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Ýstavdahl34
04-10-1997, 07:28 PM
Dear fellow subscribes,

A search in the BIOMCH-L archives revealed a
question similar to mine by Dr. Joel M. Bach
, 7 Apr 1994, but
apparently no responses were ever posted.
Therefore I turn to the whole list hoping to
gain some insight.

We are about to conduct an experiment related
to wrist motion. For capturing kinematic data
we will use a MotionStar magnetic tracking
system.

My question realtes to the placement of the
MotionStar receivers ("markers"). For
measuring extension/flexion and radiulnar
deviation we need one receiver distal to the
wrist as well as one proximal to the wrist.
In order to also measure the pronation/
supination movement, the proximal receiver
must be stabilized with respect to the
proximal radioulnar joint so that it will NOT
rotate about the pro-/supination axis.

We have tried to locate the proximal
receiver on a plaster-of-paris "socket",
custom-made for each subject, that fits
firmly over the humerus condyles as well as
the olecranon. The socket was cut so that it
would not obstruct the elbow movement. The
obvious consequence of this technique is that
the socket is displaced with respect to the
bones when the underlying muscles (as well as
the elbow joint) move, which introduces
errors in our wrist angle estimates. By
extending the socket along the ulna and
strapping this extension to the forearm, the
"extension" movement of the socket apparently
can be reduced, but at the cost of increased
deviation and pro/supination errors.

Does anyone have experience with this type of
problem (I believe yes), and if so, where/how
should our MotionStar receivers be attached in
order to minimize the crosstalk from soft
tissue and irrelevant joint movements?

Also, does anyone have experience with
linearization of magnetic tracking device
output in order to reduce errors induced by
metal objects etc.?

A summary of the responses will be posted.
Thank you very much in advance.

Best regards,

Oyvind Stavdahl (Siv.ing., Dr.ing. student)
Department of Engineering Cybernetics
NTNU, The Norwegian University of
Science and Technology Direct line: +47 73 59 43 77
O. Bragstads plass 8 Switchboard: +47 73 59 43 76
N-7034 TRONDHEIM Fax: +47 73 59 43 99
NORWAY Email: Oyvind.Stavdahl@itk.ntnu.no
http://www.itk.ntnu.no/ansatte/Stavdahl_Oyvind