In any type of motion analysis system, contacting
or noncontacting, a source of error in the estimation
of joint angle motion is due to uncertainty in the
construction of an embedded coordinate system. In
a goniometric system, the alignment of the goniometer
determines the orientation of the embedded
axis. In the present system, the body surface markers
define the embedded axes and therefore their
placement is crucial. While the effect of errors in
the definition of embedded axes on the flexionextension
angles is small, ab-ladduction and rotation
angles are affected significantly. This may be
the reason for the large dispersion reported in the
literature for the knee varus/valgus and rotation angles
and therefore these angles must be interpreted
cautiously. While it may be difficult to define the
embedded axis exactly, it is at least necessary to be