User guide

Charnwood Dynamics Ltd. Coda cx1 User Guide - Gait Analysis II - 1
CX1 USER GUIDE - COMPLETE.doc 26/04/04 69/162
P
asis2
= M
psis
+ du
x
+ (W + 20)u
y
Bilateral:
In this case there is considerable redundancy in the marker data since there may be as
many as 6 markers representing the pelvis. This allows for a more robust platform for
pelvis calculations since, for example, we are no longer dependent on interpolated
positions of out of view markers whose positions can be re-constructed like Virtual
Markers.
Once again the Gram-Schmidt procedure is used to generate a set of orthogonal axes but
this time u
y
is obtained from the medio-lateral line between ASIS markers, whilst u
x
is
obtained using Gram-Schmidt on the line passing through the mid-points between left and
right PSIS markers (i.e. the Sacrum) and left and right ASIS markers. The local z axis is
obtained from the cross product.
Thus we obtain the Sacrum reference point, etc…
P
sac
= ½(M
psisL
+ M
psisR
)
P
front
= ½(M
asisL
+ M
asisR
)
u
y
= U
y
(M
asisR
- M
asisL
)
u
x
= GS
x
(P
front
- P
sac
)
The left and right ASIS ref. points are offset anteriorly by pelvic depth d from the sacral
reference point and medio-laterally by half of the pelvic width:
P
AsisL/R
= P
sac
+ du
x
±
L/R
½ W u
y
Hip Joint Centres
These are obtained by adding local offsets to the mid-point between left and right ASIS
reference points. The offsets are calculated as proportions of pelvic width in accordance
with values allocated in Patient Data, loosely based on the standard hip-joint model
described by CAMARC. (Note:
1
the default values for the offsets presumed by the
software have been revised (August 2002) to agree more closely with the pelvis model
described by Bell
2
et al.)
Thus:
H
L/R
= ½(P
AsisL
+ P
AsisR
) - λWu
x
- νWu
z
±
L/R
µWu
y
where λ, µ, ν are the pelvic width ratios for offsets in the local x, y and z directions
respectively.
Their values are to be entered in Patient Data where the default values are provided for
guidance only.
Note that, viewed sagitally, the resultant offset from the ASIS line is posterior and inferior,
as indicated by the ‘-’ signs above.
1
The offset mechanism gives the modeler to control the positions of the hip joint centres relative to
the pelvic bony landmarks (assuming left/right symmetry). Suggested values of λ, µ and ν are for
guidance only: the modeler is invited to consult the literature on the standard linear regression
models. Prior to the revision the default values were: 0.14, 0.3 and 0.19 respectively for λ, µ and ν.
The revised defaults are 0.19, 0.36 and 0.30 respectively.
2
A.L.Bell, R.A.Brand, D.R.Pederson; Human Movement Science, 1989.