[Jeff]

12PB

Biomechanic works for Chris Welch of Zenolink.

If you want to understand what Zenolink measures, you can view the top two webinars videos at the following Zenolink website.

http://welch-e.com/welch-e/webinars....ntent=webinars

They get their clients to video a golfer using two video cameras (operating at 30-60 frames/second) at nearly right angles to each other. They then use the video data to project the golfer's movements in space from a 3-D perspective.

Here is a link showing the camera setup arrangement.

http://www.lousolarte.com/3dbiomechanics.html

Here is the type of graph they produce regarding angular speeds - derived from that top webinar.



Now, you may find this type of graph convincing, but I do not because of their method of measuring angular velocity.

This is their method of measuring angular velocity - according to an article by TPI.

"The angular velocities for the kinetic link are calculated as follows: A pelvis origin is defined as a point mid way between the hip joints; an upper body origin is defined as a point mid way between the left and right shoulder joints; a fixed spine axis is defined as the line from the pelvis and upper body origins. A plane is produced perpendicular to the spine axis at the level of the pelvis origin; call it the “perpendicular to spine” plane (the PTS plane). A vertical plane parallel to the target line is defined to cut through the PTS plane; the line this produces on the PTS plane is used as the zero reference line. A line in the pelvis from hip joint to hip joint is projected into the PTS plane. A line in the upper body from shoulder joint to shoulder joint is projected into the PTS plane. Finally a line is created from the mid shoulder point to the mid hands on the club grip (called the “composite arm” line) and it is also projected into the PTS plane. These three lines allow a rotation angle to be calculated between each one and the zero reference line. Segmental rotation speeds are calculated as the time rate of change of these angles. "

Here is a diagram showing how they project the composite arm onto the PTS plane in order to create angular velocity measurements.



The circular blue line represents the PTS plane. They then apparently project the composite arm's movement onto the PTS plane in order to calculate their angular velocity measurements. However, I think that this methodology is very problematic because the arms mainly swing downwards (more than outwards) after release of PA#4. That means that the plane the left arm is swinging along is at a large out-of-plane angle relative to the PTS plane and I think that it will cause "projection errors" of great magnitude.

Don't you think that it makes their arm speed angular velocity values incorrect? I think that one needs to measure the angular velocity of the arms along a plane that is perpendicular to the axis of rotation of the left arm, and that plane is not parallel to the PTS plane. I think that even their shoulder rotation measurements must be incorrect because the right shoulder moves downplane and doesn't move along a plane that is parallel to the PTS plane.

Jeff.