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View Full Version : Re: Shoulder net joint force in throwing!



Glenn Fleisig
10-08-1998, 05:11 AM
We appreciate Dr. Kwon's thoughts for helping us "perfect our efforts."
The main suggestion we got from his email was that the term
"compressive" force was misleading and that "longitudinal" or "proximal"
force would be a better phrase for describing the longitudinal
components of shoulder force and elbow force. Dr. Kwon does have a good
suggestion, as the terms "proximal/distal" are consistent with the use
of the directional terms "superior/inferior" and "medial/lateral" for
the other force components. However, "compressive" force can also be a
meaningful term for the force that acts to pull a distal segment towards
its proximal segment.

Regardless of terminology, the real issue is that force applied by a
proximal segment to a distal segment is a hard concept to understand.
It is primarily a tool for biomechanists and has limitations in
practical application. "Net joint force", "joint resultant force",
"joint reaction force", "muscle force", "bone-on-bone force", etc. are
defined and used in the biomechanics literature, and as Dr. Kwon warns
confusion between inter-segment force and articulation force is "a
common mistake... and... the first mistake several book authors
recommended not to make." Ideally, inter-segment forces should be
calculated as a step towards determining forces within the segment
structures (e.g., muscle force, ligament force, bone-on-bone force).
However, since bone-on-bone, ligament, and muscle forces cannot always
be calculated with current biomechanical techniques, inter-segment
forces are often the most useful information available. As
biomechanists, we should continue to diligently explain their meaning
when presented or published. For example in our 1995 and 1996 articles
cited by Dr. Kwon, we avoided using terms that the reader might
misunderstand, such as "net joint force" or "joint reaction force."
Rather we provided methods of how the forces were calculated. For
instance, in the 1996 article we explained "The sum of all forces
applied to each segment of the upper extremity was set equal to the mass
of that segment multiplied by the linear acceleration of its center of
mass... Kinetic values were reported as the force and torque applied by
the upper arm to the forearm about the elbow and as the force and torque
applied by the trunk to the upper arm at the shoulder. Kinetic values
were separated into orthogonal components..." The figures accompanying
this text showed orthogonal arrows applied to the forearm or upper arm
segment. In these figures, the entire arm is shaded, and no visual or
verbal reference is made or implied to bone-on-bone interactions.
Without teaching a biomechanics class to the reader, we don't know how
we can be any clearer, regardless of whether the component is labeled
proximal force, longitudinal force, compressive force, Fx force, or
anything else.

Dr. Kwon's issue does raise the important point that clear communication
with other scientists and clinicians is very important. Here at ASMI,
we constantly work with physicians, physical therapists, and other
clinicians to insure that they understand our information and we
understand theirs. We work together to find clinical relevance for our
biomechanical research. For example in our 1995 article, we gave
clinical interpretation to our calculated shoulder compressive force as
follows: "Andrews and Angelo found that most rotator cuff tears in
throwers were... a consequence of tensile failure, as the muscles tried
to resist distraction, horizontal adduction, and internal rotation at
the shoulder during arm deceleration. Compression force (Fig. 2) and
horizontal adduction torque (Fig. 3) were seen during arm deceleration
in this study... thus [Andrews and Angelo's belief] is consistent with
available biomechanical data."

We thank Dr. Kwon for his suggestions, and thank him for bringing the
issue of joint force into discussion again.

- Glenn


A SSSS M M IIIIIIII
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************************************************** *******
Glenn S. Fleisig, Ph.D.
work phone: 205-918-2138
work fax: 205-918-0800
address: American Sports Medicine Institute
1313 13th Street South
Birmingham, Alabama 35205
web site: http://www.asmi.org
************************************************** ********

>>> Dr. Kwon's Original Posting:
>
>Date: Tue, 6 Oct 1998 22:47:56 -0500
>Reply-To: "Young-Hoo Kwon, Ph.D."
>From: "Young-Hoo Kwon, Ph.D."
>Subject: Shoulder net joint force in throwing!
>To: BIOMCH-L@NIC.SURFNET.NL
>
>Dear colleagues:
>
>Recently, I was asked to make a presentation to some local sports
medicine
>practitioners about biomechanics of throwing. During my literature
search, I
>found a series of papers published by the American Sport Medicine
Institute
>group.
>
>Here are the papers I would particularly like to discuss:
>
>Fleisig et al. (1993). Kinetics of baseball pitching with implications
about
>injury mechanisms. The American Journal of Sports Medicine, 23,
233-239.
>
>Fleisig et al. (1996). Kinematic and kinetic comparison between
baseball
>pitching and football passing. Journal of Applied Biomechanics 12,
207-224.
>
>In these two papers, it seems the authors computed the net joint force
at
>the shoulder and transformed it to the shoulder reference frame.
>Interestingly, the authors used the term compressive force for the
>longitudinal component of the should net joint force (Figure 5 of
Fleisig et
>al., 1996; Figure 2 of Fleisig et al., 1993). The same thing goes to
the
>elbow net joint force (Figure 5 and Figure 4, respectively).
>
>To me, the term 'compression' somehow implies the actual force acting
>between the upperarm and trunk at the shoulder joint (the joint force).
But
>the shoulder net joint force is the sum of both the joint force and the
>muscle forces acted upon the upperarm from the trunk through the joint
and
>muscles. It is the force which causes acceleration of the arm, but not
the
>actual force acting at the shoulder between the trunk (glenoid fossa)
and
>the humeral head. 'Compression' means a squeezing force, and use of
this
>term for the shoulder net joint force seems inappropriate. Rather
>'longitudinal' or 'proximal/distal' would be more appropriate as they
used
>the terms 'superior' and 'anterior' for the other components.
>
>Suppose that the longitudinal component of the shoulder net joint force
is
>50 N. (The trunk is pulling the upperarm through the shoulder joint and
the
>muscles with the net force being 50 N.) This means FmL + FjL = 50 N,
where
>FmL = the longitudinal component of the muscle force, FjL = the
longitudinal
>component of the joint force (Dr. David Winter called this force as
>bone-on-bone force). The conditions such as FmL = 55 N & FjL = -5 N,
and FmL
>= 45 N & FjL = 5 N will all suffice the above condition. If FjL = -5 N,
the
>actual force acting at the shoulder joint between the trunk and the
upperarm
>is 'compression'. But if FjL = 5 N, this force becomes a tension. The
net
>joint force simply does not really reflect the actual force acting at
the
>shoulder.
>
>They also used the term 'sheer' force for the components perpendicular
to
>the longitudinal component. The same argument can be made here. Only
with
>the net joint force information at the elbow, there is no way to assess
the
>actual sheer force acting at the elbow joint.
>
>I believe this is simply a case of poor choice of terminology. Even
though
>this is a common mistake made by people with not much background in
>biomechanics, and also it is the first mistake several book authors
>recommended not to make, I don't think the authors actually
misunderstood
>the concept of the net joint force. But when people with not much
background
>in biomechanics read these papers, there is a danger of
mis-interpretation.
>They will accept the terms 'compression' and 'sheer' force literally so
that
>understand them as the actual forces acting at the joint between the
two
>articulating bones. This can be pretty serious a problem.
>
>In fact, I really appreciate all the efforts the ASMI group has put in
their
>research. I learned a lot from their papers about biomechanics of
throwing.
>I hope they could perfect their efforts by avoiding some potential
>mis-understanding of their findings.
>
>I'd like to have some feedback from the Biomch-L readership to see if I
am
>the only one who feels problematic here. I will also appreciate any
>comments, especially from the authors of the papers.
>
>Good night!
>
>Young-Hoo
>-------------------------------------------------------------------
>- Young-Hoo Kwon, Ph.D.
>- Biomechanics Lab, PL 202
>- Ball State University
>-
>- Phone: +1 (765) 285-5126
>- Fax: +1 (765) 285-9066
>- E-mail: ykwon@bsu-cs.bsu.edu
>- Homepage: http://www.cs.bsu.edu/~ykwon/
>-------------------------------------------------------------------

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