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Abstract
Measurement of three-dimensional (3D) knee joint angle outside a laboratory is of
benefit in clinical examination and therapeutic treatment comparison. Although several
motion capture devices exist, there is a need for an ambulatory system that could
be used in routine practice. Up-to-date, inertial measurement units (IMUs) have proven
to be suitable for unconstrained measurement of knee joint differential orientation.
Nevertheless, this differential orientation should be converted into three reliable
and clinically interpretable angles. Thus, the aim of this study was to propose a
new calibration procedure adapted for the joint coordinate system (JCS), which required
only IMUs data. The repeatability of the calibration procedure, as well as the errors
in the measurement of 3D knee angle during gait in comparison to a reference system
were assessed on eight healthy subjects. The new procedure relying on active and passive
movements reported a high repeatability of the mean values (offset<1 degrees) and
angular patterns (SD<0.3 degrees and CMC>0.9). In comparison to the reference system,
this functional procedure showed high precision (SD<2 degrees and CC>0.75) and moderate
accuracy (between 4.0 degrees and 8.1 degrees) for the three knee angle. The combination
of the inertial-based system with the functional calibration procedure proposed here
resulted in a promising tool for the measurement of 3D knee joint angle. Moreover,
this method could be adapted to measure other complex joint, such as ankle or elbow.