Just a quick thought,

A frequently observed event when prescribing orthoses is that the wear
pattern will change on a shoe. Podiatrists would always like to say i) this
is a good thing and I caused it and ii) it is a result of the orthotic
improving foot function. Just reading Eric's discussion made me realise
(perhaps long after everyone else has), when in the classic situation where
we have a child with medial shoeware (pronated foot, etc), then issue an
orthotic and get lateral shoeware, we have in fact seen a change in the
lever arm lateral to the STJ. In the first instance the lever arm was nice
and close and would have little effect on the STJ (perhaps nicely chosen by
the child's biomechanics) in the latter it is of course much further away.
This would of course require much greater activity of tib.post
eccentrically/concentrically to control the STJ pronation subsequently I ask
the question: Is an orthotic just simply (in this case) a tib post/tib ant
trainer. Perhaps that is why we also see the carry-over effect without the
orthotic. The other question of course is what caused the initial change
from medial to lateral shoeware, is it because the foot requires that range
of pronation and has to adapt to the orthotic (yes I know the standard
podobiomechanics answer,  but I want another)?

Just another comment, (whilst I am out on a limb so to speak, hehehe), in
foot manipulation it has been observed that following STJ/ankle manip that
there is increased inversion of the foot prior to loading in stance. Is it
possible that because we temporarily confuse the proprioceptors of the
joints that they require much more lateral heel contact/force to increase
joint sense to the brain? By doing so frequently one could  inadvertently
alter the timing/function of tib.post/ant to control pronation etc. and once
again we get the carry-over effect into gait.


For your thoughts,



Matt



-----Original Message-----
From: Fullerpod <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Tuesday, 7 April 1998 16:06
Subject: Re: Normal testable parametres and Normal Clinicalobservations


>In a message dated 98-04-06 15:17:48 EDT, Trevor Prior writes in response
to
>my comments
>
>Eric wrote
>>
>>  << In the case of the postierior tibial muscle, the locataion of the COP
>could
>>  be used.  If the orthosis causes an increase in supinataion moment then
the
>> COP,  between the orthosis and the foot would be more medial.  If the
>Supination
>> is from the posterior tibial muscle, then the COP would be more lateral.
>(Or
>>   there could be a source of supination moment from some other source.)
>> Never the less, a medial shift of the COP would translate to a smaller
>pronation
>>   moment from ground reactive force.  And this may correlate with less
>stress
>>  on the posterior tibial muscle.   >>
>
>Trevor writes:
>
>>
>>  I am not sure I agree with this. If you provided an orthotic with
>significant
>>  medial control, such that the resulting supination of the foot caused
>lateral
>>  instability, then the CoP would be lateral. The position of the CoP
deends
>on
>>  the equilibrium of the oppposing forces ala Kevin.
>>
>>
>
>How does an orthotic provide significant medial control?  By increasing
force
>medially.  You cannot simultaneously increase force on the medial and
lateral
>sides of the STJ axis.  The maximum amount of force on the foot is body
>weight, in stance.  The force from the medial side plus the force from the
>lateral side must add up to body weight.
>
>There are three possible situations that could occur here.  The first is a
>situation where the COP is directly beneath the STJ axis.  I call this a
>balance foot, because the forces medial to the STJ axis balance the forces
>that are lateral to the STJ axis.  This foot will appear more "supinated
than
>average."  Addition of a varus heel wedge in this foot will cause
supination
>and this will cause a lateral shift of force on the forefoot so that the
foot
>will re equilibrate.  Essentially we increased the lever arm of forces
medial
>to the axis and this required a shift, away from the axis, of the forces
>lateral to the axis.  The center of pressure will stay in the same place
and
>Ground reactive force still balances about the STJ axis.
>
>The second situation is where the COP starts lateral to the STJ axis and
this
>causes a pronation moment from the ground.  In this case there must be a
>supination moment from some structure within the foot (possibly the
posterior
>tibial tendon.)  A varus heel wedge (medial control) would decrease the
>pronation moment from the ground and decrease the need for tension in the
>posterior tibial tendon.  In this case the COP would move medially.
>
>The third option is that COP starts lateral to the STJ axis.  In this case
>there is a supination moment from the ground.  (A patient prone to peroneal
>tendonitis.  The peroneals can be noted to contract in stance.  Kevin,
thanks
>for pointing this foot out to me.)  A varus heel wedge (contra - indicated)
in
>this foot would shift the COP even more medially and then we would see
>additional peroneal contraction to try to re-establish STJ position where
the
>patient would feel comfortable.  The COP would stay medial.
>
>The only scenario that I can imagine where the foot would supinate so much
>that there would be a lateral shift in the COP would be in a foot where
there
>was increased supination moment from above the floor/ orthosis.  (For
example,
>the posterior tibial tendon.)   This all theoretical musings and very hard
to
>measure because it would be difficult to correlate EMG activity with joint
>moments, although there are those studies that have calculated the moment
from
>ground reactive force by attempting to measure the position of the STJ axis
>during gait.
>
>Cheers,
>
>Eric
>



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%