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unknown user
11-12-1990, 07:32 AM
From: Edmund Chao, Rochester, MN

Fabio Catani and Herman Woltring

Date: November 12, 1990

Subject: Issues related to joint moment calculation

Tom Cahalan has shown me the letters sent by Fabio Catani and the response
letter by Herman Woltring and I read them with great interest. As a
noncomputer user, I seem to have missed many of the interesting communications
through the computer BITNET system and, luckily, I have Tom Cahalan to keep me
posted on some of the subjects that I have always had an interest in.

I am deeply sympathetic towards Dr. Catani's problems, and I wish to address
the issue of joint moment calculation in general and acknowledge that the
suggested solution and calculation guidelines provided by Herman are the right
way to go. First of all, we should all realize that the so-called "joint
moments" currently used in the field are calculated mainly from the externally
measured forces and the inertial effect of the musculoskeletal system. Hence,
these are the quantities to be supposedly more reliable without the concerns of
the Inverse Dynamic Problem and the Redundancy in muscle and joint constraint
force determination. In order to calculate such moment, accurate determination
of joint center (2-D) or joint axis (3-D) of rotation is extremely important,
as stated by both Catani and Woltring. Unfortunately, the limited papers I
have read in the literature about joint moment analysis have not gone into
details concerning how these moments were calculated. In fact, when this
subject was presented at some society meetings, I occasionally asked questions
related to this concern. On this issue, I wish to raise the following
fundamental questions.

1. The calculation of joint moment has many sources of error in addition to
the relative motion of the markers due to soft tissue movement. The more
critical issue here is that all the potential measurement errors, including
ground reaction force magnitudes, the location of such forces and their
directions, plus the determination of the center of mass as well as the
inertial properties of the limb segments are all additive in accumulating
the potential error of such calculation. These do not include the problem
of locating joint axis of rotation yet. I realize that one may only con-
cern relative changes instead of the absolute value. But, if the calcula-
tion will be subjected to so many sources of error, one should question
the accuracy and reliability of such quantities (i.e., joint moments in
different planes).

2. In all seemingly exciting clinical applications, one should provide an
error bound of all the moment data based on the experimental method and the
theoretical calculation utilized. Otherwise, only relying on statistical
analysis, the results and concluding remarks can be grossly misleading. The
concerns on data normalization with respect to walking velocity or cadence,
on the temporal occurrence of critical joint moment of concern and on the
consideration of the entire pattern of moment as a function of time rather
than looking at discrete instances must be addressed critically. We are
all waiting to have the most exciting and reliable technique for clinical
application of gait analysis, but we must also be extremely careful not to
fall flat on what we have tried for so long to reach such goal.

3. Finally, the determination of instantaneous axis of rotation in 3-D rigid
body kinematics or the hypothetical 3-D instantaneous center of rotation
should be utilized to calculate joint moments. The joint articulating
surface contact point (even if one could determine it reliably) does not
necessarily represent the center of rotation, since the type of articula-
ting surface motion can vary from sliding to spinning type of motion rather
than rolling without slipping.

Therefore, I wish to propose that the biomechanicians in the field who are
interested in rigid body kinematics and kinetcs be more critical of the
currently reported joint moment papers. There is no doubt in my mind that the
present instrumentation and technology are able to determine approximately the
instantaneous helical axes and refer them back to each connecting skeletal
segment for the purpose of calculating joint resultant moment resulting from
externally applied forces and the inertial effects. The method outlined by
Herman appears to be very elegant and practical. The concept of three-
dimensional instantaneous center of rotation proposed by me about ten years
ago was only meant to represent a spatial trajectory illustrating the path of
instantaneous helical axis movement of two loosely connected rigid bodies.
Strictly speaking, such concept has rather limited utilization, and for the
purpose of calculating instantaneous joint moment, one must utilize the
instantaneous helical axis concept described by Herman. Please keep up with

For those who may be interested in knowing the potential application of joint
moment to clinical orthopedics, the following excellent references are highly
recommended:

1. Podromos, et al., JBJS, 67-A:1188, 1985
2. Wang, et al., JBJS, 72-A:905, 1990
3. Berchuck, et al., JBJS, 72-A:71, 1990

issue of repeatability of gait kinematic, kinetic, and EMG data in normal
the repeatability issue has been addressed, but unfortunately, the issues of
accuracy and reliability were not questioned. Such issues should be carefully
considered, especially when one wishes to calculate the joint moments in the
frontal plane and for more distal joints (such as the ankle and knee) because
of the potential error magnification and the inherent technical limitation of
the motion analysis system due to visual field resolution.

Sorry I don't have Cappozzo's paper discussing joint moment determination as
presented in the Bristol ESB meeting, 1987. I would very much like to read
that to see if Cappozzo's opinion would in any way coincide with my concerns.
We are dealing with a rather important subject, because our orthopedic clinical
colleagues tend to take the surface value of a biomechanical article, and we
must ensure that the data presented do indeed reflect the actual physiological
phenomena quantitated as accurately as current instrumentation allows. If the
method used to determine the joint moment is not theoretically correct or is
technically inaccurate, one must be extremely carefull before making strong
prognostic or therapeutic statements. I hope that this discussion and my
personal opinion will stimulate a lot of debate as well as clarification. If
joint moment can be well established for clinical application, everyone in the
field should utilize it to its maximum extent. I will be eternally happy to
see theoretical/objective gait analysis make a giant step forward in its
practical utilization in orthopedics.

Sincerely,

Edmund Chao
Rochester, MN

P.S. Dr. Chao may be contacted through Tom Cahalan