Inertial Magnetic Measurement Systems (IMMS) are becoming increasingly popular by
allowing for measurements outside the motion laboratory. The latest models enable
long term, accurate measurement of segment motion in terms of joint angles, if initial
segment orientations can accurately be determined. The standard procedure for definition
of segmental orientation is based on the measurement of positions of bony landmarks
(BLM). However, IMMS do not deliver position information, so an alternative method
to establish IMMS based, anatomically understandable segment orientations is proposed.
For five subjects, IMMS recordings were collected in a standard anatomical position
for definition of static axes, and during a series of standardized motions for the
estimation of kinematic axes of rotation. For all axes, the intra- and inter-individual
dispersion was estimated. Subsequently, local coordinate systems (LCS) were constructed
on the basis of the combination of IMMS axes with the lowest dispersion and compared
with BLM based LCS.
The repeatability of the method appeared to be high; for every segment at least two
axes could be determined with a dispersion of at most 3.8 degrees. Comparison of IMMS
based with BLM based LCS yielded compatible results for the thorax, but less compatible
results for the humerus, forearm and hand, where differences in orientation rose to
17.2 degrees.
Although different from the 'gold standard' BLM based LCS, IMMS based LCS can be constructed
repeatable, enabling the estimation of segment orientations outside the laboratory.
A procedure for the definition of local reference frames using IMMS is proposed.
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