October 21, 2007
A simple problem, with your arm out t-pose palm flat (facing down) do this:
- Rotate you arm down to the side, then forward (You’ll notice the bicep faces up)
- Now go back to your arm out. Rotate it forward (the bicep will face to the right or left now)
We have an oddity here, and I think it’s one of the founding principles of biomechanical rotation, it ‘resolves’ itself. When two rotations meet the union causes another plane of freedom to be introduced – i.e the top of the bicep will twist 45 degrees with your arm going forward and -45 going backwards. Looking at this from a math perspective its spherical rotation. (a quaternion)This spherical rotation similar to a quaternion is what stops the arm from twisting itself off its joint. The muscles are are treating its ball and socket joint as a spherical rotation or in other words a quaternion. Now this may not seem interesting but this is before there’s been any rotation of the elbow.I.e the twist of the upper arms is brought on by the constant rotation of the shoulder and its resolution or twist, is brought on by the rotation of the elbow. This is why it can be hard to get a frame of reference for the twist. I’ll see if i can update this with some pics.