EXPLORING
THE EFFECT OF SENSORY INTERACTION ON BALANCE
Introduction and Background
An individual’s ability to maintain standing balance
is a result of forces which are applied, via contraction of
trunk and leg muscles, at the interface between the individual’s
feet and the ground surface. These forces must counteract
all external forces including gravity, so contractions are
coordinated in order to control postural sway and to keep
the body’s centre of mass within its supporting base,
hence preventing the individual from falling. Postural behaviour
is both reactive and predictive: the neuromusculoskeletal
system is adapted for making quick, automatic responses to
disturbances of equilibrium. These disturbances may be external,
for example being pushed, or internal; voluntary movements
which change the individual’s centre of mass are accompanied
by postural adjustments which serve to stabilise the body.
Postural orientation and equilibrium arise as a result of
the communication of three main sensory systems, whose inputs,
though independently collected, interact to result in the
appropriate muscular response. Studies have suggested that
the cerebellum, basal ganglia and cerebral cortex are involved
in the processing of these inputs. The three sensory systems
are described below.
The somatosensory system provides information via the sense
of touch which indicates the position of the body in space,
and elements of bodily contact. It includes receptors in the
skin, muscles and joints that communicate changes in pressure,
stretch, and position respectively. The effect of ‘pins
and needles’ is an example of receptors in the skin
failing to communicate changes in pressure from the bottom
of the foot.
The visual system provides information about the surroundings
and location and about the speed and direction of movement
of the individual. The eyes send regular signals to the brain
about what is ahead and to the side of a person, and the brain
sends impulses to the muscles if any corrective movement is
needed to avoid accidental injury.
The vestibular system is located in the semi-circular canals
of the inner ear. It consists of gel-like fluid, which signals
to the brain regarding the position and movements of the head.
In normal circumstances these signals act to reduce swaying
and ensure dynamic balance. However, when the vestibular system
is compromised, the fluid may erroneously signal to the brain
that the head is moving even though it is motionless. This
may result in dizziness, nausea and often inability to stand.
The various inputs are weighted differently according to
task conditions and prior experience. The brain needs to gather
information regarding the position of the centre of mass,
which is derived through the three sensory inputs. In children,
visual information tends to dominate the system, but in general
the preferred sensory input for the control of balance in
humans is somatosensory information from the feet in contact
with the support surface. This somatosensory information triggers
and shapes postural responses to unexpected disturbances of
stance equilibrium. Vestibular information is nevertheless
weighted more heavily than somatosensory information in certain
other task conditions,especially when somatosensory information
is not available, for example the early response to sudden
freefall and when the support surface is compliant rather
than firm. In this case the CNS relies on the orientationally-accurate
vestibular inputs.
Sometimes an individual encounters intersensory conflict;
this is when signals received from the different systems relay
inconsistent information to the brain. For example, when standing
motionless yet observing a moving train, an individual’s
visual input would be inconsistent with his somatosensory
and vestibular inputs. Usually intersensory conflict, which
is a regular day-to-day occurrence, is easily overcome; in
the described situation a healthy individual’s CNS would
disregard the visual motion input and rely instead upon the
other, orientationally-correct inputs.
Structures in the brainstem and the cerebellum are believed
to be responsible for the integration of multisensory inputs
and subsequent delivery of descending commands to the lower
spinal circuits for postural control. This is backed up by
observing severe balance problems in individuals whose midline
cerebellar regions have been damaged.
Various abnormalities in sensory organization may exist in
certain individuals. Below are three instances of such abnormalities,
to which balance problems are often attributed.
- The individual's dominant sensory system is inappropriate.
If an individual relies heavily on vision or somatosensory
inputs when these inputs are inaccurate, an inappropriate
balance response or a failure torespond will be the result.
- The individual’s vestibular input is too low –
this makes it difficult to resolve intersensory conflicts.
- The individual's vestibular input is too high – in
this case it has too much influence on perception of orientation.
Aim of the Study
The aim of this study is to investigate the influence of sensory
interaction upon postural stability. The details of an individual’s
postural stability can be assessed clinically by carrying
out an experiment where an individual attempts to maintain
standing balance under various sensory conditions.
Brief Methodology
An individual’s ability to maintain standing balance
is assessed by observing and quantitatively noting hispostural
sway in the standing position (feet side by side).
This ability is measured under different combinations of abnormal
conditions, these conditions being (a) foam standing surface,
(b) blindfolded, and (c) visual-conflict headgear worn. The
six different combinations are detailed below:
1) Flat surface, eyes open (normal) 4) Foam surface, eyes
open
2) Flat surface, blindfolded 5) Foam surface, blindfolded
3) Flat surface, headgear 6) Foam surface, headgear
Data Interpretation
Sway determination (least to most): Minimal sway = 1, Mild
sway = 2, Moderate sway = 3, Fall = 4.
- Of the 58 individuals tested, 14 were male and 44 were female.
- There were 6 males between the ages of 25 and 29, but the
rest of the males were spread relatively evenly among the
age categories (1-3 males in each). Overall there were 11
males under the age of 34, and 3 over the age of 35.
- The females were generally spread evenly among the age categories
(4-6 females in each), except the age group 35-39 where there
were 13 females. Overall there were 23 females under the age
of 34, and 21 over the age of 35.
- There were no individuals tested above the age of 50, or
below the age of 15.
Brief summary of results:
Condition 1 – Information available
from all 3 sensory systems
No patients showed any more than minimal sway (sway factor
1) in normal conditions.
Condition 2 – Visual information unavailable
9 men and 17 women (of mixed ages) showed mild sway, 1 woman
(aged 40-44) moderate sway.
Condition 3 – Inaccurate visual information
17 people showed mild sway, 4 showing moderate sway (3 women
– mixed ages and 1 male – aged 15-19).
Condition 4 – Inaccurate somatosensory
information
16 people showed mild sway, no moderate sways. Of those showing
mild sway, 9 were aged over 35 and 7 under 34.
Condition 5 –Vestibular information
is available and accurate, somatosensory info is inaccurate,
visual information is unavailable
All but two (females, aged 30-39) showed at least mild sway,
including 2 falls (both women, one aged 20-24 and the other
aged 35-39) and 21 moderate sways. The two not showing mild
sway under condition 5 showed mild sway under condition 6).
Condition 6 - Vestibular information is available
and accurate, somatosensory info is inaccurate, visual information
is inaccurate
18 moderate sway, 30 mild sway, 10 only minimal sway. Of those
10, 8 were women, and all of the ten showed at least mild
sway under condition 5.
DISCUSSION
What would be expected Vs What we observed
Condition 1 - normal conditions, so all
individuals would be expected to maintain balance quite easily.
The experimental data showed this to be the case.
Condition 2 - visual information unavailable,
but the healthy individual would be able to rely on somatosensory
information and easily maintain balance. With the exception
of a few mild and moderate sways, this was shown to be the
case.
Condition 3 – visual information available
but inaccurate, so the healthy individual would again rely
on the correct somatosensory information to retain standing
posture. The results for this condition were similar to those
for condition 2 i.e. most people could maintain balance quite
easily.
Condition 4 – somatosensory information
was inaccurate but visual and vestibular information were
available and accurate, so the healthy individual should maintain
balance quite easily – the results in this condition
were similar to those for condition 3 and 4 i.e. most people
could easily rely on the former two inputs to maintain balance.
Condition 5 – in this condition the
individual must rely solely upon his or her vestibular system,
so we would still expect most people to maintain balance but
would expect some people to show some loss of stability. This
was in fact the condition showing overall least stability
– there were two falls and many individuals showed some
degree of sway.
Condition 6 – similarly to condition
5, the individuals must here rely on the vestibular inputs
to maintain balance – again there was pronounced swaying
and the results were indeed similar to condition 5.
Further analysis
The aim of the experiment, when carried out in the clinic,
is to 1) determine which of the three sensory systems the
patient is most dependent on and 2) how well the patient reacts
when the sensory systems show conflict. Most of the individuals
in this study were able to maintain balance in conditions
1-4, but most showed some mild instability under conditions
5 and 6, which correlates with theory described in the introduction
as under these last two conditions the patient must rely primarily
on inputs from the lesser-weighted vestibular system. However,
no patients could easily maintain stability over all six conditions;
this is contradictory to the statement that ‘most healthy
adults easily maintain stability over all six conditions’.
Hence it can be assumed, if that statement is taken as correct,
that either the subjects tested were not healthy, the test
was carried out incorrectly (producing too much sway in the
subjects) or the sway was quantified incorrectly (a minimal
sway recorded as a mild sway etc). Or perhaps a better paradigm
on which to base results would be: ‘In conditions 5
and 6, normal individuals will sway more but will not become
unstable’.
Joe Bloggs, a 40-44 year-old woman, stood heel-to-toe and
performed the experiment under the six conditions. She had
no problems on a flat surface (some mild sway while wearing
the headgear) but on the foam surface she showed moderate
sway when eyesight was not impeded, and was unable to prevent
from falling under conditions 5 and 6. This could be explained
by the fact that standing heel-to-toe requires input from
the vestibular system – the vestibular input given during
heel-to-toe standing is different from the input given when
the individual is standing in a stable feet side-by-side position.
So in condition 4, ‘Joe’ is just about able to
prevent from falling because she can rely on her visual input,
but in conditions 5 and 6 there is no accurate data from any
of the 3 systems hence even after 2 attempts she is unable
to maintain standing balance.
The effect of age - The ageing process brings
with it intrinsic changes such as a degrading musculoskeletal
system, degrading sensory function, and gait changes. When
an elderly person starts to fall, the detection by the sensory
systems that a disturbance of balance is taking place is slower
than in a younger individual so the corresponding corrective
muscular movements may not take effect in time. In this experiment
there was no one tested over the age of 50 so it is difficult
to assess the effect of the conditions under study on a degraded
sensory system. With these results age appeared not to be
a factor as there were no clear trends linking increased age
to increased instability or vice-versa.
A further study should include individuals between the ages
of 50-70.
Posture and balance in Podiatry –
as we have already seen, the brain receives somatosensory
information, a very important sensory input for balance, through
the feet. Foot physicians can study an individual’s
posture and balance and use their findings to diagnose and
treat gait dysfunction and posture disorders. Different parts
of the foot can be stimulated, inhibited or supported to improve
an individual’s stability and mobility.
Limitations in the experimental method
Problems in quantification of postural sway were
suggested earlier. A new ‘modified clinical test of
Sensory interaction on balance (mctsib)’ boasts a far
better quantification of sway – the subject’s
sway velocity is measured, by a computerised system, as the
ratio of total distance moved by the COG (centre of gravity)
in degrees, to time. (A sensitive platform calculates the
centre of pressure and hence the centre of gravity.) This
gives a far more accurate idea of the individual’s sway
and better quantification of his/her reaction to the varying
conditions, hence further insight into the patient’s
preferred sensory system.
- BIBLIOGRAPHY
- Anatomy & Physiology 2nd Edition –
Seeley R, Stephens T, Tate P
- Clinical Disorders of Balance, Posture &
Gait – Bronstein A, Brudt T
- moon.ouhsc.edu/dthompso/balance/
- ‘Stepping Up’ Fall prevention efforts
– Kay Van Norman www.alsuccess.com
- portlandphysio.co.uk
- www.eng.auburn.edu
- abcnews.com
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