Some Clinical Applications - Kinematic Analysis of Human Movement - page 89

spine, in a horizontal plane containing the sternal marker.
The alignment of the sternal and lower thoracic markers is
done using a thoracic compass equipped with a level,
guaranteeing the definition of the horizontal anteroposterior
axis attached to the thorax for each subject.
The movements analyzed are simple movements of
flexion/extension of the head, right and left lateral bending,
right and left rotation. The movements are carried out in the
same order for all subjects. The subjects perform 10 cycles
for each movement, at maximum amplitude and at the speed
that they prefer.
A “headset” orthonormal coordinate system is defined
with axis Z c perpendicular to the plane of markers pointing
upwards, axis Y c in the plane of markers, pointing forwards
and axis X c pointing to the right. For the “Thorax” coordinate
system, axis Y t joining the marker on the T6 and the sternal
marker, by construction, is horizontal and pointing forwards.
The axis X t is constructed as perpendicular to the plane
formed by the T3, T6 markers and Y t axis, pointing to the
right. Axis Z t completes the direct trihedron and points
upwards. The relative movement of the head with regard to
the thorax is described by splitting the rotation matrix
between both segments into three successive elementary
rotations (see Chapter 3), as recommended by the ISB
[WU 02, WU 05], however the description of the movement of
the cervical spine, i.e. of the head relative to the thorax, is
not standardized. The order chosen for the sequence is X , Y ,
Z . The first rotation corresponds to a flexion-extension
movement about axis X t, linked to the Thorax coordinate
system. The second rotation occurs about the floating axis Y ,
defined at each instant as the common perpendicular to the
two other motion axes. This rotation is interpreted as lateral
bending. The third rotation occurs about axis Z c, linked to
the headpiece and corresponds to a right and left rotation.
The neutral position (zero of angles) is defined by the posture