SEG_PROGRESSION_ANGLE is the projected angle of the segment (e.g. projection of the long axis of the segment onto the ground) with the path of the center of mass of the segment over a window of 51 frames centered at the current frame.

Compute_Model_Based_Data /RESULT_NAME=LFT_PROGRESSION_ANGLE /FUNCTION=SEG_PROGRESSION_ANGLE /SEGMENT=LFT /REFERENCE_SEGMENT= /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 ;

Step 1) Store the Center of Mass position for the segment (relative to ground) for the 25 frames before the current frame, the current frame, and 25 frames after the current frame. (51 frames total) Step 2) At each of the 51 frames transform the Center of Mass location from ground coordinate system into the local coordinate system of the segment at the frame. Step 3) Once you have the 51 points of progression in local space discard the Z coordinate. This leavies you with 51 data points in the segment's XY plane. Step 4) Compute the slope of the X-Y data over the 51 frames via linear regression. Step 5) Take the atan of the slope.
This gives you the SEG_PROGRESSION_ANGLE which is basically angle of progression of the Center of Mass over the period of + and - 25 frames in the XY plane of the segment.