View Full Version : Re: Torque or moment about a joint

Van Den Bogert, Ton
11-07-2007, 09:17 AM
Robert-Soutas Little wrote (in a very helpful posting):

> The term torque is used interchangeably with moment in many
> applications, but is generally associated with twisting.

It would be great if we could agree on this, it would make our
terminology more consistent. Paolo de Leva already mentioned a letter
published by John Paul in the 1980s, which was titled "Torques produce
torsion", basically making the same argument. It is appealing, but
let's think carefully.

In tissue biomechanics, I don't think there is a problem. Nobody would
talk about "bending torque". The terminology of beam theory seems to
apply nicely to bones.

The confusion exists in movement biomechanics, where we often quantify
muscle actions by a "joint moment" or "joint torque". Which term is
best? These are moments about an axis that is perpendicular to the long
axis of the bone, so it is definitely not a torsional moment. It is
easy to see how the term "torque" came to be used here. With inverse
dynamic analysis, we model the system as rigid links with rotational
motors at the joints representing the muscles. And motors produce
torque, right?

The term "joint moment" has some potential for being misinterpreted
also. If it were a real motor producing this moment, it would induce a
bending moment of equal magnitude. But since these are muscles that
span a considerable length of the bone, the actual bending moment is
often much less than what the "joint moment" would suggest.

Considering all this, I tend to favor "joint moment" over "joint
torque". It is more general and it does not offend those who use
"torque" only for torsional loads. I have been guilty of using both
terms, and I like the idea of always using just one, if there is a
consensus. Unlike literature, scientific writing does not really
benefit from synonyms.


Ton van den Bogert
Department of Biomedical Engineering
Cleveland Clinic Foundation

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