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Herman J. Woltring
06-05-1991, 10:47 PM
Dear Biomch-L readers/posters,

Following Ton van den Bogert's posting the other day, I'd like to add a few
considerations, in the hope that this debate will not confine itself to one
just between your two `moderators' !

(1) While net joint force and moment may be `abstract mathematical entities',
i.e., the mathematical sum of all `physical' forces and moments (muscle force,
ligament tension, interbone contact forces, etc. plus their moment effects),
I would suggest that net joint *power* is quite physical, i.e., the effective
power generated at / transmitted through a joint. The versatility and redun-
dancy of the NMSK system entail that different muscles and muscle parts may
be called upon to generate the *same* net kinetics and kinematics in order to
counter fatigue and other detrimental factors in individual motor components.
When considering (pathological) movement from a functional point of view, on
the IDH (Impairment - Disability - Handicap) scale, Ian Stokes' and Ton van
den Bogert's approach seems to lean more towards Impairment, while mine might,
perhaps, be biased more towards Disability and perhaps even to Handicap?

Furthermore, Ton wrote in a posting some time ago that high contact forces
in the ankle joints of his horses (if I recall correctly) do not seem to have
much detrimental effect, and I have suggested some years ago that high force
*transients* (kinetic `jerk' -- 3rd derivative related) may be important,
similar to metal fatigue effects. Of course, 3rd derivative estimation from
noisy position/attitude data is even more difficult that conventional estima-
tion of 1st and 2nd derivatives...

Indeed, the skating biomechanics researchers at the Faculty of Human Movement
Sciences at the Free University in Amsterdam typically assess joint and seg-
ment power in their studies, as has been done by David Winter in Waterloo and
his students; thus, I am not so sure about Ton's `sometimes' when he writes
that I am

"(...) deliberately limiting the discussion to *net* joint
kinetics, i.e. the model consists of rigid links with one force
and one moment transmitted by each joint. These variables are
calculated, plus sometimes the joint powers (moment * angular
velocity). The analysis essentially stops there, (...)"

I should like to see comments from others on this list on their views on the
utility of net forces, moments, *and* powers in FMA (Functional Movement
Assessment). Any volunteers ?

(2) While I agree that the net moment reference point does not *have* to be
the (2-D or 3-D) ICR *if* linear power terms are not forgotten, it seems
useful to consider it seriously in anticipation of the kind of applications
for which both Ton and I deem the ICR meaningful. Thus, experience on its
assessment and use can be obtained, and databases can be accumulated that
can be used once we are able to routinely acquire the type of data that are
necessary for more comprehensive analyses. In addition to Ton's arguments
in favour of the (2-D) ICR, I might mention that another argument is the
notion of knee joint stability in stance, where the normal from the knee
joint's Centre of Rotation to the GRF (Ground Reaction Force) vector or its
projection onto the sagittal plane is used as a measure for such stability
considerations (with the assumption that inertial effects are negligible
during stance). If the ICR can be modulated by different forms of muscular
co-contraction (those forces cannot be gleaned from the external kinematics
and GRF data only), we may have a useful gait assessment parameter in the
ICR and its projection onto the (equally instantaneous) GRF vector.

(3) When Ton writes about the `net joint force' in his more comprehensive
analysis not being equal to the one in `net joint kinetics', I think that
he is creating a Babylonian, linguistic confusion. I strongly suggest to
reserve the term `net joint forces, moments, and powers' for the free-body
analysis of my previous posting, with one total force and moment vector per
joint, and to use different terms for other physical things or mathematical
abstractions.

(4) The moment arm as the partial derivative d(muscle length)/d(joint angle)
applies in the planar case where the (straight) muscle is perpendicular to the
IHA (Instantaneous Helical Axis, normal to the plane under consideration).
In the general 3-D case, the issue is complicated because of the possibility
that perpendicularity no longer holds, because of a translation component
along the IHA, and because there is not a single `joint angle'. If a muscle
and an IHA are parallel, for example, the muscle has no possibility to cause
further rotation *about* the IHA, but only of *changing* the direction and
position of the IHA, and these are different phenomena. In general, a muscle
may show a mixture of both properties.

Herman J. Woltring, Eindhoven/NL.