I agree with Jesus Dapena for all comments in his recent posting,
exept for the following two points.

1) Briefly, I wouldn't like to use the labels XZ in 2D reference
systems, as Jesus suggested in COMMENT A. But I wouldn't like to use Y as the
vertical axis in 3D either, according to the ISB standardization proposal.
Solution: just use XY in 2D, and Z for the vertical axis in 3D.
In fact, most people is probably used to the latter 'unwritten' conventions.

It is evident that 2D and 3D terminology cannot be unified without
creating problems either in 2D or in 3D. I don't see why anybody should
be confused by the (absolutely trifling) difference: just LEARN the
difference, and there will be no problem in reciprocal understanding.
3D terminology for axes should be standardized, but there's no real need to
link it to the 2D terminology (that is already, although not officially,
standardized, and not only in the field of Biomechanics).

2) I agree that the notation Xg, Yg and Yg for the global reference
system axes, is not a good choice, as Jesus wrote in COMMENT B.
But what does the O in Xo, Yo, Zo (the notation suggested by Jesus) stand for?
The global reference system coordinates could be more simply called
X, Y and Z, since all other local or 'special' or 'partial' reference systems
will have their special definition (Xt,Yt,Zt for the one attached to the
thigh, for example). Therefore, there's no need to use a special label for
the global reference system too.

Paolo de Leva
Biomechanics Lab
Bloomington, IN 47405