Friday, August 15, 2008

Skelton Again

Once again, much has been made about James Skelton's 165-pound frame. I wanted to take a look at this from a physics perspective. Let's take a quick look at the physics of a crouching catcher. Below is the ugliest free body diagram you will ever see, but it is of a crouching catcher from his knees up. It is a very simple model, but one that should show the relationship between the catcher's weight, height, and knee reactions.
Acting downward is the weight (W) of everything from his knees up, including equipment. This acts at the center of mass of everything, I've got this acting roughly at the anterior side of the top of the torso. It should be further to the right, but Microsoft Word made it difficult to put things in the right place. We'll say this value is 85% of the player's weight plus 15 pounds of padding. For Skelton, this is 155.3 pounds. For a player like Dusty Ryan (6'4", 220), this will be 202.o pounds.
The reaction force at the knee (R) is the compressive support provided by bone contact. This is typically the force that will be acting on the meniscus of the knee, which essentially internal padding within the knee joint.
The Quadriceps (Q) are acting in tension to create a moment balance for the body. If not for the quadriceps, the knee reaction and weight would cause the body to move counterclockwise. Note that if the center of mass is to the right of the knee, the hamstrings would provide the rotational support. The force of the quadriceps acts through the patellar tendon.
The distance from the center of mass to the knee (a) is going to be a function of the catcher's height and his crouching mechanics. The taller a catcher or the less his torso bends, the higher the value a will be. For now, we will assume this to be 1/10 of the player's height. For Skelton, 7.1 inches, while Ryan's value will be 7.6 inches.
The distance (b) is the distance from the center of the knee to the patellar tendon. The mechanics of the knee allow this distance to increase when needed. While this value varies significantly, we'll say it is 2 inches.

By doing a force/moment balance, you can find that R will be equal to W*(a+b)/b, and Q is equal to W*a/b.

For James Skelton, the force at his meniscus is 714.38 pounds. For Dusty Ryan, the force at his meniscus is 969.6 pounds. As far as the quadriceps, James Skelton's quads will need to impart 559.6 pounds of force; Dusty Ryan's quads will need 767.6 pounds.

While there is going to significant error in these calculations, the simple fact is that heavier and taller catchers will put more stress on their knee while crouching. This has already been shown in Dusty Ryan's case, as he missed much of last season with a torn meniscus.

The scouts are wrong about James Skelton. His lack of bulk does not portend durability problems. The people who see him as skinny are confusing the confusing bulk with strength. It is entirely possible that Skelton's leg strength could prevent him from being durable, but let's keep strength and bulk separate, because they are.

1 comment:

J. Baughman said...

Quick tip on Word - if you hold down the Alt key while dragging an object, you can deviate from the grid that Word otherwise forces you to snap to. Might be helpful in the future...

Great analysis.