Measuring Movement in 3D

Translations are typically measured in the global reference frame (up/down, toward/away from the target, toward/away from the ball). Instructionally, this tends to work well for translations, but it does not work well for rotational movements. We will take a look at why it is important to use local reference frames as we look at segment orientations.

When standing upright, the axes of the segments are roughly aligned with the room axes – so side tilt and forward tilt will create tilts toward the target or the ball.

However, in golf posture, we do not stay aligned. 

As you can see, as we tilt forward, then rotate, and then side tilt, none of our torso axes align with the room axes – the side tilt performed at the end of the video now results in tilting away from the ball.

Another reason for measuring segment rotations in relative coordinate systems is related to how we control the movement of our bodies. From a motor control perspective, we do not control our movements based on the orientation of the room; we move based on the orientations of our bodies. In other words, our bodies know what tilting forward or backward is, but our bodies do not know where the ball is. Our brain takes that global information (ball location) and then puts together a series of commands telling the body which relative motions will result in hitting the golf ball.