User guide

Charnwood Dynamics Ltd. Coda cx1 User Guide - Gait Analysis II - 1
CX1 USER GUIDE - COMPLETE.doc 26/04/04 65/162
Derivations of 3D Segments and Joints from Marker Placements
The geometrical properties of each body segment are derived, generally speaking, from
three non-collinear points bearing particular anatomical relationships to the given
segment. For a full description, including inertial properties, additional anthropometric
data are sought in respect of mass proportions, centres of mass and radii of gyration,
along with certain static geometries, such as joint or segment widths or depths. These
extra data are to be entered into the Patient Data file by the clinician.
The anatomical relationships for each of the markers in the standard Segmental Gait
marker set, using the given marker names, are as follows:
Pelvic Frame Markers
Unilateral data acquisition:
‘Sac.Wand’ : Positioned near the end of the sacral wand (distance from sacrum not
important).
‘PSIS’ : Positioned on the side of the pelvic frame, near the PSIS.
‘ASIS’ : Positioned on the side of the pelvic frame, as far forward as possible
(usually further forward than the Pelvis ASIS).
The line joining the ‘PSIS’ and ‘ASIS’ markers defines the anterior-posterior axis of the
Pelvis.
The medio-lateral axis direction is defined by the perpendicular to the PSIS - ASIS line, in
the plane which contains all three Pelvic frame markers.
The inferior-superior Pelvis axis is defined by the perpendicular to the plane containing all
three Pelvic frame markers.
The Sacrum reference point is defined to be offset from the ‘PSIS’ marker along the
lateral axis, at a distance from the marker of half the Pelvis width plus a fixed offset
(20mm).
‘Front’ : optional marker on a forward wand, in-line with the sacral wand.
If present, this marker allows the pelvis orientation to be tracked more
accurately when one of the other Pelvic frame markers is obscured (e.g. by
arm swing).
Bilateral data acquisition:
‘R.Sac.Wand’ &
‘L.Sac.Wand’ : Positioned near the end of the sacral wand, directly opposite each
other.
‘R.PSIS’ & ‘L.PSIS’ : Positioned on the sides of the pelvic frame, near the back.The line
joining these two markers should pass through the PSISs.
The Sacrum reference point is defined to be at the mid-point of the
line joining these two markers.
‘R.ASIS’ & ‘L.ASIS’ : Positioned on the sides of the pelvic frame, as far forward as
possible (usually further forward than the Pelvis ASIS points), and
equally so on both sides.