extension, vestibular, proprioception
4Introduction
Maintaining bipedal postural stability is fundamental to the survival of humans, and 
multiple sensory sources are utilized to facilitate postural control, including: the visual, 
vestibular and somatosensory systems.  Within these sensory systems further subdivisions 
or specific modalities can be identified, each providing unique and/or redundant 
information.  This creates an intricate sensory integration task for maintaining postural 
equilibrium. 
At any given moment or scenario an individual’s postural stability state can be viewed 
from the perspective of sensory noise (uncertainty) within each contributing sensory 
system.  Varying levels of sensory noise exist within and between individuals.  For 
example, sensory noise in vestibular inputs can increase with age-related hair cell 
degradation and can be internally generated from head positions or movements which 
deviate from the optimal orientation of the vestibular apparatus (Asseman and Gahery 
2005; Paloski et al. 2006).  In fact, the sensory noise generated by a 30 degree static neck 
extension is sufficient to reduce postural stability (Paloski et al. 2006).  Likewise, sensory 
noise within ankle proprioception and plantar exteroception generated by a sway-
referenced support surface, foam surfaces, anaesthetized feet, etc. all negatively impact 
postural stability (Shumway-Cook and Horak 1986; Kogler et al. 2000; Horak et al. 2002; 
Meyer et al. 2004).  
5For clinical assessment of balance disorders, the Equitest computerized dynamic 
posturography system (NeuroCom International, Clackamas, OR) provides sensory 
organization test (SOT) conditions, which specifically introduce sensory imprecision to 
ankle/foot somatosensation for assessing potential sensory deficiencies.  These tests 
generate sensory noise within the feet/ankles through anterior-posterior (AP) support 
platform rotations, sway-referenced to body center-of-mass (COM) sway.  Similar 
ankle/foot sensory noise is also thought to be produced by a foam support surface 
(Shumway-Cook and Horak 1986).
 
Therefore, the aim of this study was to examine the differential effects of various sensory 
noise sources within postural sensory systems.  Ankle/foot somatosensory noise was 
generated by a sway-referenced and a foam support surface, while vestibular noise was 
created with neck extension.
  
6Methods
Subjects
Twelve subjects (6 males, 6 females: age range 23 to 47 years) participated in this study.  
Each participant was in good general health, passed a NASA-modified United States Air 
Force Class III-equivalent medical examination, and reported no history of balance or 
vestibular abnormalities.  All subject selection criteria and experimental procedures were 
approved by the Johnson Space Center Committee for Protection of Human Subjects.  All 
subjects provided written informed consent prior to inclusion.
Apparatus
A computerized dynamic posturography system (Equitest, NeuroCom International, 
Clackamas, OR) was used to evaluate balance control.  Subjects wore a safety harness and 
stood on the platform; their ankle joints were aligned with the support surface rotational 
axis.  Audio communications and a constant level of pink noise to mask ambient auditory 
cues were provided to subjects through headphones.  Pudgee foam pads (polyurethane 
open-cell gel-foam, Dynamic Systems, Inc., Leicester, CA) were used to create foam 
support surfaces.  Three sizes of left and right footprints were cut from thin (0.95cm, 
~480kg/m3 density) and thick foam pads (7.62cm, ~240kg/m3 density).  
Procedure
All subjects performed a set of balance control trials during which they were instructed to 
maintain stable, natural upright posture with arms folded across the chest.  Each trial (20 s 
7duration) was performed with absent vision (eyes closed), one of two somatosensory 
conditions (fixed support (SOT2), sway-referenced support (SOT5)) and one of two 
vestibular conditions (head erect, head statically pitched back 20° (P)).  During SOT5 
conditions the foot support surface was sway-referenced by rotating the force platform in 
the sagittal plane in direct proportion to the subject’s estimated instantaneous COM sway 
angle.  Four trials were performed for each of the four experimental conditions, totaling 
16 trials.  This protocol was repeated on three consecutive days, each with a different 
foam support surface: no foam, thin foam, and thick foam.  The presentation order of the 
foam support surface condition was randomized.
Analysis
COM was calculated by low pass filtering the center-of-pressure waveform obtained from 
the NeuroCom support surface force plate.  AP sway, medio-lateral (ML) sway, AP 
velocity, ML velocity, equilibrium score (EQscore), and minimum time-to-contact 
(TTCmin) were subsequently calculated from the COM waveform.  AP and ML sway 
represented the mean absolute sway amplitude, S , of COM,
xx
N
S i
N
it
t
-= å
=1
1
,
and AP and ML velocity, a first derivative of COM positional data, represented the mean 
absolute velocity
8i
N
it
v
N
V
t
å
=
=
1
1
,
in their respective directions.  tN  is the number of data points in a 20 s trial.  The 
EQscore was calculated from peak-to-peak COM sway amplitudes, and TTCmin 
represented the minimum value of TTC calculated from the COM directional velocity and 
the distance between COM and the stability boundary to which it was moving towards 
(Forth et al. 2006).   Repeated measures analysis with Bonferroni adjustments was used to 
determine differences between conditions.
9Results
 (Insert figure 1)
For SOT2 conditions, thin foam created no significant impact on postural stability 
measured by EQscores (p>0.05), although thick foam generated significantly lower 
EQscores than no foam conditions (p=0.0001) (Figure 1).  In contrast, different levels of 
foam did not modify the EQscore for SOT5 (p>0.05).  Comparisons of specific conditions 
revealed that thick foam (SOT2-thick foam) and a sway-referenced support (SOT5-no 
foam) produced comparable EQscores, but simultaneous application (SOT5-thick foam) 
produced significantly lower EQscores than thick foam alone (SOT2-thick foam) 
(p=0.0001).  Neck extensions failed to affect the EQscore for SOT2P or SOT5P, apart 
from a substantial reduction in EQscore for SOT5P-thick foam (p=0.002).  No subjects 
registered a fall in any condition.
Similar trends were also observed for the TTCmin and measures of AP sway, ML sway, AP 
velocity and ML velocity.  Except, significantly less AP sway was generated by the 
addition of thick foam to SOT5 than SOT2 (p=0.003) (Figure 2).  Neck extensions 
generated similar patterns of decreased stability in AP sway, ML sway, AP velocity and 
ML velocity.  This included sufficiently increased SOT5-thick foam AP sway to be 
consistent with instability patterns induced by neck extension in the other measures.  
(Insert figure 2)
A specific comparison between SOT5 and SOT2-thick foam showed the sway-referenced 
10
support surface generated significantly greater AP sway than foam (p=0.048), although 
AP velocity was equivalent (p>005).  In contrast, thick foam generated greater ML sway 
(p = 0.043) and ML velocity (p=0.002) than SOT5.
11
Discussion
Foam surfaces, sway-referenced support, and neck extension all present sensory noise 
which reduces postural stability.  In the present study, thick foam and sway-reference 
support surfaces generated equivalent levels of instability, however greater instability was 
gained from combining the two simultaneously.  This suggests foam and sway-reference 
support surfaces generated sensory noise in different sensory receptors.  Sway-referencing 
of the support surface is primarily designed to increase noise in ankle proprioceptors, and 
likely only generates a secondary level of imprecision in plantar haptic receptors.  Thick 
foam, on the other hand, probably creates substantial noise in foot haptic receptors and 
less disruption to ankle proprioceptors.  Though, the eversion/ inversion movements of the 
ankle joint facilitated by this compliant surface must also be considered. 
Furthermore, the differential sensory modality effect of thick foam and sway-referenced 
support may be attributed to the multidirectional nature of sensory inaccuracy created by 
thick foam, compared with the single direction of inaccuracies generated by sway-
referenced support in the AP plane.  Indeed, thick foam generated greater ML sway and 
velocity, while sway-referenced support surface generated greater AP sway.  These 
findings are consistent with Allum et al. (2002) who demonstrated subjects orientated 
sideways on a sway-referenced platform, experiencing a roll plane sway-reference, 
produce trunk movements analogous to foam conditions (Allum et al. 2002).  Thus, ML 
sensitive receptors of the ankle/foot are more affected by a thick foam surface than AP 
12
sway-referenced support surface.  
Interestingly, in the current study we also demonstrated sway-referenced support (SOT5) 
and thick foam (SOT2-thick foam) conditions reached similar AP velocities, whereas 
SOT5 generated greater AP sway.  A source of high AP velocity while on thick foam may 
result from foam compression during sway and corrective movements, although a 
complementary rise in AP sway would also be expected.  Alternatively, SOT5 AP velocity 
may have been muted due to a mechanical limitation of sway-referencing which was 
insensitive to high frequency, low amplitude sway oscillations.  Also, the elevated AP 
sway for SOT5 may be due to a strategy adopted to extract more proprioceptive 
information from the sway-referenced surface by initiating controlled or moderate velocity 
sway.  
Neck extensions had minimal effect on postural stability until they were combined with 
simultaneous thick foam and SOT5.  Rather than conflicting with previous work (Kogler 
et al. 2000; Paloski et al. 2006), this minimal impact may have resulted from a subject 
sample that included high performers who either generated less sensory noise with neck 
extension, or may have relied less on the internally generated vestibular noise in the 
sensory integration.  To answer these questions is beyond the scope of this study.  
However, it is interesting to note that neck extensions were substantially disruptive to 
stability when combined with simultaneous foam and SOT5.  This finding implies neck 
extension does present sensory inaccuracies, despite them only being exposed with further 
13
compromised redundant systems.  Neck extension combined with foam or SOT5 alone did 
not present such instability, neither did the combined foam and SOT5 without neck 
extension.  This finding is inconsistent with assertions of greater vestibular contributions 
for sway-referenced support than foam surfaces (Jeka et al. 2004), as selective instability 
for conditions containing neck extension coupled with SOT5 would be expected.  Indeed, 
the substantial drop in postural stability for simultaneous thick foam, SOT5 and neck 
extension may indicate the level to which ankle/foot sensory contributions were utilized, 
despite the sensory noise during neck extension conditions with foam only or SOT5 only 
conditions.  
In contrast to thick foam, thin foam never generated enough noise to affect postural 
stability even with additional noise from a sway-referenced support or neck extension.  
The sensory input for postural control was only compromised by thick foam.  
Nevertheless, a benefit of this result is that thin foam could be used during postural testing 
to alleviate foot discomfort created by the hard support surface of force plates, without 
altering posture measurements (e.g. for bed rest and diabetic patients).  
A limitation of this study was the differing densities of the foam, which restricts 
comparisons of foam thickness in isolation.  Thus, all comparative foam results reported in 
this study represent a combination of differing thickness and densities.  Another limitation 
of this study was that all conditions were performed with eyes closed.  The complete 
elimination of vision, a sensory contributor to posture, may alter the integration of 
14
remaining senses as they become inaccurate.  The drawback, of course, being vision is a 
potent redundant sensory source that often needs to be removed to yield instability for 
measurable differences.
 
However, in the absence of vision, these findings exposed differences in the seemingly 
similar destabilizing factors, foam surface and sway-reference support surface.   We 
highlighted the potential differences in receptors utilized and provided a window into the 
sensory integration of noisy redundant inputs.  Further work that can measure or 
manipulate specific sensory noise levels will greatly expand the understanding of sensory 
integration for postural control. 
15
References
Allum JH, Zamani F, Adkin AL, Ernst A (2002) Differences between trunk sway 
characteristics on a foam support surface and on the Equitest ankle-sway-
referenced support surface. Gait Posture 16:264-270
Asseman F, Gahery Y (2005) Effect of head position and visual condition on balance 
control in inverted stance. Neurosci Lett 375:134-137
Forth KE, Metter EJ, Paloski WH (2006) Age associated differences in postural 
equilibrium control: A comparison between EQscore and minimum time to contact 
(TTC(min)). Gait Posture DOI 10.1016/j.gaitpost.2005.12.008 
Horak FB, Dickstein R, Peterka RJ (2002) Diabetic neuropathy and surface sway-
referencing disrupt somatosensory information for postural stability in stance. 
Somatosens Mot Res 19:316-326
Jeka J, Kiemel T, Creath R, Horak F, Peterka R (2004) Controlling human upright 
posture: velocity information is more accurate than position or acceleration. J 
Neurophysiol 92:2368-2379
Kogler A, Lindfors J, Odkvist LM, Ledin T (2000) Postural stability using different neck 
positions in normal subjects and patients with neck trauma. Acta Otolaryngol 
120:151-155
Meyer PF, Oddsson LI, De Luca CJ (2004) The role of plantar cutaneous sensation in 
unperturbed stance. Exp Brain Res 156:505-512
Paloski WH, Wood SJ, Feiveson AH, Black FO, Hwang EY, Reschke MF (2006) 
Destabilization of human balance control by static and dynamic head tilts. Gait 
16
Posture 23:315-323
Shumway-Cook A, Horak FB (1986) Assessing the influence of sensory interaction of 
balance. Suggestion from the field. Phys Ther 66:1548-1550
17
Figure 1.  The equilibrium score (±sem) for SOT2 and SOT5 at three levels of foam, with 
(P) and without and neck extension.
Figure 2.  The AP and ML sway and velocity (+sem) for SOT2 and SOT5 at no foam and 
thick foam, with (P) and without neck extension.  


The Effect of Sensory Noise Created by Compliant and Sway-Referenced Support Surfaces on Postural Stability

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