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unknown user
03-12-1990, 01:40 AM
Greetings once again,

In Herman's recent message which summarized the postings by several of us,
he mentioned that he was concerned with the statement I made about finite
screw axis defining the axis about which the actual joint rotation is occuring
at a given instant in time. Well, he is right. As written in the posting,
my statement is incorrect. I was thinking one thing and writing another. For
our purposes here, we use two different types of finite and instantaneous
screws. We call them absolute and relative screws. The absolute is the screw
motion which defines the position of a body relative to a base coordinate
system. Essentially, it is the motion the body would go through if it started
out coincident with the base coordinate system and ended up in its final
position. This is the screw used by ourselves to calculate the screw attitude
angles and I believe this is the same definition used by Herman. The relative
screw is the axis about which the body would move when it goes from a position/
orientation at time t to a position/orientation at time t+1. This screw axis
can be defined in terms of any coordinate system. In our case we usually look
at it in terms of the proximal segment local coordinate system (LCS), although
the distal segment LCS and lab global coordinate system (GCS) versions are also
calculated. This is the screw axis that I was refering to in my posting. This
I believe is the axis about which the joint rotates at a given instant in time.
Of course, the relative instantaneous screw axis should be a more correct
description and in the limit as the sampling rate becomes infinitesimally small
the finite and instantaneous screw axes should coincide. Note: I'm refering to
the direction of the screw axes not the entire screw (i.e., the rotation and
translation along and about the axis and well the screw's position).

Both the absolute and relative screws are useful as can be seen in Herman's
application of the absolute screw and in Leendert Blankevoort's works on joint
kinematics where he has used both absolute and relative screws I believe. The
relative screw description is what has been used to define the pierce point of
the axis of rotation of the knee on to a sagittal plane as following a C shaped
curve located in the midst of the femoral condyles.

Sorry for the confusion. I hope this clarifies what I meant rather than
what I said :-)

One last comment. I fully agree with Leendert Blankevoort's comments about
translation definitions. We've tried a number of different definitions
usually based upon the locus of the instantaneous screws of a given motion and
it is definitely more difficult to define translations than rotations are far
as I'm concerned. With translations not only is the placement of the embedded
coordinate systems critical, but also the orientation. This is an even more
difficult task for trying to come up with a standardized definition.

Dwight Meglan
The Ohio State University
Gait Analysis Lab
meglan%gait1@eng.ohio-state.edu