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View Full Version : Joint force terminology. A reply to Hinrichs' message.

Paolo De Leva, P.e. Institute,rome - Sport Biom.
05-25-1993, 01:02 AM
On May the 12th, Rick Hinrichs posted a message to
the list, about "joint force terminology". After 13 days, I
eventually found the time to write my thoughts about the
issue, which puzzled me quite a bit when I read Rick's
message.
Rick wrote that he prefers not to use the expression
"joint reaction force", either for indicating the net joint
force (F1) or the component (F2) which is left after
removing muscle forces from F1.
However, it's also my opinion that we shouldn't care
if two authors use the same name for two different forces,
as Rick wrote in your message. In fact, the name could be
appropriate and exact for calling one force, and totally
wrong when used for the other force. In this case, however,
I think that the expression "joint reaction force" is not
completely appropriate for either force.
Newton's third law states that no force exists
alone. Whenever a force is applied to a body, there's always
a companion anti-force applied to another body. I think
Newton himself called the first force "action", and the
anti-force "reaction". However, it's not always easy to
decide which is which. The ground force elicited by a runner
or a jumper has been always called reaction. In fact, a man
voluntarily generates the force applied to the ground, and
men are always The Actors, the ones who perform the
"actions". And since the force applied to the ground is the
action, the force elicited from the ground must be the
reaction.
But what happens for example in a tug-of-war
context? Which is the team performing the action, and which
is generating the reaction? No way you can decide it non-
arbitrarily. And it actually doesn't matter, does it? I
believe it's a valid point herein just to show that in some
cases there's no way to reasonably tell an action from a
reaction, and vice-versa. You just know that there are two
equal and opposite forces.
When applied to the joint, the action-reaction
dilemma leaves unresolved ambiguities as well. A muscle,
ordered or controlled by the CNS, turns chemical energy into
mechanical energy through a highly complex process,
therefore muscle force (at the selected insertion point) can
reasonably assume a primary role, and be called an "action"
force.
However, bone-on-bone forces, or forces generated by
ligament stretching cannot be consistently defined either
actions or reactions. Sometimes bone-on bone and/or ligament
forces can be elicited by a muscle pull, and since we call
the muscle pull an "action", then they should take the role
of "reactions". However, sometimes bone-on-bone forces
and/or ligament forces are just a consequence of the motion
of the adjacent segment(s), or a consequence of external
forces (actions) exerted on the adjacent segment(s) (there
are various cases, but there's no point in doing herein an
accurate list of possibilities). In the latter cases, they
should probably be considered to be "action" forces,
although this fact is not frequently acknowledged. For
example, the bone-on-bone forces exerted by the femurs on
the tibial plate during standing are probably best defined
as action forces, because a large component of them depends
on the weight of the segments above the knees, hence the
tibial plate has almost the same role as the ground for the
feet, which is a passive role, reasonably associable with
the concept of being acted upon by "action" forces.
Also, according to Feltner and Dapena, there are
motion-dependent forces during a baseball pitch, which
produce the elbow extension without the need of a strong
triceps contraction. A large part of those forces are
evidently generated by ligaments which just happen to be
doing their job of preventing the forearm from being thrown
away together with the ball! And that can be described as
the application of an "action" force to the proximal end of
the forearm (unless you think that centrifugal forces have a
primary role relative to centripetal forces: if you do, then
you can hardly convince me of it, as well as I cannot
convince you of the contrary, so it stays still true that
there's no way to define those forces as "reactions").
Because of the undetermination of the term
"reaction", I conclude the expression "joint reaction force"
is not appropriate to indicate either bone-on-bone,
ligament, or resultant joint forces. Probably, sometimes
(but only sometimes) bone-on-bone and ligament forces can be
considered to be reactions. Never, however, as far as I can
see, the net joint force is a sum of reaction forces only. A
component of it is always an action (muscle pull, most
commonly) (eventual examples of the contrary are welcome).

As to the proper terminology for F1, I agree with
Rick that either the word "net" or "resultant" should be
included in its name, to avoid undetermination and
consequent ambiguity of terminology. The expressions "net
joint force" or "resultant joint force" seem to be both
appropriate. However, I also think that the simpler
abbreviation "joint force" is a legitimate name for F1. In
fact, that's the linear version of the "joint torque". We
are used to such abbreviations in physics: "velocity"
instead of "instantaneous linear velocity", and so on.

As to F2, again Rick makes a good point when he
writes that "bone-on-bone forces" are sometimes only a
component of it, since forces exerted by ligaments and other
fibrous and non-fibrous tissues surrounding the joint have
sometimes a major role in the joint dynamics or statics. I
cannot think of any name for F2, as defined above. I only
know that "bone-on-bone force" is not the right name for F2.
However, "bone-on-bone force" is a very appropriate and
descriptive name for just bone-on-bone forces (the name even
contains its own definition, what a lust!).
Therefore, I suggest to eliminate altogether the
concept of F2, defined as what is left after removing muscle
forces from F1. Instead, F2 might be defined as the
resultant of the distribution of forces exerted by one joint
surface on the other, and named "bone-on-bone force", or
"bone-on-bone resultant force", if you just want to be
unnecessarily precise. Then an F3 should be added, defined
as the resultant of the forces produced by stretching of
fibrous tissues, such as ligaments and joint capsule (I am
not sure of the English name of the latter structure,
though), plus, if you want, a negligible F4, defined as the
force produced by stretching of other soft tissues
surrounding the joint (except for the muscles or tendons),
such as skin, vessels and nerves.
A good name for F3, as defined above, could be
"ligament force" ("connective", "linking", or "binding
force" refer to the function, but are just not good because
the same function is partly carried out by muscles and other
tissues). I don't have a simple name for F4.
I can, anyway, suggest a name for F3+F4. When I was
writing the above phrase, I thought the expression "passive
binding [or connecting, or linking, I am not sure of the
English exact term for the concept] force" can somehow
exclude the muscle function, which can be considered to be
"active".

When I started writing, I thought it would be a
short message, but things revealed themselves to be more
complex than I initially thought. I hope the length of the
message didn't bother you listservers too much. Thanks for
your attention,

With kind regards,

Paolo de Leva
Istituto Superiore di Educazione Fisica
Biomechanics Lab
P. Lauro De Bosis, 6
00194 ROME
ITALY

Tel: 39-6-5743523
FAX: 39-6-3613065

e-mail address: DELEVA@RISCcics.ing.uniRoma1.IT