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To the Biomch-L readership:
Aki Salo brings up an excellent question. I think some people may
simply assume that the external points do represent the positions of the
internal joints, but (as Aki says) that would be a pretty bad assumption.
A second, and more valid, use for surface markers could be to use
them to calculate 3D coordinates, and then make a 2D (TWO-dimensional)
analysis. For instance, you could make a 3D filming of a person running,
and then consider only the Y and Z coordinate values (i.e., forward and
vertical), ignoring the X values (mediolateral). This turns the 3D study
into a 2D study, but it has an advantage over the traditional one-camera 2D
studies: you have no perspective errors; you have truly accurate 2D data.
I saw this done in a paper coming out of Calgary (I think), and I felt it
was a very good idea for their purposes.
Of course, the best use of surface markers in a true 3D study would
require software to enable the prediction of the locations of the internal
landmarks from the surface ones, but that presents problems as well, such as
the need for a large number of markers, and possible diffulties due to skin
movement.
---
Jesus Dapena
Department of Kinesiology
Indiana University
Bloomington, IN 47405, USA
1-812-855-8407 (office phone)
dapena@valeri.hper.indiana.edu (email)
Aki Salo brings up an excellent question. I think some people may
simply assume that the external points do represent the positions of the
internal joints, but (as Aki says) that would be a pretty bad assumption.
A second, and more valid, use for surface markers could be to use
them to calculate 3D coordinates, and then make a 2D (TWO-dimensional)
analysis. For instance, you could make a 3D filming of a person running,
and then consider only the Y and Z coordinate values (i.e., forward and
vertical), ignoring the X values (mediolateral). This turns the 3D study
into a 2D study, but it has an advantage over the traditional one-camera 2D
studies: you have no perspective errors; you have truly accurate 2D data.
I saw this done in a paper coming out of Calgary (I think), and I felt it
was a very good idea for their purposes.
Of course, the best use of surface markers in a true 3D study would
require software to enable the prediction of the locations of the internal
landmarks from the surface ones, but that presents problems as well, such as
the need for a large number of markers, and possible diffulties due to skin
movement.
---
Jesus Dapena
Department of Kinesiology
Indiana University
Bloomington, IN 47405, USA
1-812-855-8407 (office phone)
dapena@valeri.hper.indiana.edu (email)