Colleagues, All, (I get confused here as to what is the appropriate -proper- one word opening salutation is/should be) 



To Bart and others who work and/or have access to gait labs, and/or can influence/convience gait labs (hy-po-tee-size here that Bart will hate me for this, giving him more work, but what are friends for...), if finite element analysis software was integrated as one more analytical tool, many of the questions being asked, and many of the answers/replies being offered via deductive analysis in these discussions could/would be verifyed, confirmed and or denied, as well as more questions being rasied. Food for thought, my 2 cents.



Norman



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	From: A group for the academic discussion of current issues in podiatry on behalf of [log in to unmask] 

	Sent: Wed 9/22/2004 10:10 AM 

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	Subject: Re: GRF on Forefoot, What Moves?

	

	

	Hi all,  Kevin Replied to Simon's excellent comment on the importance of moment of inertia.



		Good point about the moment of inertia of the individual segments, Simon.  Therefore, using the concept that the angular acceleration of each joint will be dependent on the moment of inertia of the segment and the stiffness/compliance of that segment, then when a force is applied to a distally located segment, such as the 5th metatarsal head, the ankle joint, subtalar joint and midtarsal joints will all move simultaneously in response to the force on the 5th metatarsal head but will move at different rates.

		This certainly makes sense to me.  However, since the masses of the segments we are speaking about are so small compared to the magnitude of the external (i.e. ground reaction force) and internal forces (i.e. resisting forces from muscles, ligaments and tendons) involved, the differences in the moments of inertia between each segment in question will probably be insignificant in affecting the angular accelerations especially considering how great the external forces and internal forces  being applied to those segments are.



	For the best analysis of this question, one must look at all of the forces.  In static stance, gravity is pulling the body toward the center of the earth and ground reactive force is a second force that brings the net force to zero in static stance.  So, when ground reactive force pushes up on a particular structure, there are forces acting on the more proximal joints simultaneously.

	 

	Even if the pushing on the foot with a finger, when the patient is seated in a chair, their will be simultaneous forces from proximal structures.  The inertia of the body creates a force from above on proximal structures.  Even if the chair is on wheels.

	 

	This goes back to the discussion of how to calculate a moment at a joint.   (Moment = moment of inertia x angular acceleration and moment = force times distance)  We can use a hinge axis model to calculate the distance from the line of action of the force or we can use a force couple model to calculate the distance.  Most of the time they will yield the same result.  The hinge axis model ignores the proximal force.  Whenever a force causes a rotation there will be a force couple involved.  (One of the forces can come from inertia)  

	 

	So a force applied to the foot will create a moment at all of the joints of the foot.  The acceleration that occurs will be determined by the magnitude of the moment and the moment of inertia of the body part(s) that will be accelerated.  Which joint moves the most will be determined by the ratio moment / moment of inertia.

	 

	Regards,

	 

	Eric Fuller



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