Originally Posted by Si
...I have no idea if by rotary forces you mean at an anatomical joint or as applied to the ski...
After the huge thread I got embroiled in a couple of summers ago about the exact definitions (and I mean *really* exact) of angle of inclination, degree of angulation, angle of bank, and a few other related quantities, I've been diligently trying not to get sucked into another "definitions" thread.
However, I've got to come down fully on Si's side with respect to the general concern he expresses above.
I am a stickler for detail when it comes to precise definitions. As someone who has put together models and analysis of multi-jointed structures, it is clear to me that in discussions of skiing, rarely if ever is the above distinction recognized. Even less frequently is it stated which of the two possibilities someone has in mind during a discussion of ski technique.
In scientific/engineering terms, what it comes down to is torques and forces around different axes, and how these relate to each other.
Just as in the inclination/angulation thread, I feel that the common usage of terms for torques and angular movements are dreadfully qualitative and poorly understood by most recreational skiers and instructors. The difference between a qualitative definition and a quantitative definition is that the former clearly could not be used to define/construct a humanoid robot whose legs moved in the precise way the designer might be trying to specify, whereas with a quantitative definition, one could do so.
HOWEVER, since the old angulation/inclination definitions thread, I've learned a lesson. For 95% of all skiing or ski instruction, exact mechanical definitions doesn't make an iota of difference. For example, if you are trying to show a student the difference between a twisting motion of the ski (in the plane of the snow) and rotation of the femur in the hip (and it's possible effect on edging), you just demo the two possibilities. For the first, you stand up tall with straight legs, and for the second, you drop your butt towards a chair-sit position and rotate your leg in the hip socket. If you try to make such distinctions verbally each and every time you talk about "rotary", you will sound overly verbose and pedantic, and simply turn many people off.
The problem is that for the remaining 5% of the cases, for example, on Epic, as well as at high levels of instruction / coaching, such distinctions between related/similar movements occasionally becomes of paramount importance. Hopefully, most occurrences will be accompanied by text or other context which will clue you in, but in too many cases, shortcut wording will be used to keep the discussion from dragging, and then nitpickers like Si and I are left scratching our heads until we figure out exactly what the speaker intended.
As I see it, at it's most fundamental level, this issue of precise definitions on Epic arises because discussions here fluctuate from a typical, on-the-snow level of precision to practically the level of a scientific journal article in biomechanics, and the precision of usage of these terms varies accordingly, often, even within one thread. Geeks are only happy at one end of the spectrum, normal people only at the other end.
Just my $0.02,
Tom / PM