XY is not the only anomaly to occur in biomechanics based on axis systems. I
give two examples, the first being totally harmless and the second that has
considerable potential for confusion.

1. Elbow angle. Some people interpret full extension between the upper arm
and forearm as zero degrees. The angle between the two segments is, of
course, 180 degrees. I personally don't like the former scenario but it is
only a 180 degree transformation and has been justified because some people
find it easier to work between a range of motion of zero and 150 degrees
(approximating full flexion) rather than between 180 and 30 degrees.

2. Sagittal plane motion. This terminology is often used to refer to a 2D
kinematic analysis. The 2D in 2D kinematic analysis is an XZ-plane or
vertical plane analysis where the XZ-plane in this case is an earth-fixed
axis system (nb. I'm using mechanics, engineering, mathematics, physics
notation, not the ISB recommendations). The sagittal plane is a part of a
body axis system (technically I suppose it is a principal body axis system
whose axes are orientated such that they coincide with the principal axes of
inertia of the body. By definition, the origin of this system of axes and
the centre of gravity must be one and the same). Worse still, the sagittal
plane is defined in a static posture and (correct me if I'm wrong) the
notation system was not designed for dynamic analyses (principal axis
systems apply to rigid bodies with invariant mass distribution only; ie.
should probably be used in reference to the anatomical or fundamental
standing position only, as it is in anatomy). This anomaly is more of a
concern than the first example because it mixes all three of the main types
of axis systems; ie. earth, body and path axis systems. When people talk
about sagittal plane motion they probably either mean an axis system based
on the path of the CM (that could be an instantaneous path or a global,
intended path of the CM over a set distance; eg. a 10m walkway) or they
could mean the XZ-plane of an earth-fixed axis system (actually a
calibration reference frame for 3D analyses or camera orientation reference
system for 2D kinematic analyses).

XY as the vertical plane is probably close to the harmless end of the
continuum. If people want to use it they should realise that they are not
conforming to commonly used notation systems used in nearly all of the major
sciences and, therefore, a considerable portion of the 3D biomechanics

Russell Best
Victoria University