Comments on: Kinematic Analysis: Wrist to Elbow Relationship

By: Stephen Strasburg, Mechanics, and his “Timing Flaw” | Driveline Baseball

Tue, 08 Mar 2011 22:48:57 +0000

[...] We’ve been doing these types of measurements for more than a year here at Driveline Baseball in our biomechanics lab, as pointed out in January 2010 in the article Kinematic Analysis: Wrist to Elbow Relationship. [...]

By: Kyle

Kyle — Fri, 22 Jan 2010 22:44:37 +0000

“My thought is that the difference between the two columns should not be an absolute value.”

Agreed.

By: Trip Somers

Trip Somers — Fri, 22 Jan 2010 22:40:53 +0000

I assume you mean that you’re looking for negative numbers in the delta-x column. The interesting thing is that if a negative delta-x were paired with a negative delta-x2, the negative delta-x holds less significance because they are moving in the same direction (even if it is the wrong direction).

My thought is that the difference between the two columns should not be an absolute value. This way the delta-deltas would not only show a magnitude, but it would indicate which joint is accelerating faster. A negative value would show that the elbow is accelerating faster, while a positive value would show that the wrist is accelerating faster.

An absolute value seems to blur the wrist/elbow relationship.

I believe this is in line with your goal, but I could be wrong.

By: Kyle

Kyle — Wed, 20 Jan 2010 18:15:30 +0000

The column with no negative values is the absolute value of the difference between the deltas. I think that this isn’t necessarily the correct thing to measure, because what I mostly look at this chart for is exactly what you pointed out – negative numbers indicating that the wrist is moving more slowly (this is probably inevitable to some degree from a kinesiological sense) or even backwards (the real problem).

I just wanted to publish the entire data from where I estimated the start of the driveline. But you’re right; how the arm lays back into MER is the phase I’m most concerned with. The other stuff is just confusing the issue.

By: Trip Somers

Trip Somers — Wed, 20 Jan 2010 06:55:34 +0000

Your final column does not show negative values.

For example, data point #26 has -6, 6, and 12, while data point #47 has 11, 0, and 11. The relationship is clearly different at these two points, but the final column says it’s nearly the same.

My impression is that positive numbers would be good, indicating that the wrist is moving in the right direction faster than the elbow, while negative numbers would indicate the wrist moving more slowly than the elbow or even backwards.

Can you explain why there’s no differentiation made?