User's Manual

Document MV0319P.N
© Xsens Technologies B.V.
MVN User Manual
110
Figure 66: Global and local coordinate frames in MVN Studio
19.6 Anatomical model
19.6.1 Definition of segment axes
To describe motion capture data in clinically meaningful data, e.g. joint rotations, it is necessary to define
a coordinate reference frame attached to the body segment. The ISB proposal for standardization for
joint coordinate systems is currently based on positions (optical markers) of bony landmarks. For most
segments on the body, the origin of the reference frame and direction of axes are described in Wu and
Cavanagh (1995)
[
i
]
Wu et al. (2002)
[
ii
]
and Wu et al. (2005)
[
iii
]
.The MVN Fusion Engine uses the
calibration poses to determine the direction of the axes of each segment. Figure 67a shows the origins
of the segment axes in the neutral anatomical pose. The origin of the segment is always in the proximal
joint center which is the functional rotation point. The origin is chosen such that by connecting all joint
centers a character (skeleton) is formed, see Figure 67b. Their positions for the calibration poses in the
global reference frame can be found in the <segments>section and <frame type=”tpose/npose”>when
exporting as MVNX file.
Segment
A
Segment
B
Sensor
B
Y
A
B
X
A
B
Z
A
B
X
B
B
Y
C
B
Z
B
S
X
A
S
Y
A
Body frame
Sensor frame
G
Z
G
X
G
Y
Global frame