unknown user

03-12-1990, 12:40 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

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