Dear Biomch-L readers:

I agree with Paolo de Leva that "net joint force" or
"resultant joint force" or simply "joint force" are all good names
for the sum of all the forces made by one segment on its neighbor.

As Paolo points out, defining "action" and "reaction"
sometimes is a chicken-and-egg problem, so I believe that it probably
is best to stay away from using those terms ("Joint REACTION force"
probably is not a good name for anything.)

Paolo proposed breaking down the net joint force into three
or even four parts. That could be alright, except that it is already
difficult enough to break the net joint force into TWO parts! Almost
always we will keep calculating (as before) the values of the total
force and of its two usual parts [the traditional "muscle-produced
force" and "bone-on-bone force" -- the latter actually including
bone-on-bone, as well as (erroneously, considering its name) ligament
forces and other forces]. The TERMINOLOGY is what we are trying to
change. If we followed Paolo's suggestion (and broke the total force
down into 3 or 4 parts), then we would almost always have to report
the value of a parameter defined as "F2+F3+F4", and that would be
awkward. So instead, I would rather have a FIRST breakdown of the
total force into two parts (the "traditional" breakdown), and later
on possibly breaking those two parts into further subdivisions (along
the lines that Paolo proposed), but ONLY IF actual numerical values
can be calculated for all of the separate elements in those further
subdivisions. In other words, I think it would still be useful to
have a name for the entire "non-muscle" part of the joint force.

That brings us back to Rick's original question, what names
should we use for the total force and for its two traditional parts.
Paolo's term "passive binding force" gave me an idea.

How about "passive joint force" for the non-muscular part of
the net joint force? Even though it can be argued that the
bone-on-bone force (and even the "passive binding forces" that Paolo
proposes) sometimes could be called "active" instead of passive, we
would probably agree that their "activeness" is much less direct than
the activeness of a muscle. So, in a relative sense at least, we
could refer to the bone-on-bone forces and also to the forces made by
ligaments and by other structures (Paolo's F3+F4) as "passive"
forces, and to the muscle forces as "active" forces.

So here is my terminology proposal:

(a) "net joint force" or "resultant joint force" or simply
"joint force": The sum of all the forces made by a segment on its
neighbor. This force can be broken down into two components, b and

(b) "active joint force": The sum of all the muscular
forces made by the segment on its neighbor through the muscles.

(c) "passive joint force": The rest of the force made by
the segment on its neighbor (through bones, ligaments, capsule and
other structures).

Jesus Dapena