A joint angle can be represented as a 3×3 rotation matrix that describes the transformation between two coordinate systems.

A Model based item describing the knee angle as a rotation matrix is:

Compute_Model_Based_Data /RESULT_NAME=RKNEE_ANGLE /FUNCTION=JOINT_ROTATION /SEGMENT=RSK /REFERENCE_SEGMENT=RTH /RESOLUTION_COORDINATE_SYSTEM= ! /USE_CARDAN_SEQUENCE=FALSE ! /NORMALIZATION=FALSE ! /NORMALIZATION_METHOD= ! /NORMALIZATION_METRIC= ! /NEGATEX=FALSE ! /NEGATEY=FALSE ! /NEGATEZ=FALSE ! /AXIS1=X ! /AXIS2=Y ! /AXIS3=Z ; The command will return 9 columns of data for each point in time. Each row represents a position in the 3×3 matrix.

NOTE: If looking at the signal in the Data Viewer, the columns will be numbered 0 through 8. When accessing the columns using the Evaluate_Expression command, the columns can be accessed as 1 through 9. For this reason, the image above shows numbers 1 through 9. It is always important to pay attention when a signal or command is 0 or 1 based.