Paul,

You wrote:
 There are three components to foot motion.
> They are the lateral foot : calcaneus, cuboid, toes 4 and 5
> the medial foot all the rest.
> The lateral foot allows
> Pronation and adduction thus allows the standing foot to move to the 
> medial
> foot.
> It inhibits supination and abduction.
> The medial foot that allow running, jumping, climbing etc. allows 
> supination
> and abduction and inhibits pronation and adduction.
> Finally the middle cunieform allows none of these motions. The line of 
> action
> of the middle cueieform is talus neutral.
> This is why we have 5 bone in the mid foot.
> The heel has for standing the calcaneus.
> The foot has the talus.
> Easily summed up.
> All of these positions mentioned by others are totally reliant on gravity 
> and
> muscle tone.
> Joint position is fixed and thus so called neutral is never neutral 
> despite the
> best offers by dictionary.
> If you do not beleive me, come to our workshops.


We must differentiate between foot motion and STJ motion in order to discuss 
the axis and nature of STJ motion.  I consider STJ motion to be only that 
motion which occurs between the talus and calcaneus relative to each other. 
The ankle joint, which has a triplane axis contributes greatly to foot 
motion and makes appreciating the specific motion at the STJ all the more 
difficult.  The MTJ contributes to a great deal of forefoot motion.  There 
are two forms of STJ motion.  They are open chain and closed chain.  The 
simplest way to appreciate the axis of the STJ is in the open chain when you 
move the foot via the calcaneus about a static talus.  But when you move the 
STJ in the open chain, since there is relative motion between the 
talonavicular joint and the calcaneocuboid joint, some MTJ motion must 
occur.  Therefore, using the forefoot as a reference indicates to some 
extent what the STJ is doing, but forefoot motion is significantly 
influenced by midtarsal and other joint motion.  Unless you completely 
isolate the two osseous components that comprise the STJ, that being the 
talus and the calcaneus, you are seeing motion which is the result of other 
joints.

I have a crude theory (based on speculation more than anything!) that it may 
be possible the STJ is actually a compression joint and when it moves it 
disarticulates, in that the talus and calcaneus separate.  Under normal 
circumstances much of what we think of as STJ motion may actually be 
movement of the rearfoot as a unit (talocalcaneal unit).  At the distal 
aspect, the motion occurs via the MTJ.  When there is a demand for greater 
motion, then the STJ disarticulates.  It is much easier to move the STJ in 
the open chain since the compression forces at the STJ are at a minimum. 
When the foot is bearing weight, there is more compression force acting on 
the STJ, and therefore less actual STJ motion occurs.  Have you ever noticed 
that is seems impossible to produce the same degree of STJ or foot to leg 
motion in stance that you can produce in a non-weightbearing condition?

Anatomically, the STJ is actually two entirely different joints.  The 
posterior portion is the talocalcanean joint or true STJ (posterior, 
superior facet of the calcaneus with the inferior facet of the talus.  The 
anterior portion of what we call the STJ is actually the talocalcanean 
portion of the talocalcaneonavicular joint (the under side of the head of 
the talus articulates with the anterior and middle faces of the calcaneus, 
which may be fused to form a single facet).  It is impossible to move the 
talus relative to the calcaneus without separation between the two bones. 
They have a close coupled position which may anatomically be what we call 
the neutral position and any motion from that position seems to cause an 
uncoupling of the talus and the calcaneus.  Both articular surfaces between 
the calcaneus and the talus must disarticulate when the talus moves relative 
to the calcaneus.

That's my junk theory!

I'm not sure I can really address the bulk of your reply because I 
personally seem to have a difficult time comprehending or following your 
method of discussing biomechanics.

Respectfully,
Jeff Root
 

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