The following message is the outcome of email discussions on the topic of
standardisation by Fred Yeadon, Ton van den Bogert and Giovanni Legnani.
Since the standardisation proposal is "on the table" interested parties
should (a) accept it as it stands (b) propose changes. We think that changes
are needed and as an initial step have made some comments on the proposal.

Comments on the document:

Standardization and Terminology in Biomechanics.
Volume 1. Section A.
Recommendations for standardization in the reporting of kinematic data.

It should be borne in mind that the purpose of standardisation is to have a
common convention for REPORTING data so that the results of different
researchers can be interpreted and compared more easily. Any discussion on
this document should bear this in mind and avoid issues of what conventions
are best used for data analysis. It is the reporting of results that is the
present focus of the standardisation document.

In the "Recommendations" document:-

Part 1. X, Y, Z axes for displacements

It should be recognised that there IS general agreement on the directions of
the three Cartesian axes in that they are mutually perpendicular, one axis is
vertical, and the directions of the remaining two horizontal axes are not
usually contentious. There is some disagreement as to the NAMING (XYZ) of the
three axes. There is also the issue of the senses of the axes in that there is
general agreement that "up" is positive whereas "left" may be positive or
negative. The system proposed in the standardisation document seems to be
supported by people who have initially used 2D analysis and have added the a
horizontal Z axis for 3D in order to preserve consistency between 2D and 3D.

Unfortunately the majority of researchers in biomechanics seem to favour a
system in which the Z axis is vertical. This does not augur well for the
acceptance of the standardisation proposal in its present form.

However, since there is general agreement on the directions of the axes and the
focus is on the reporting of data, standardisation could take the form of naming
these axes with words rather than with letters. For example, it might be
possible to use "forward", "lateral", and "vertical" to name these directions.
The point is not that particular names be cast in stone but that it become
common practise to use appropriate words rather than XYZ.

Another issue is that the XYZ system used should fit well with the system used
to describe rotations. This will be addressed after the comments on Part 4 and
Part 5.

Part 2. Definition of segmental reference frames

The system proposed in the standardisation document is consistent with the
system proposed for the absolute reference frame in Part 1. However, as noted
above, most researchers are not in agreement with the adoption of the proposed
Part 1. An alternative will be proposed after considering relative orientation
in Part 5.

Part 4. Absolute Orientation

In the standardisation document it is proposed that the absolute orientation of
a body segment be defined by successive rotations about lateral, longitudinal
and frontal axes. Since the system for defining the orientation of the whole
body has not yet been published the present proposal prejudges the issue.
Recommendations have been made to the Standardization Committee on whole body
orientation as detailed in:

Yeadon, M.R. 1990. The simulation of aerial movement. Part I: The determination
of orientation angles from film data. Journal of Biomechanics 23, 59-66.

In this system successive rotations are made about the lateral, frontal and
longitudinal axes. This order of rotations has the advantage that the Cardan
angles of somersault, tilt and twist agree with the nomenclature used by
gymnasts and coaches. Since the focus is on the reporting of data such
considerations of the established use of terminology are of great relevance.

Part 5. Relative Orientation

The description of the system proposed here appears to be somewhat ambiguous but
our interpretation is that successive rotations about lateral, frontal and
longitudinal axes bring one segment from initial alignment with a more proximal
segment to its final relative orientation. That is we interpret the proposal
as stating that the system described in the cited publication (Grood and Sunday,
1983) is to be adopted. If this is so then we are in agreement with this part
of the proposal and expect that most researchers will also find it acceptable.
On the other hand this definition is in conflict with that proposed in Part 4,
suggesting again that Part 4 be changed.

Part 1 and Part 2.

It would be attractive to think that articles could use Tables in which axes
were labelled with words rather than X, Y and Z but since this is unlikely to
occur there is a case for having a standard system. One rationale would be to
let the definition of orientation dictate the axes nomenclature by requiring
sequential rotations about the X, Y and Z axes. This leads to the adoption of
a system with Z vertical (longitudinal).

One final note of caution on the subject of standardisation:

There is a danger if recommendations were to be adopted formally by an academic
journal. Any standardised system will need to be able to adapt to various needs
and changing circumstances. For different types of arm movements it may be
helpful to use different systems of describing orientation. It will not be
helpful if systems are cast in stone from on high. Researchers need the freedom
to choose what they think is best bearing in mind the conventions prevalent at
the time. For example even if whole body orientation is "standardised" in some
way, it should not be imposed on all activity. What is a sensible system for
gymnastics may not necessarily be sensible for high jumping. It should rest
with the researchers of high jumping to arrive at a useful system bearing in
mind what already exists.

Fred Yeadon
Ton van den Bogert
Giovanni Legnani

M.R. (Fred) Yeadon