View Full Version : Shoulder net joint force in throwing!

Young-hoo Kwon, Ph.d.
10-06-1998, 01:47 PM
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

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

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 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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