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View Full Version : WDYT about classification of postural strategies of a standinghuman?



Johannes Reich
11-18-1999, 07:07 AM
*** Thesis ***

Postural strategies of a standing human are mainly classified as ankle- or
hip-strategies in the current literature. In my opinion they should be
classified as either static or dynamic with the following definition of
both terms: A static strategy means that the human controller has to rely
on angle dependent backdriving torques in the ankle while a human
controller using a dynamic strategy contents himself with only
angle-velocity dependent torques or no torques at all in the ankle


*** Reasoning ***

Although two legged standing seems to be a simple daily life activity, a
sufficient understanding is still lacking. The reasons are manifold: The
biomechanical system is highly nonlinear, inherently instable and the
biological controller has to deal with relevant time lags, redundancies on
several levels and noisy sensors and actuators.

The stabilizing mechanisms are not based on long loop reflexes, as was
thought earlier. Fitzpatrick et al. (1996) estimated their gain to be
approximately 1. Consequently they should play only a minor role in
postural stabilization against disturbances.

The situation seems to be much more complex. What seems clear is that
different strategies are used. Based on experimental work three different
strategies have been distinguished in the literature: Ankle-, hip- and
step-strategy. Which one is choosen depends on the circumstances
(instruction, magnitude of disturbance, personal preferences, etc.)

Against small, unexpected disturbances on firm ground most humans behave
as an inverted pendulum, stabilized by a torque in the ankles against the
disturbance (e.g. Nashner 1976) wherefore this is also called
ankle-strategy.
The most often cited condition for success, that the projection of the
center of mass has to remain over the supportive area (the feet), is valid
only if the ground is indeed firm and the foot segment is nonrigidly
connected to the floor.
With larger disturbances or with a smaller supporting area (e.g. standing
on a narrow ridge, Otten 1999)) most humans stabilize themselves
dynamically by using the inertia of their body segments and their
mechanical coupling. E.g. facing a precipice and being pushed forward most
humans would accelerate their upper body forward - in contrast to backward
as with the ankle-strategy - in the direction of the precipice and flail
their arms in order to move their center of gravity behind the edge.
Because the main movement of the trunk takes place around the hip joint
this is also called hip-strategy (Horak and Nashner 1986). Standing in the
opposite direction would restrict movements of the trunk so that the main
movements would be probably performed by the arms.
In case that a successful stabilization in place is not possible anymore,
often a step is taken as the third strategy (probably not at the
precipice;).

As can be seen the current classification of strategies is based on a
somewhat arbitrary selection of experimental conditions. In my opinion, a
classification based on the underlying mechanisms of control would be much
better.

The ankle-strategy on firm ground obviously depends on torques in the
ankles, which could well be supplied by a passive, i.e. static
arrangement. Such a controller could be successful even in a more general
situation, where the backdriving forces were ankle-angle dependent.

The characteristic feature of the response to larger disturbances is not
the hip movement, but the fact that one does not resist the disturbance
but yields to it. Some body segments are even accelerated in the direction
of the disturbance, to move the projection of the center of mass via the
mechanical coupling in the desired direction. During such a dynamic
stabilization the projection of the center of mass does not have to remain
over the supportive surface - in contrast to a static one -, only in a new
equilibrium that may be reached at the end this condition has to be
fulfilled again.
A perpetual violation of the static criterion is seen easily in a case
where one balances successfully on a narrow ridge without reaching an
equilibrium, i.e. the system is stabilized dynamically in a probably
chaotic attractor instead of a fixpoint.

Neglecting the step-strategy, a sensible classification of postural
strategies would therefore be based on the terms "static" and "dynamic",
emphasizing that motor activity cannot be subdived per se into a (static)
postural and an additional (nonpostural) dynamic component.

Unfortunately, there is no uniform use of the terms "static" and "dynamic"
in the context of postural strategies. E.g. Westcott et al. (1997) defines
as static balance "the ability to maintain a posture" and as dynamic
balance "the ability to maintain postural control during other movements".
Another example is the term "dynamic posturography" that refers to a
paradigm of Nashner et al. (1982) were people had to stand on a movable
plattform while different sensory channels could be disturbed in a
possibly contradicting way.

For this reason I would like to define the terms "static" and "dynamic" as
I did in the thesis above: A static strategy means that the human
controller has to rely on angle dependent backdriving torques in the ankle
while a human controller using a dynamic strategy contents himself with
only angle-velocity dependent torques or no torques at all in the ankle

WDYT ?


*** Literature ***

# Fitzpatrick, R.C, Burke, D. and Gandevia, S.C. (1996). Loop gain of
reflexes controlling human standing measured with the use of postural and
vestibular disturbances. J. of Neurophysiology, Vol. 76, 3994-4008.
# Horak, F. and Nashner, L. (1986) Central programming of postural
movements: Adaptation to altered support surface configurations. J. of
Neurophysiology, Vol. 55, 1369-1381.
# Nashner, L.M. (1976) Adapting reflexes controlling the human posture.
Experimental Brain Research, Vol. 26, 59-72.
# Nashner, L.M, Black, F.O. and Wall, C. III. (1982) Adaptation to altered
support and visual conditions during stance: pattients with vestibular
deficits. J. of Neurosciences, Vol. 2, 536-544.
# Otten, E.(1999) Balancing on a narrow ridge: biomechanics and control.
Philosophical Transactions of the Royal Society of London, Series B, Vol.
354, 869-875.
# Westcott, S.L., Lowes, L.P. and Richardson, P.K. (1997). {Evaluation of
postural stability in children: Current theories and assessment tools.
Physical Therapy, Vol. 77(6), 629-644.


--
Johannes Reich | Email reich@uni-duesseldorf.de
Neurolog. Therapiecentrum | Tel. ++49 (0)211 7816 159
Hohensandweg 37 | Fax. ++49 (0)211 576152
D - 40591 Duesseldorf |

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