systems, problems with tracking closely spaced
markers make measurement of joint angle motion
labor intensive. Therefore, for routine clinical use,
the external marker system must be simple and yet
rigorous enough to define the relative motion of the
rigid body segments in three dimensions. Despite
the vast literature related to lower extremity kinematics,
a detailed description of the external marker
system for computing the motion at the pelvis, hip,
knee, and ankle joints during gait is not available.
The definition of the axes or planes about which the
limb rotations take place as well as the methods to
construct these axes and planes based on body surface
markers are also lacking. In this paper, we
present a simple marker system that can be easily
implemented for routine clinical gait evaluations.
We describe in detail the definition of axes and
planes as well as the techniques for constructing
them. We present the results of a sensitivity analysis
designed to demonstrate the limitations associated
with the joint angle measurement system.
DEFINITION OF PARAMETERS
In gait analysis, human body segments are modeled
as rigid bodies and the relative rotation is as-