Upper body fixed values
A shoulder offset value is calculated from the Subject measurement value entered, plus half the marker diameter. Elbow, wrist and hand offset values are also calculated from the sum of the respective thickness with the marker diameter divided by two.
A progression frame is independently calculated in just the same way as for the lower body. C7 is tested first to determine if the subject moved a distance greater than the threshold. If not, the other thorax markers T10 CLAV and STRN are used to determine the general direction the thorax was facing in from a mean of 10% of the frames in the middle of the trial.
Note that in principle it could be possible to arrive at different reference frames for the upper and lower body, though the circumstances would be extreme.
The head origin is defined as the midpoint between the LFHD and RFHD markers (also denoted 'Front').
The midpoint between the LBHD and RBHD markers ('Back') is also calculated, along with the 'Left' and 'Right' sides of the head from the LFHD and LBHD midpoint, and the RFHD and RBHD midpoint respectively.
The predominant head axis, the X axis, is defined as the forward facing direction (Front - Back). The secondary Y axis is the lateral axis from Right to Left (which is orthogonalized as usual).
For the static processing, the YXZ Euler angles representing the rotation from the head segment to the lab axes are calculated. The Y rotation is taken as the head Offset angle, and the mean of this taken across the trial.
For the dynamic trial processing, the head Offset angle is applied around the Y axis of the defined head segment.
The orientation of the thorax is defined before the origin. The Z axis, pointing upwards, is the predominant axis. This is defined as the direction from the midpoint of the STRN and T10 to the midpoint of CLAV and C7. A secondary direction pointing forwards is the midpoint of C7 and T10 to the midpoint of CLAV and STRN. The resulting X axis points forwards, and the Y axis points leftwards.
The thorax origin is then calculated from the CLAV marker, with an offset of half a marker diameter backwards along the X axis.
Shoulder joint center
The clavicles are considered to lie between the thorax origin, and the shoulder joint centers. The shoulder joint centers are defined as the origins for each clavicle. Note that the posterior part of the shoulder complex is considered too flexible to be modeled with this marker set.
Initially a direction is defined, which is perpendicular to the line from the thorax origin to the SHO marker, and the thorax X axis. This is used to define a virtual shoulder 'wand' marker.
The chord function is then used to define the shoulder joint center (SJC) from the Shoulder offset, thorax Origin, SHO marker and shoulder 'wand'.
The clavicle segment is defined from the direction from the joint center to the thorax origin as the Z axis, and the shoulder wand direction as the secondary axis. The X axis for each clavicle points generally forwards, and the Y axis for the left points upwards, and the right clavicle Y axis points downwards.
Elbow joint center
A construction vector direction is defined, being perpendicular to the plane defined by the shoulder joint center, the elbow marker (LELB) and the midpoint of the two wrist markers (LWRA, LWRB).
The elbow joint center is defined using the chord function, in the plane defined by the shoulder joint center, the elbow marker and the previously defined construction vector.
Wrist joint center
After the elbow joint center is calculated, the wrist joint center (WJC) is calculated. In this case the chord function is not used. The wrist joint center is simply offset from the midpoint of the wrist bar markers along a line perpendicular to the line along the wrist bar, and the line joining the wrist bar midpoint to the elbow joint center.
After the wrist joint center is defined, the Humerus is calculated with its origin at the EJC, a principal Z axis from EJC to SJC, and a secondary line approximating to the X axis between the EJC and the WJC.
The radius origin is set at the wrist joint center. The principal axis is the Z axis, from the WJC to the EJC. The secondary line approximating to the Y axis is taken as the Y axis of the Humerus segment.
The hand is defined by first defining its origin. The chord function is used again for this, with the WJC, FIN marker and Hand Offset. The midpoint of the wrist bar markers is used to define the plane of calculation.
The principal Z axis is then taken as the line from the hand origin to the WJC, and a secondary line approximating the Y axis is defined by direction of the line joining the wrist bar markers.