Hi Eric,

>>>   A varus wedge effect device will apply force on the more medial 
>>> side of the cup and the cup contacts the ground further laterally.  
>>> The equal and opposite reaction from the foot applied to the 
>>> orthosis and the force from the shoe on the heel cup of the orthosis 
>>> create a force couple that will bend the device.  In these devices I 
>>> will tend to add rearfoot posts.
>>
>> Doesn't this depend on the amount of post? Without a post, there is a 
>> contact point of the heel cup in midstance. This is related to the 
>> forefoot of the orthotic. This shouldn't be influenced much until 
>> there is enough material under the medial side to contact the ground.
>
> I'm not quite sure what you mean.  I have felt a difference, with my 
> own feet, between a symmetrical device, an unposted heel varus wedge 
> device and a posted heel varus wedge device.  The unposted device will 
> shift the center of pressure, especially when the anterior edge of the 
> orthosis is on the ground.   Is this what you mean?  It is my sense 
> that a posted device can effect the foot before forefoot loading.  
> This effect can occur with a partial rearfoot post on the device or 
> whole post.  What do you mean by amount of post?

Eric, I agree that a posted device can effect a foot before loading. 
When you post, it will bring the foot to the midstance position quicker. 
By that I mean if you are posting to neutral (assuming your orthotic is 
100% efficient), you will be in neutral earlier than without a rearfoot 
post. If you are posting to a pronated postition and assuming 100% 
efficiency, then the foot will assume the pronated postion earlier. 
There is a greater lever arm (R) from the axis of the STJ to the 
contacting part of the calcaneus. The lateral aspect of the post acts as 
an extension to the lateral aspect of the calcaneus. Since this is 
occuring at heel contact, the portion of the post posterior to the 
center of the calcaneus, is having the influence at this time.
At midstance, if the posting is less than the amount required to 
maintain neutral position and the orthotic is made to neutral position, 
then the rearfoot post will not contact the ground on the medial aspect 
when the device is placed on the ground. The post will act in principal 
to give a variable stiffness to the device. Initially it will be easier 
to bend the the device, as the distance between the rear most part of 
the medial device to fore most part of the device to contact the ground 
will be greater than after the device bends, and the post contacts the 
ground to shorten this distance which stiffens the device.
If the post is equal to the amount required, (for instance a 6 degree 
rearfoot post with a 6 degree rearfoot varus), then in neutral position, 
the post will be flat on the ground. This now acts to shorten the 
effective bending moment (? or is it couple), and stiffen the device. It 
is interesting to note that this effect is done by the part of the post 
that is anterior to the center of the calcaneus.

Regards,

Stanley

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