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Ton Van Den Bogert
11-19-1989, 02:44 AM
Dear BIOMCH-L readers,

In last week's posting, Herman Woltring raised
the question of how to define body segment axes and joint rotations.
This is indeed a fundamental problem in basic as well as applied
biomechanics, which can only be solved by some sort of standardization.
My own experience on this subject comes from 2-dimensional multibody
modelling, and (to a lesser extent) from clinically oriented 3D
kinematic analysis on horses.

1. Multibody models are commonly used for (inverse) dynamic analysis of
movement, using measured kinematics as input. Usually, these models
assume hinge or ball joints between the body segments. I think that
this is the reason why in these models the 'joint centers' are often
used to define the long axis (being one of the coordinate axes) of the
segment. This definition also slightly simplifies the equations of
motion. In 3D models a second axis must be defined somehow, usually the
lateromedial axis.
Kinematic data, obtained from landmarks placed at arbitrary points on
the segments, can be used for dynamic analysis if the positions of these
points with respect to the segmental axes are known. For my 2D work, I
obtained this information from radiographs showing the 'joint centers',
as well as the kinematical landmarks (marked by steel rings).
I think this method has a few serious drawbacks:

- It relies heavily on the assumption that joints have a fixed center of
rotation, and that these points can somehow be identified.
- For 3D applications, the definitions are not sufficiently strict, and
transformation of marker data to rigid body kinematic variables
depends on marker coordinates with respect to the segmental reference
frame. It is difficult to obtain this information.

Possibly it would be wiser to define standard segment axes with help of
three well-defined points on the outside of each bone. If these axes do
not coincide with the traditional anatomical axes, this will have to be
accepted.

2. For clinical applications, it is practically inevitable to use
segment axes based on external markers. For this reason, I tend to
prefer methods of kinematic analysis that give results that are (within
certain limits) independent of marker placement. This is the best way
to ensure that data obtained from different recording sessions or
different individuals can be compared safely. In fact, this seems to be
the way most software supplied with kinematic analysis systems works.
A typical analysis method is to put two markers (proximal and distal)
on each body segment, and make sure that during the recording the
walking direction is along one of the coordinate axes (e.g. the Y-axis)
of the laboratory reference frame. Joint angles are obtained from
projections of the 'stick diagrams' on the sagittal (YZ) and frontal
(XZ) planes. Incorrect marker placement will only produce a constant
error in the angles, and it is easy to extract parameters from the
signals that are insensitive to this.
Of course, the example given above does not provide data for all six
degrees of freedom (DOF) of the body segments. In my opinion, this loss
of information is not too serious for clinical applications because the
set of kinematic variables describing human movement is by no means
independent. A more practical reason for this 2x2D approach is, that a
complete 3D analysis of, for example, the femur requires a simultaneous
view of markers on the medial and lateral condyles. The system we are
currently using (CODA-3) does not allow this, you would need a video-
based system with many cameras.

Summarizing:

- For basic research (requiring full 3D kinematic data, 6 DOF per
body segment) standard segmental axes should be adopted, based on
the same palpable bone landmarks where the markers are attached.

- In clinical biomechanics, we should only measure those variables that
are reliable and reproducible. If a 3D analysis produces less reliable
results or requires too much effort, stick to a 2D (or 2x2D) approach.

These are my own personal opinions, and I would welcome further
discussion on BIOMCH-L about this subject.

Ton van den Bogert
Dept. of Veterinary Anatomy
University of Utrecht, The Netherlands.