Both the Coda and Helen Hayes (Davis) definitions of the pelvis segment are tilted forward approximately 20 degrees from the horizontal. In describing the orientation of the pelvis or for describing the hip joint angle, however, it is often convenient to define a pelvis angle that has a coronal plane parallel to the floor, e.g., a vertical segment with zero tilt.

An example pelvis segment. The segment coordinate system's coronal plane is not aligned with the floor.

This example demonstrates how to create a virtual (kinematic only) pelvis segment that uses the same tracking as your default Coda or Helen Hayes pelvis but has a segment coordinate system whose coronal plane is parallel with the floor. This results in normalizing the hip joint angle.

We need to define landmarks along the Iliac Crest which are positioned vertically above the hip joint landmarks.

Step 1. Created a right Iliac Crest landmark.

NOTE: The Z edit box contains the subject metric 0.5*ASIS_DISTANCE as there are no strict dimensions, but it is convenient to base it partly on the subject's size. This will end up being the length of a virtual pelvis segment but will not be used for any inverse dynamics calculations, so the distance does not actually matter because only the orientation of the segment coordinate system will be used for segment and joint angles.

Step 2. Create a left iliac crest landmark.

We will now use the landmarks created above to define our virtual pelvis segment.

Step 3. Create a kinematic-only segment named RPV_2.

Step 4. Define the new pelvis segment as having:

1. its proximal end at the midpoint of the iliac crest landmarks defined above;
2. its distal end at the midpoint of the hip joint centre landmarks; and
3. using the same pelvis tracking markers as the original pelvis (LASI, RASI, SACR for a Coda pelvis).

Your model now contains 2 pelvis segments named Pelvis and RPV_2. Pelvis segment RPV_2 has a z-axis that is now directed vertically upward, which means that its angle relative to the lab will have no anterior tilt.

The pelvis segment angle and hip joint angle can now be defined relative to segment RPV_2.