[log in to unmask] writes:
<< ps - where are those papers that show flaws/errors in the studies on core
stability? It would be interesting to read them. >>
***For a start, how is it possible for the "core" to stabilise in the absence
of attachment or influence from adjacent or more distal structures?
Stabilisation is profoundly dependent upon context and physical enviroment.
The mechanisms involved in static and dynamic "core stabilisation" involves
a much wider variety of cybernetic strategies than are suggested
simplistically by far too many of the "core stabilisation" gurus, including
the compensatory stepping response and change of support response. My
comments are also based upon the balancing and coping strategies used by
spinal patients with high lesions, who obviously cannot use "core stability"
methods to carry out their daily activities.
Continued reference to "spinal or core stability" based upon any belief that
stability of the spine can be defined and studied in the absence of its atta
chment to adjacent and more distal structures of the body is very flawed and
irrelevant. Stabilisation of any biological structure has to be understood
and analysed with respect to its physical environment and its context, as
well as the neurocybernetic processes involved.
Here are a few of the hundreds of articles that are relevant to discussion of
this issue:
-----------------
The role of limb movements in maintaining upright stance: the
"change-in-support" strategy
Maki BE, McIlroy WE
Phys Ther 1997 May; 77(5): 488-507
Change-in-support strategies, involving stepping or grasping movements of the
limbs, are prevalent reactions to instability and appear to play a more
important functional role in maintaining upright stance than has generally
been appreciated.
Contrary to traditional views, change-in-support reactions are not just
strategies of last resort, but are often initiated well before the center of
mass is near the stability limits of the base of support. Furthermore, it
appears that subjects, when given the option, will select these reactions in
preference to the fixed-support "hip strategy" that has been purported to be
of functional importance.
The rapid speed of compensatory change-in-support reactions distinguishes
them from "volitional" arm and leg movements. In addition, compensatory
stepping reactions often lack the anticipatory control elements that are
invariably present in non-compensatory stepping, such as gait initiation.
Even when present, these anticipatory adjustments appear to have little
functional value during rapid compensatory movements. Lateral destabilization
complicates the control of compensatory stepping, a finding that may be
particularly relevant to the problem of falls and hip fractures in elderly
people.
Older adults appear to have problems in controlling lateral stability when
stepping to recover balance, even when responding to anteroposterior
perturbation. Increased understanding and awareness of change-in-support
reactions should lead to development of new diagnostic and therapeutic
approaches for detecting and treating specific causes of imbalance and
falling in elderly people and in patients with balance impairments.
----------------
Cross-correlation analysis of the lateral hip strategy in unperturbed stance
Lekhel H, Marchand AR, Assaiante C, Cremieux J, Amblard B
Neuroreport 1994 Jun 2;5(10):1293-6
Subjects standing heel-to-toe on either hard ground or soft support were
instructed to stand upright keeping optimal balance. Lateral accelerometric
measurements at head, hip and ankle levels were subjected to conjugate
cross-correlations analysis in order to determine the co-ordinated movements
or strategies.
The results strongly suggest that there exists a hip lateral strategy which
is very similar to the hip strategy previously described in fore-aft body
oscillations. This lateral hip strategy was only observed when the greatest
body oscillations were observed, namely on the soft supporting surface, and
its descending sequence of co-ordinated movements is consistent with the idea
of a top-down organization of postural control during movement or difficult
stance conditions.
-----------------
Task constraints on foot movement and the incidence of compensatory stepping
following perturbation of upright stance
McIlroy WE, Maki BE
Brain Res 1993 Jul 9; 616(1-2):30-8
Our understanding of the postural control responses in the event of external
perturbation has focused almost exclusively on the early automatic
adjustments. The present study addresses another postural reaction that is
functionally important: compensatory stepping. The purpose was to identify
the relative importance by comparing the prevalence of compensatory stepping
with and without instructions constraining the subjects' responses.
Subjects stood on two force plates which were mounted on a "moveable"
platform. Their posture was perturbed by the translation of the platform
either forward or backward at various accelerations. Following a practice
period, seven subjects each performed under two different tasks:
"constrained" (keep feet in place) and "unconstrained" (no specific
instructions given). The primary focus of the analysis was on responses to
forward platform translations.
Analysis revealed that the frequency of stepping tended to be higher in
"unconstrained", as opposed to "constrained", tasks. The frequency of
stepping was also related to the interaction between the tasks and the order
in which they were given. Specifically, subjects stepped most frequently when
they received the "unconstrained" task first. The frequency of stepping also
increased as the magnitude of the platform acceleration increased. Time of
onset of stepping, as defined from the force plate measures, began as early
as 160 ms in one subject and averaged 250 ms across all subjects.
These relatively fast response times suggest that step initiation often
occurs well before the limits of stability are reached. A novel and
unexpected finding was the identification of a third response type,
intermediate to stepping and (bilaterally symmetrical) non-stepping responses.
---------------
Does frontal-plane asymmetry in compensatory postural responses represent
preparation for stepping?
Maki BE, Whitelaw RS, McIlroy WE
Neurosci Lett 1993 Jan 4;149(1):87-90
The bilateral symmetry of feet-in-place responses to postural perturbations
in the anterior-posterior direction has not been well studied. This paper
presents evidence that right- and left-leg responses that appear to be
approximately symmetrical in the sagittal plane may actually involve an
asymmetry in the frontal plane, namely, a lateral weight shift.
Comparison with trials where subjects stepped suggests that these lateral
weight shifts represent early preparations for stepping responses that are
aborted before the foot is actually lifted. Thus, it would seem that
compensatory stepping involves a sequence of discrete modifiable stages,
rather than a single immutable motor program. Moreover, postural responses
that appear to be similar in the sagittal plane may actually be seen to
involve quite different postural strategies, i.e. in terms of preparation for
stepping, when viewed in the frontal plane.
---------------
Influence of lateral destabilization on compensatory stepping responses
Maki BE, McIlroy WE, Perry SD
J Biomech 1996 Mar; 29(3): 343-53
Previous studies of compensatory stepping, in response to postural
perturbation, have focussed on forward or backward stepping; however, the
ability to step in other directions is of equal functional importance, since
the perturbations encountered in daily life may often include a lateral
component. The primary objective of this study was to determine how lateral
destabilization affects the compensatory stepping response, in terms of: (1)
swing-leg selection, (2) preparatory unloading of the swing leg, and (3)
spatial and temporal characteristics of the swing trajectory. A novel
multi-directional moving platform was used to apply transient perturbations
in eight horizontal directions, in 10 healthy young adults. Perturbation
magnitude was varied unpredictably over a wide range and subjects were
instructed to try not to step, so as to discourage preplanned 'volitional'
foot movement.
The predominant strategy, seen in 96% of stepping responses to lateral
destabilization, was to swing the leg that was unloaded by the perturbation.
This strategy allowed a much more rapid foot-lift but required a longer and
more complex swing trajectory, compared to responses where the
perturbation-loaded leg was swung. When compared to forward and backward
steps, the addition of a lateral component to the perturbation led to a 20%
(90 ms) reduction in time to foot-off, a 20% (7 cm) increase in step length
and a 70% (110 ms) increase in swing duration, on average.
The results clearly demonstrate that compensatory stepping responses to
non-sagittal perturbations are strongly influenced by biomechanical
constraints and affordances that do not affect the forward and backward
stepping behaviour that has been studied traditionally. These findings
underscore the need to assess postural responses in multiple directions, in
order to understand more fully how balance is maintained in the exigencies of
everyday life.
----------------
Thresholds for step initiation induced by support-surface translation: a
dynamic center-of-mass model provides much better prediction than a static
model
Pai YC, Maki BE, Iqbal K, McIlroy WE, Perry SD
J Biomech 2000 Mar; 33(3):387-92
The need to initiate a step in order to recover balance could, in theory, be
predicted by a static model based solely on displacement of the center of
mass (COM) with respect to the base of support (BOS), or by a dynamic model
based on the interaction between COM displacement and velocity. The purpose
of this study was to determine whether the dynamic model provides better
prediction than the static model regarding the need to step in response to
moving-platform perturbation.
The COM phase plane trajectories were determined for 10 healthy young adults
for trials where the supporting platform was translated at three different
acceleration levels in anterior and posterior directions. These trajectories
were compared with the thresholds for step initiation predicted by the static
and dynamic COM models. A single-link-plus-foot biomechanical model was
employed to mathematically simulate termination of the COM movement, without
stepping, using the measured platform acceleration as the input. An
optimization routine was used to determine the stability boundaries in COM
state space so as to establish the dynamic thresholds where a compensatory
step must be initiated in order to recover balance. In the static model, the
threshold for step initiation was reached if the COM was displaced beyond the
BOS limits.
The dynamic model showed substantially better accuracy than the static model
in predicting the need to step in order to recover balance: 71% of all
stepping responses predicted correctly by the dynamic model versus only 11%
by the static model.
These results support the proposition that the central nervous system must
react to and control dynamic effects, i.e. COM velocity, as well as COM
displacement in order to maintain stability with respect to the existing BOS
without stepping.
-----------------------
Dr Mel C Siff
Denver, USA
http://groups.yahoo.com/group/Supertraining/
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