Subject: DISCUSSION-FORUM on the ISB Recommendations for
Standardization in the Reporting of Kinematic Data

In the October-1995 issue of the Journal of Biomechanics appeared on
pages 1257-1260 the "ISB RECOMMENDATIONS FOR STANDARDIZATION IN THE
REPORTING OF KINEMATIC DATA" presented by G. Wu and Peter R. Cavanagh
on behalf of the ISB-Standardization and Terminology Committee. The
recommendations are followed by an EDITORIAL COMMENT written by R.A.
Brand.

Among other proposed definitions are the recommended directions of the
global (resp. local segmental) right-handed Cartesian axes: the
+Y-axis pointing upward and parallel with the field of gravity
(+Yi-segmental axes pointing towards the proximal joint of a
segment), the +X-axis pointing in the forward direction to the right
(the +Xi-axes in the anterior direction), and the +Z-axis sidewards
and perpendicular to the XY-plane (and similarly the +Zi-axes).

This proposed convention ignores and contradicts all international
standards and commonly accepted practices in physics, mechanical
engineering, Lagrangian dynamics, applied mathematics and, most
important, also in biomechanics. The following spatial (and hence also
segmental) Cartesian coordinate system(s) is (are) commonly adopted:
the X-axis points sidewards (in the direction of the ISB-proposed
Z-axis), the Y-axis in the forward direction, and the Z-axis upwards
in the vertical direction.

Here follows a small sample of English (American)-language and
internationally renowned textbooks and works in which this spatial
coordinate system is used: L.K. Branson:ENGINEERING MECHANICS, Simon
and Schuster, New York, 1970, p. 3; R.Resnick and D. Halliday:
PHYSICS (parts I and II), John Wiley, New York, 1966, p. 302; D.A.
Wells: LAGRANGIAN DYNAMICS, Schaum Publ., New York, 1967, p. 181; H.
Goldstein: CLASSICAL MECHANICS, Addison-Wesley, 1969, p. 94; J.L.
Synge, B.A. Griffith: PRINCIPLES OF MECHANICS, Mc Graw-Hill, New
York, 1959; P. Allard, I.A.F. Stokes, J. Blanchi: THREE-DIMENSIONAL
ANALYSIS OF HUMAN MOVEMENT, Human Kinetics, 1995, p. 147; J.
Wittenburg: DYNAMICS OF SYSTEMS OF RIGID BODIES, Teubner, Stuttgart,
1977, p. 21.

I am unable to understand why the ISB can make recommendations which
not only grossly contradict international standards but also ignore
the e-mail discussions of a few years ago, in which a number of
colleagues (including me) proposed this universally accepted
coordinate system which has already been used in the creation of
three-dimensional human body models. In addition, the ISB-recommended
coordinate system necessitates an awkward sequence of rotations in
obtaining the rotation matrix: rotation about the Z-axis first,
followed by Y and X, while the common and logical sequence is (e.g.
for Cardan angles) X, Y, Z. Finally, it should be pointed that most
manufacturers of force plates also use this reference frame for
defining ground reaction force components.

Any biomechanist, who uses the ISB-proposed coordinate system in
publications submitted to international engineering, mechanics,
mathematics or similar journals runs the risk of being heavily criticized
for not adhering to international conventions.

I feel that this issue is certainly worth being discussed in this
forum. In fact, in my opinion, it would have been wiser to discuss the
ISB-recommendations within the scientific community before their
publication in the Journal of Biomechanics.

H. Hatze, Ph. D.
Professor of Biomechanics, University of Vienna, Austria.