The Physics of Catchers’ Knees

The knee is a complex marvel. (via Alan Turkus)

The knee is a complex marvel. (via Alan Turkus)

To be a catcher is sort of like being a cowboy, without the romance of the Old West. It’s hard, dirty and often dangerous work. And old catchers, like old cowboys, tend to walk a little more gingerly than the rest of us.”

— Dan Rather

Deep knee bends for a second or two ten times every evening is probably good for you. However, crouching behind the plate for five to ten seconds about one hundred and fifty times a night can’t be healthy.

If we are going to examine this from the perspective of physics, we need to realize that when you are at rest, your knees exert an upward force on the rest of your body to counter the weight of everything above them.

When Buster Posey stands up trying to understand why nobody covered third base, his knees support about 80 percent of his weight. So each knee supports about 90 lbs. Remarkably, this also is true when Posey is crouched behind the plate. When he is down waiting for the pitch, something has to keep the upper portion of his body from collapsing into the dirt. What else could it be but his knees?

Initially then, it might seem that catching shouldn’t be any more troublesome for the knees than standing. Since that is clearly not the case, we need to investigate more deeply.

Catchers-knees-2Let’s be sure we understand this from the point of view of physics. The first step is to remove all features of the catcher–including his All-Star status–and focus only on a caricature of his body as shown in the figure.

His upper body is isolated by the gray oval. The forces acting on the upper body are 80 percent of his weight pulling down and the knees pushing up. Since the catcher is at rest, at least for the moment, these forces must cancel out. That is, they are the same size, about 180 lbs.

Now, let’s look at the situation when the catcher is-–as Mike Krukow likes to say–“in the squuuaaaat.” The sketch again uses a gray oval to isolate the upper portion of the body. Again the two forces acting are 80 percent of his weight downward and the knees pushing upward. So again, these two forces must be equal when the catcher is at rest.

Catchers-knees-3However, there is a distinct difference between the two situations. When Posey is standing, his weight is directly above his knees. When he is catching, most of his weight is behind his knees. If fact, his weight is concentrated directly above his feet so he doesn’t tip.

Another way to think about it in the standing case is, the force exerted by the knees is directly in line with the weight of the upper body. In the crouch, the forces are equal but not aligned.

Engineers call two equal but non-aligned forces a “couple.” I’ve often wondered if this is due to the poor quality of the relationships engineers have with their spouses. Anyway, the key point is, couples create rotations.

A Hardball Times Update
Goodbye for now.

Try it yourself with that pencil in front of you. Exert equal but oppositely directed forces on opposite ends. Rotation! Wait a second. The pencil rotates but Buster remains still as he goes through the signs.

Exactly! The total force exerted by the knees must equal 80 percent of the weight, but there must be a combination of forces in each knee to insure that upper body doesn’t rotate. We need to look more carefully at the knee joint.

Catchers-knees-4The knee joint is an amazingly complex collection of ligaments, tendons, cartilage, and several other words I don’t really understand. One of the best descriptions I have found was on YouTube. That’s the beauty of physics. I can say something about what’s going on without understanding very much…I don’t think that came out quite right.

Anyway, back to the details of the knee joint. I trust if I mess anything up too badly our readers with expertise here will help clear things up.

The sketch shows the knee slightly bent. While there are probably many forces exerted by all the various parts of the knee acting on the upper portion of the body, the primary two are shown.

The upward force is due to the bigger bone in the lower leg (tibia) pushing upward on the big bone in the upper leg (femur). The downward force is the tibia pulling down on a ligament (patellar ligament), which wraps over the kneecap (patella) and pulls on the femur.

Catchers-knees-5Now that all the jargon is over, the three forces on the upper part of the body are shown in the sketch. These forces must balance so that Buster doesn’t move up or down, and they must keep him from tipping over, as well.

You can think of these forces as forming a see-saw. The upward force is the one at the center about which the see-saw pivots. The downward force is the heavy kid who needs to be near the pivot to balance the lighter kid that is further away. This lighter kid in this analogy is the 80 percent of the weight.

Look what’s happened now! The upward force must be much bigger than 80 percent of the weight because it must also balance out the downward force.

How much bigger? Let’s look at the see-saw.

Catchers-knees-6In physics we call it the “Law of Levers,” but it might as well be the “Law of See-Saws.” The rule is that the product of the distance and the force on one side must equal the product of distance and force on the other. So,

d Fdown = D (80 percent of the weight).

Looking at my own scrawny legs, I would guess that d is about an inch and D is around six inches. The math gives a downward force of around 1080 lbs. (540 lbs. each) for Mr. Posey. Since the upward force is the sum of the downward force and 80 percent of the weight, the force that Buster’s tibia is pushing against his femur has gone up from a measly 180 lbs. when standing to about 1260 lbs. (630 lbs. for each knee) in the squat.

Fortunately, there is some padding between the tibia and the femur called the “meniscus.” You might have heard that word before in reference to knee injuries. The meniscus must be able to withstand the 1260 lbs. of compression force. In addition, it must deal with the friction caused by the rotation of the femur across the tibia each time Buster goes into or out of his squat.

The beating inflicted upon the meniscus and other pieces of soft tissue in the knee is substantial. Hence the invention of “Knee Savers”–you know, those triangular pads attached to the back of the lower legs that prudent catchers wear.

Catchers-knees-7

Since we now understand how the forces work in a see-saw, we can add the upward force on the upper body from the Knee Saver as shown above. There are many ways to think about how this additional force reduces the upward force the tibia exerts on the femur, reducing the pressure on the meniscus.

You can think about the force from the Knee Saver canceling part of the weight of the upper body, thereby reducing the size of the downward force and, as a result, the upward force.

Another way to think about the Knee Saver force is that it supplies additional counter-clockwise rotation on the see-saw, reducing the need for the counter-clockwise rotation from the downward force, thus reducing the downward force and thereby reducing the upward force, as well.

Finally, you could just look at the Knee Saver force as providing some of the upward force so the upward force at the knee can now be smaller. No matter how you look at it, the Knee Saver is appropriately named, as it reduces pressure on the meniscus.

Returning to the cowboy theme, Johnny Bench summarized the lot of a catcher by saying, “A catcher and his body are like the outlaw and his horse. He’s got to ride that nag till it drops.”

References & Resources

  • Thanks to Chris McSharry for his thoughtful comments on the anatomy of the knee.
  • In addition, Melissa Mache got me started on this article with a helpful conversation.


David Kagan is a physics professor at CSU Chico, and the self-proclaimed "Einstein of the National Pastime." Visit his website, Major League Physics, and follow him on Twitter @DrBaseballPhD.
23 Comments
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Liam
6 years ago

7 X more force on the knee while in the squat?! Can only imagine the kind of force on weightlifters (although they their center of mass is likely closer to directly over their knees).

Not that I’m doubting a physics professor, but from my own measly understanding of the subject there does seem to be something left out in these calculations. I would assume that there would also be a force acting on the catcher’s ankle– the athlete is pushing down into the ground with his calf too, no? This would reduce the stated force on the knees.

Either way, catchers are something else.

Thanks for the article.

-LAF

RedsoxManiac
6 years ago
Reply to  Liam

Think about a wooden futon frame legs. You can say the dimensions are about 20 square inches and about 4 inches high. If we were to think of the coupling forces at the joints, I could easily envision it breaking if the legs were much more skinnier, as if I folded the futton into a “crouching” position, the weight would be disrupted with more force on the top than the bottom can handle (vs. when weight is evenly distributed when it is flat). Essentially, skinnier legs would break the bottom legs easily, thereby the legs having a huge effect in holding weight of the furniture.

I agree w/ Liam. There is some force from the shin and feet holding its position that disperses how much force is being put on the knees, and think this equation is missing something. I do believe it has to be the factor of being in that position X moving out of that position forcefully (getting up, making a throw) X longevity in the position (lack of rest, more likely for injury under constant stress).

The moving forcefully (plyometric) has the biggest to do with deterioration if the leg is fatigued or loses strength (which happens over the course of the season). You see this with 2nd baseman, who play a little closer than SS, but have to pivot their bodies in all sorts of positions as a small change in movement or direction puts them at a different angle to throw to the 1st basemen.

SS usually glide straight right to left toward the 1st basemen, and when they get the ball their movement is right on a much straighter angle to the 1st basemen, then a 2nd basemen that has to turn their hips on many throws. Not to mention 2nd basemen covering double-plays and attempted steals. That’s another position where players lose their talents quickly due to deterioration of the knees, and I suspect explosive movements throughout the game to be the cause.

A cure would possibly having a 6-7th+ inning catcher instead of catching days off. The body heals easier if it spreads out effort more equally vs intensively with spotted rest, especially over a very long span. Interval training isn’t even meant to be done without meaningful rest, so unless these catchers are only playing every other day, best way to keep catchers knee intact is to keep them crouched for no more than 5-6 innings at a time.

Hopper
6 years ago
Reply to  Liam

The professor is correct. What happens at each joint is it’s own equation. If you had three 50 pound sticks stacked up such that you have 2 joints you have to evaluate each joint separately. The upper joint supports 50 pounds and the lower one 100 (the knee vs the ankle). If you add 50 pounds to the top then each joint has to support an extra 50 pounds. The “knee” now supports 100 and the “ankle” supports 150. For the catcher, the ankle supports all the weight that the knee supports plus the weight of the lower leg (which is a few pounds more). The knee is more interesting as it goes through a larger range of motion. When the knee flexes it takes on multiplied forces due to the shifting of the fulcrum.

Hopper
6 years ago
Reply to  Hopper

Should be “is its own equation”.

BobDD
6 years ago

Why is it that in some cultures men relax by squatting? Does that mean that there are some bodies that can relax that way and not receive the same wear and tear?

And what about the subset of catchers who are down on one knee?

Kinesiologist
6 years ago
Reply to  BobDD

You certainly see it throughout Asia, from the young to the old – on the side of roads or in the fields. If you can maintain flexibility throughout your life, with emphasis on core strength and balance, you’ll minimize damage to your knee in the long run.

Mechanical Engineer
6 years ago
Reply to  BobDD

If we assume your thigh doesn’t touch your calf, then all of your weight goes through both of you knees every time.

If you are flexible enough, and have thin enough thighs and calves, then your thighs can come into contact with more of your calves. This shares the force between your knees and the contact between your thighs and calves. In this case, your ankles and feet feel the same forces regardless of your position.

ALSO, and this is probably the biggest influence, the closer your butt and torso are to the vertical axis of your knees, the less torque on your knees. Imagine the “D” in the professor’s example being smaller. Another way to picture it: imagine your knees at 90 degrees (where D is as large as it can be) and your knees folded, let’s say at a 45 degree angle (having your butt lower than 90 degrees).

Jetsy Extrano
6 years ago

Dr. Nathan has worked through the static forces and those are remarkable, but the dynamic forces to pop up out of a crouch are even greater.

BobDD, I don’t know, do they squat so that their heels are contacting their butt? Because that starts to take body weight there.

BobDD
6 years ago
Reply to  Jetsy Extrano

Examples:

http://www.desireemoodie.com/wp-content/uploads/2014/07/cowboy-squat-newmexico-439442-h.jpg

http://originalstrength.net/wp-content/uploads/2015/11/men-squatting.png

http://4.bp.blogspot.com/-AAL4AeF6Wyg/U7_SN8YEX8I/AAAAAAAALe8/LJvOpMAj7Lo/s1600/chinamen.jpg

http://www.leofuchsarchives.com/paul-newman/at-home-intimates/2356596

All of these poses appear to be relaxing to the individual, so it makes me wonder. However, I used to catch in a youth league and I found it much more taxing than any other position – still, it seems to be far easier for some than for others, which is why I’m raising the question.

Mechanical Engineer
6 years ago
Reply to  BobDD

I couldn’t view the first site, but the last three show a lot of contact between the butt/thigh and calves. This *probably* feels especially relaxing because they are just using different muscles. They are stretching their quads, and not using those muscles to stand. All of their weight is being supported by the joints, not the muscles, in those pictures. This is similar to a rock climber keeping their arms extended, as opposed to climbing with flexed biceps — resting the muscle by having the joint bear weight.

CowboyUp
6 years ago
Reply to  BobDD

In the first picture I assume they are squating as it is easier to hold a plate of food while resting your forearm on your knee when there are no tables or chairs around. And I don’t think they are ‘real’ cowboys based on the age of the car and the fact that at least two of them are wearing dress ties.

kneedles
6 years ago

Would be nice if the apostrophe in knee’s was removed from the photos. If it’s just one knee pushing upward, an apostrophe is suitable: the knee’s pushing upward. But if it’s more than one knee we’re discussing, then that should be knees, maybe even knees’, but not knee’s.

Ed
6 years ago

Let’s not forget the umpire who does TWICE as many squats per game.

Hopper
6 years ago
Reply to  Ed

But not as deeply.

Kevin Tashadow
6 years ago

The best catchers’ heels are flat on the ground, to give best balance and weight distribution. Definitely not against the butt. This means that they need good long Achilles tendons – any injury to those and you just aren’t going to be as good a catcher any more.
When coming out of the squat, particularly explosively to gun down a runner stealing to second, most of the force is exerted by the quads, with some help from the calf muscles.
I’m still catching at age 65 but, the older I get, the faster I was.

Dr28kumar
6 years ago

Here in India there are people who squat to relax or have a conversation. They do not seem to have any problem. How does one explain this

GMH
6 years ago
Reply to  Dr28kumar

Do the squatters in India spring up out of the squat 140 times over the course of about 90 minutes? Do they spring up out of a squat about 15 to 20 times over the course of about ten minutes? Do they have to drop from the squat to their knees while sliding laterally to block a pitch in the dirt traveling 140 feet per second? Do they have to spring up out of their squat and sprint 90 feet to back up a throw approximately a dozen or more times in a span of 90 minutes? Do they have to field a bunt, an errant throw or pitch, or throw out a base-runner? And do all of the above while wearing body armor?

Jonny Bench
6 years ago

As a former catcher when Im boning a late of the evening, I’d rather be in the upright position.

Jonny Bench
6 years ago
Reply to  Jonny Bench

*lady of the evening. Sorry Im on meth

BobDD
6 years ago
Reply to  Jonny Bench

*and you’re kneedy for attention

CubbyBear
6 years ago

Re: Umpires
Due to the position roataion, each umpire will only “cover the plate” one in every four games (on average).

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5 years ago

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Douglas Farrago MD
5 years ago

This was well done. I had worked it out the same way years ago (1988) when I invented and then patented the Knee Saver (1991)