Introduction- For a long
time, markers have been fixed upon moving subjects. The trajectories of these
markers were directly used to study the gait. More recently, industrial systems
of motion analysis have been designed (Woltring 1980, Ferrigno 1985). They allow
a model a priori of the gait, in the sagittal plane or the three dimensional
space, fixing one marker approximately on each joint and linking two successive
markers by sticks (Andrews 1982). This gives a global view of human gait.
The new generation of systems returns marker trajectories with a good accuracy.
But this accuracy becomes illusive if one intends to acceed to the articular
motions and point out the anomalies in motion. For this several problems have
to be solved: first, we have to determine the optimal number and location of
markers on the subject then identify two phenomenons masking the internal structure
motion: the slipping of the skin versus muscular tissues, and the moving of
these muscular tissues, versus the underlying bones.