Dear Biomch-l subscribers:

I would like to know if there is a preferred terminology within the
biomechanics community for forces which act at a given joint. In particular
I am referring to what David Winter (in his 1990 book "Biomechanics and Motor
Control of Human Movement") refers to as "joint reaction force" (hereafter
referred to as F1) and "bone-on-bone force" (hereafter referred to as F2).

F1 (Winter's "joint reaction force") is what I prefer to term "resultant
joint force" or "net joint force" which is a vector sum of all forces
crossing the joint, including muscle forces. No knowledge of muscle origins,
insertions, lines of action, or moment arms is necessary to compute this
force. This is the simplest force to compute at a joint using inverse
dynamic analysis. I have also heard this called "intersegmental joint

F2 (Winter's "bone-on-bone" force) is what is left after muscle forces have
been removed from F1. For example, in a simple static equilibrium example of
the tibiofemoral joint, knowing an external force applied to the tibia and
its moment arm (for example from an isokinetic dynamometer), assuming the
quadriceps muscle is the only muscle active and knowing its insertion point
and angle of pull on the tibia one can compute the force in the patellar
tendon of the quadriceps and the F2 using simple static equilibrium equations
(I do this regularly in teaching examples). However, I do not like calling
F2 "bone-on-bone force" because the shear component in the above problem,
for example, is sustained not by the bones, but mainly by ligaments (ACL or

The problem is this: Nordin and Frankel (in their 1989 book "Basic
Biomechanics of the Musculoskeletal System" which I use as a textbook for a
course on skeletal biomechanics) use the term "joint reaction force" to refer
to F2 whereas Winter uses the same term to refer to F1. Because of this I
prefer to avoid using "joint reaction force" altogether. If I say "resultant
joint force" most people will understand what I am referring to (F1).
However I like to avoid "bone-on-bone force" when referring to F2 for reasons
stated above. So what should I call F2?

I would like to conduct an informal survey and will report the results to the
list. Tell me what your preferred terms are for F1 and F2 above. Thanks for
your help.

With warm regards,


| Richard N. Hinrichs, Ph.D. | email: Hinrichs@ESPE1.LA.ASU.EDU |
| Associate Professor | or atrnh@ASUACVAX |
| Dept. of Exercise Science | or atrnh@ACVAX.INRE.ASU.EDU |
| Arizona State University | Phone: (602) 965-1624 |
| Tempe, AZ 85287-0404 USA | FAX: (602) 965-8108 |