To the Biomch-L readership:

I agree wholeheartedly with the Yeadon/Bogert/Legnani
counterproposal for standardization.

Comment #1: Cardan angles in the order X-Y-Z (i.e., successive
rotations about the mediolateral, anteroposterior and longitudinal axes)
should be a good and logical way to express segment orientations. I believe
that this should be the "standard", which should be used ***almost***
always. At the same time, we have to keep in mind that such an order (like
any other order) will produce "gimbal lock" at certain orientations. (This
is where some of the angles "go crazy" because there are two very different
combinations of numerical values of the three angles that produce very
similar physical orientations of the segment.) Gimbal lock should be
avoided. A different Cardan angle order will achieve this. That is why I
consider it very important also that the new proposal allows for deviations
from the standard when necessary. There is no angle order that will prevent
gimbal lock everywhere. Any angle order has a "ugly orientation" that
produces gimbal lock. That is why we should not force any given order on
every situation.

Comment #2: I would like to recommend that the X axis generally
should point toward the right, and the Y axis forward, instead of the
alternative (X axis forward and Y axis toward the left). This is not really
a very critical issue, but here is why I think that X toward the right and Y
forward is somewhat better:

When I am analyzing a person walking or running forward, I generally
think of myself as an observer that is traveling with the subject. I feel
that this helps me to gain a more intuitive understanding of what the
subject is experiencing. When I am considering the values of forces or
velocities in the horizontal plane, I think of myself as the subject looking
downward at this plane. Then, if I am using the X=right/Y=forward
convention relative to the direction of motion, when I look at the ground I
will see the positive X direction pointing toward the right and the positive
Y direction pointing forward, which is how I generally draw these axes when
I make any graph on a piece of paper: I am in familiar territory. But if I
use the alternative convention X=forward/Y=left, then when I look at the
ground I see the X direction pointing forward and the Y pointing toward the
left, which is more unfamiliar. I think most people would probably feel
more comfortable with X=right/Y=forward.

Jesus Dapena