In Pose the angle of falling is measured from the ball of the foot through the hips. I never really considered the angle of the trunk from the hips through the head. I was recently contacted by a fellow Pose coach Jason Hoy. He was wondering about the relationship of the angle of the trunk to Pose running technique.
One thing I find is that I have a lack of explanation for the visual differences among Pose runners. One extreme is Michael Johnson, Pose coach Valerie Hunt, and Pose coach Jacky L. The other side is Romonav, Jack, etc.
When viewing the videos the Jacky L from the side she always looks completely vertical. When watching Jack, Dr. R, etc you can clearly see a whole body lean.
I admitted that I did not know, so I contacted Dr. Romanov, and asked him for an explanation. Below is part of Dr. Romanov's Response.
[In Summary] , when we have an accelerated running our trunk could have this leaning condition. In running where speed is consistent, the trunk could be in very upright position. Plus it's complicated with the runner's body structure. If a runner has a pronounced arch in the lower back, then his upper body very likely will be in upright position.
At the end of the day, everything is coming to the position of the GCM ready to fall, with continuous speed (constant angle of falling) or accelerated running (with increasing Angle of falling), these conditions can be visually indicated by the trunk position.