Sensitivity Analysis
Accurate definition of the embedded axes is essential
to reliable estimation of three-dimensional
motion at each joint. In the present Eulerian system,
the definition of the flexion-extension axis as
well as the rotation axis is crucial. The flexionextension
axis, about which the first rotation in the
Euler sequence is assumed to take place, is defined
with respect to body surface markers. If the actual
flexion-extension motion does not take place about
this axis, then the computed joint angles, i.e., flexion/
extension, ab-/adduction, and internaVexterna1
rotation, would all be in error. To quantify the effects
of errors in the definition of the flexionextension
axis, a sensitivity analysis was performed
using knee joint angle data from a representative
subject. The orientation of the flexion-extension
axis in the transverse plane at the knee joint was
analytically varied, from + 15 to - 15" at 5" intervals
and the resulting joint angle patterns were recalculated.
Similar analyses were performed at the
hip and ankle joints; however, only the results for
the knee joint will be presented here.