Anne Mannion's recent posting (12/02/94), in which she was looking for users
of specific motion analysis systems in Northern Ireland, has sparked an
interesting discussion within our lab:

The appropriateness of magnetic-based systems for the measurement of
relatively small translations, in combination with moderate to large
rotations typical of those occurring in some human joints, is perhaps not
established at this time. These systems seem to be growing in popularity,
nonetheless, perhaps because of their relatively low price tags and ease of
use. From our work in computer aided surgery and clinical biomechanics we
have experience measuring motion using direct mechanical (LVDT based systems
and goniometers), magnetic-based, and opto-electronic based motion analysis
systems. In our experience the magnetic systems suffer from relatively low
accuracy, trouble measuring dynamic events and an extreme sensitivity to
ferro-magnetic materials in the proximity of the measurement devices.

An example from the situations we are considering is: assume the lumbar
functional spinal unit to have a cranial/caudal stiffness of 2000 N/mm and
typical daily spinal load of 2 times body weight or approximately 1500 N.
The axial motions of interest are then likely to be less than 1.0 mm.
However the magnetic tracking system we have is inappropriate for this
application since it has a translational accuracy of 2.54 mm.

We thought this might be an appropriate topic for discussion on BIOMCH-L.
Have any of you had similar experiences with these systems? Does anyone
have ideas about what applications these systems should be restricted to (if
any) or about how the limitations of these systems can be overcome?

-Lutz Nolte-

| Lutz-P. Nolte | Tel: +41 31 632 8722 |
| M.E.M. Inst. for Biomechanics | Fax: +41 31 381 0259 |
| University of Bern | Internet|
| Murtenstr. 35, P.O.Box 30 | |
| CH-3010 Bern, Switzerland | |