Dr Mel Siff Blog

Many Internet users are already aware of my concern about the inappropriate
or misleading use of biomechanical terms such as “force-couple”, which is
recognised in mechanics as something that is very different from a “couple”,
as we shall see later.

Recently I encountered the following Internet commentary on this same topic,
so it is apparent that the misuse of biomechanics terminology may be
escalating, simply because some folk are relying in good faith on textbooks
or information sources that may not be accurate or complete. This is why it
is so important that teaching texts should be as accurate as possible.

<The definition of a force couple is two parallel forces of equal magnitude,
but opposite direction, applied to a structure at equal distances from the
center of the mass. Muscles behave very differently in isolation than they
do in multi joint activities. We always talk about force couples at the
shoulder joint. But they also occur elsewhere in the body. Think about what
glenohumeral motions occur in the transverse plane during the throwing
motion. Internal and external rotation, and shoulder horizontal adduction
are two of them. The question is, can we use this particular force couple to
help neutralize the traction forces at the shoulder during follow through?>

***This definition refers to a “couple”, not a “force couple”. The word
“force” is redundant, since the use of a “couple” in biomechanics is defined
on the basis of forces being involved. Including the word “force”simply
introduces ambiguity and inaccuracy. A “force-couple” is a specific
combination of a force and a couple that is used in mechanics and statics as
the equivalent of a force acting on a body. Thus, in analysing the
kinesiology of a physical system, a force may be replaced by a “force-couple”
or conversely, a “force-couple” in a system may be replaced by a force.

If one examines the action of the thoracoscapular, glenohumeral, pelvic and
other physical subsystems, it will be seen that understanding the difference b
etween a “couple” and a “force-couple” can facilitate the analysis of such
systems. Unfortunately, the use of the term “force-couple” seems to have
been seriously overused and misapplied by simplistic popularisation of
Schafer’s definition of a couple by Paul Chek and others, namely:

“FORCE COUPLES A mechanical couple represents a special case of a pair of
parallel forces of equal magnitude that act in opposite directions some
distance apart and tend to produce rotation…..”
(Schafer RC “Clinical Biomechanics – Musculoskeletal Actions and Reactions”
1987 )

Simply omit the use of the word “force” and all likelihood of confusion with
an entirely different mechanical concept will be avoided.

————————–

Further comments read as follows:

<The definition of a force couple is two parallel forces of equal magnitude,
but opposite direction, applied to a structure at equal distances from the
center of the mass. Muscles behave very differently in isolation than they
do in multi joint activities. We always talk about force couples at the
shoulder joint, but they also occur elsewhere in the body. Think about what
glenohumeral motions occur in the transverse plane during the throwing
motion. Internal and external rotation, and shoulder horizontal adduction
are two of them. The question is, can we use this particular force couple to
help neutralize the traction forces at the shoulder during follow through?

A force couple helps keep the pelvis from drifting laterally during all
phases of pitching in baseball…..>

***This commentary and my earlier remarks about “force couples” leaed us to
some interesting questions:

1. Are we justified in taking about “force couples” or “couples” at the
shoulder and other joints?

2. In each case of joint action mentioned, is it entirely correct that there
are “two parallel forces of equal magnitude, but opposite direction, applied
to a structure at equal distances from the center of the mass.” ?

3. Should we be referring to moments, couples or other more intricately
linked systems involving non-collinear forces? Can we reduce the many
muscles, multi-articular systems to collinear forces and, thence, to couples?

4. Are we justified in analysing these linked systems in terms of active
muscular action without considering the role played by collinear and
non-collinear passive connective tissues at different stages of movement?

5. Are we justified in analysing the above systems as true “couples”,
because this presumes that no translation whatsoever occurs?

6. Since several forces, moments or couples can be involved in all joint
action, should we not analyse movement in terms of “force-couples” (in their
accurate sense as special combinations of forces and couples) rather than
“couples” (of only two equal and opposite collinear forces)?

Dr Mel C Siff

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