Dear all,

Kevin said


>Co mpliance/stiffness of a segment about an axis of rotation is
>determined by the amount of applied force (or moment) acting on that
>segment versus the linear or angular displacement of that segment
>about the axis of rotation.  Therefore, the moment of inertia may
>affect the apparent stiffness of a segment, but probably not
>significantly at low magnitudes of angular acceleration, such as
>would be present in the foot during the stance phase of gait.
>

Good reply.
  But I would only add that theoretically the term
compliance/stiffness is  basically a *static* measurement and as such
has nothing to do with moment of inertia which is a *dynamic*
variable. Compliance/stiffness  is not related to velocity or
acceleration of movement but only to force and its  resulting
displacement. With what velocity or acceleration this displacement is
occurring as a result of the application  of  force, is not
important. For a known force, only  the *magnitude* of the the
displacement matters.

But with visco-elastic material (as tendon and ligaments) velocity
will play a role when determining the compliance of a material or
joint. But again this is because the viscous character of the
material  creates internal or friction when deformed and this
friction increases with velocity of deformation.. But this resistance
is *internal* and is not related to one or other inertial effect.
  And if inertia is involved in a measuring rig for measurement of
material stiffness, its effect should be taken into account in the
calculations.

Hope this helps,
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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