At 04:00 PM 5/17/2007, you wrote:
>From:    Paul Conneely <[log in to unmask]>
>Subject: Re: Tensegrity
>
>Eric et al
>
>Firstly The S1/S2 disc in most of us is nothing but a remnant and the two
>bones are fused.
>
>If you move up to L5/S1 then what is written makes more sense.
>
>Food for thought: In the lumbar spine...
>1. there is an annulus that surrrounds the center of the disc. The fibres
>of the annulus are set at about 120 degrees to each layer just like the
>cross ply in you car's tyre.
>2. the annulus takes up rotational properties
>3. in the centre of the disc is the toxic ball called the nucleous 
>pulposis. This takes up compression.
>
>Thus there is a tensegrity unit.

Aren't tensegrity units supposed to have compression members that 
attach to the tension members.  How does the annulus attach to 
nucleus pulposis?  This does not look like the tensegrity structures 
shown in the website suggested by Kevin Miller.  It looks more like a 
very thick balloon that supports compression.


>Having said this, Stuart Macgill from Canada has been working on the 
>spin for years and is THE world authority.
>
>In his experiments using extremely fresh cadavars (like a few hours) where
>they have dissolved all the tissues from the lumbar spine from L3 down
>except for the ligaments has shown when the spine is kept erect and a
>pound (8KG) load is applied to the top of the L3 vertebrae the L3
>vertebrae is visually deformed within 15 minutes. I have seen it happen,
>it is hard to believe, but true.
>
>So how do we support our own body?  the muscles, fascia have a huge role
>in keeping us vertical ie. antigravity.

Could you expand on the experiment?  I'm not quite sure what your 
point is.  The tissue around the vertebra is dissolved in 
something.  Doesn't this make the bone weaker too?



>Finally I cannot see how the foot has trusses etc. to make explanations.
>
>I have been to all the web sites and studied them thus the delay in 
>writing (those mentioned on this web base) and I have yet to see a 
>beam that looks like a bone.  If they do look like bones, the 
>experiments that show a beam supported at each end and then a force 
>is supplied mid shaft etc. look silly when a bone is the thinnest at 
>the mid shaft.  If a beam was made in this fashion, I doubt it would 
>last very long. I have yet to see a bridge made in the likes of a 
>bone.  Any one else had a thought on this subject?
>

A series of bones and ligaments can be treated as a beam.  For 
example the calcaneus, cuboid and fifth metatasrsal along with their 
plantar ligaments will behave as beam when a vertical load is 
applied.  The windlass mechanism as described by Hicks describes the 
medial column as a tied arch.

Perhaps trying to attach a name like truss or beam to the situation 
is confusing the issue.  The concept that needs to be understood is 
bending moment from applied load.  Take a foot that bears weight at 
the plantar surface of the calcaneus and metatarsal heads.  When 
standing the vertical load of gravity acting on the rest of the body 
is applied by the tibia to the top of the talus.  This load is not 
directly over either the calcaneus nor the met heads.  This sets up a 
situation where the external load will try to flatten the arch.  The 
arch does not flatten because of internal forces in the foot.  These 
internal forces are similar to the internal forces in a beam.

To eamine these forces further you can do a free body diagram on half 
of the foot.  This is straight out of basic engineering 
textbooks.  Take the talus and calcaneus.  There is a downward force 
from the tibia on the top of the talus.  There is an upward force 
from the ground on the bottom ot the calcaneus.  These forces are not 
directly pointed at one another so a force couple is created.  A 
force couple creates a moment that will tend to cause rotation unless 
resisted by another moment.  The magnitude of the moment from the 
force couple is equal to the magnitude of the force times the 
distance between the two forces.  So the talus and the calcaneus will 
tend to plantarflex in response to the external forces described 
earlier.  In static stance these bones are not plantar flexing so 
there must be a moment created by the part of the foot removed from 
the diagram that opposes this plantar flexion moment.  Tension in the 
spring ligament and compression at the talo navicular joint will 
create a dorsiflexion moment on the combined talus and calcaneus 
(rearfoot unit)  Also an anterior pull at the insertion of the 
plantar fascia and compression of the talo navicular joint will also 
supply a dorsiflexion moment on the rearfoot unit.  The tension from 
the forefoot must be exactly equal to the compression from the 
forefoot.  This must be true because there cannot be a net anterior 
to posterior force from the forefoot on the rearfoot, because if 
there was a net force there would be an acceleration.  So if only the 
spring ligament (plantar talonavicular ligament) were preventing 
plantarflexion of the rearfoot then the tension in the ligament would 
have to be much higher than the tension in the plantar fascia if the 
plantar fascia was the only structure preventing plantar flexion of 
the rearfoot.  This because the distance between the compression and 
tension forces is smaller for the spring ligament than the plantar fascia.

This is also the reason that a high arched foot will have lower 
tension in the fascia than a low arched foot given identical external 
loads.  This is also why in a single foot, when the STJ is supinated 
there will be less load in the fascia than when STJ is pronated with 
identical external loads.

So, it doesn't really matter if you call it a truss or an arch or a 
beam.  You just have to understand how the foot supports the load.

Regards,

Eric Fuller 

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