Core Stability Myths and Research

[Guest guest]

A physical therapy student responded on a physio list to my recent post on

" core stability " and core guru claims, thus:

<< I haven't seen this " rising tide of critical analysis. " Do you have any

specific articles? I'm currently doing a paper on spinal stability for

school so I am familiar with some of the research (still reading)......

Most if not all the articles you presented on postural stability were top

quality research from some of the top people in the area. However, I believe

that you misinterpreted the application of the research in many cases and in

at least one instance misinterpreted the research itself.

You talked about the importance of the ankle and hip in postural stability.

Postural stability and spinal stability are related but not the same thing.

You could theoretically fall down and still maintain spinal stability (in the

medical sense of stability we'd be in trouble if we couldn't). I'm sure all

the physiotherapists on this list learned about ankle and hip strategies in

school. I think most people will agree that peripheral stability is

important and many of the exercises we give challenge it but it is not the

same thing as core stability.

The article you misinterpreted was out of Waterloo. I believe that Winter

and/or Patla were co-authors. It talked about stiffness being the initial

response to postural perturbations. You stated that this showed that

reflexes weren't involved and thus can't be trained (or something to that

affect). What the authors were saying was that the very initial part of the

response was due to preset stiffness not the whole response. A ton of

research has been done in this area and any study doing emg I'm sure has

shown an increase in muscle activity. This increase in muscle activity has

to be due to a reflex of some type. >>

This was my response:

*** When you refer to " stiffness " , are you referring to the colloquial use of

the term, as in stretching and flexibility, or are you referring to

mechanical stiffness as the dynamic response to external loading on a

mechanical system? Anyway, if the latter, mechanical stiffness is NOT

" preset " in the body, but depends on the degree of activation and tension in

the various soft tissues (I examined this issue in great detail in my PhD

research). Stiffness is also frequency and temperature (and fatigue)

dependent, so I am curious to know how it can be " preset " in the body.

Nowhere did I dispense with the role played by various stretch reflexes in

control of any neuromuscular actions either locally or generally. I have

NEVER stated that reflexes cannot be trained - on the contrary, I have

written extensively on operant, concurrent and respondent conditioning of

reflexes, based upon work going back as far as the great Pavlov. You seem to

be seriously misunderstanding or misinterpreting what I have written.

Incidentally, my " misinterpretation " of the mechanisms involved in " core

stabilisation " is also based upon a very extensive collection of additional

articles, such as the following small sample, which address the issue that

static and dynamic stabilisation of the body and its components depends on 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. More of my

misinterpretation is 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.

I trust that your continued reference to " spinal stability " is not being

based upon any belief that stability of the spine can be defined and studied

in the absence of its attachment to adjacent and more distal structures of

the body, because that sort of modelling 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.

Though you dismissively proclaim: " I'm sure all the physiotherapists on this

list learned about ankle and hip strategies in school " , this statement does

not mean that they all understand or have scientifically analysed the

processes involved, because some of the most erudite workers in the field of

motor control still do not presume to understand the processes involved to

any great level of certainty.

Here are those few additional articles that I mentioned earlier. If your

school project on " spinal stability " so far ignores these and related

findings, then it undoubtedly will need some more material other than the

Waterloo papers that you touched upon.

-----------------

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, 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, 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.

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Dr Mel C Siff

Denver, USA

Supertraining/