Hello Eric

I mostly appreciate your contributions  as clarifying the topic in
discussion. And relying on simple physics is hereby your favored
strategy and also your trade mark. But I am quite puzzled when
reading your explanation below:

>>>>>
>   When the ground reactive force on the hallux is greater than the
>mass of the hallux, the ground reactive force on the hallux must be
>passed to a more proximal structure.  The only structure capable of
>supporting compression just proximal to the hallux is the
>metatarsal.  (If the hallux weighs 1 Newtons and ground reactive
>force on the hallux is 12 Newtons then force from the proximal
>structures acting on the hallux will be 11 Newtons.  If the
>metatarsal is pushing on the hallux with 11 Newtons of force, then
>the hallux is pushing on the metatarsal with 11 Newton's of
>force.  [Newtons third law for every action there is opposite
>reaction]

>>>>>



As we all know,  the vertical GRF's under the foot  are distributed
under all foot structures in contact with the ground. And if the
hallux yields to the GRF under, the other foot structures still in
contact with  the ground, especially the 1st met, will  be loaded
proportionally to the hallux's yielding. But I do not understand what
the weight of the hallux and  how action-reaction influence vertical
loading of the 1st metatarsal.
I am afraid I am missing something here.


You continue:

>>>>
So, even if the metatarsal were off of the ground, it would still be
bearing ground reactive force as it is passed through the hallux.
Even though it is up in the air, and thought to be hypermobile, it
is sill stabilized by ground reactive force from below and body
weight from above.  I think this puts a hole in the use of
hypermobility to explain bunion formation.
>>>>



I cannot see how a structure in the air is bearing GRF. However, any
structure does bear joint reaction forces at its proximal and distal
end. And these, of course,  will be influenced by the GRF: so will
joint reaction forces acting on the tibial plateau and the femoral
condyles  increase or decrease when the GRF acting on the foot
increases or decreases. Quantification of this GRF influence  can be
calculated by solving the motion equations of the respective
free-body analysis, but there cannot be a direct involvement of GRF's
on structures not in contact with the ground or hanging in the air.
So, I do not get your point here  and  I  am afraid there may some
more readers in the same position.

Eric, sorry for forcing you to work on a weekend, but we would
appreciate  you clarifying your rationale here, in this way  enabling
all of us to  enjoy your insight into the phenomenon.

Regards,
Bart
--
*******************************************************************
  Bart Van Gheluwe
Laboratory of Biomechanics
Vrije Universiteit Brussel -Fac. LK
Vakgroep BIOM
Pleinlaan 2, 1050 Brussel, Belgium
Tel.: 02/629.27.33 (31)
Fax: 02/629.27.36
*******************************************************************

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