Have you ever swung a bat by the barrel end? Go ahead. Give it a try. I’ll wait. You should notice that it is dramatically easier to swing the bat from the barrel end than the handle end. In general, it is easier to rotate an object that has more of its mass near the axis of rotation than an object that has more of its mass further from the axis.
The “rotational inertia” of an object is a measure of how hard it is to get the object to rotate. Objects that have a large rotational inertia have proportionally more of their mass farther from the axis of rotation than objects that have more of their mass closer to the axis. In other words, if you swing a bat by the barrel end, it has less rotational inertia than when you swing it by the handle end.
If batters can get the bat into the hitting zone as fast as possible, they can wait as long as possible to decide whether to swing and where to expect the ball. If that is their goal, then they should hold the bat by the barrel end. I suspect you’ve already realized that if a batter swung the bat by the barrel against a big-league fastball, the handle would snap like a twig.
Clearly, batters aren’t interested just in getting the bat into the hitting zone quickly. They are also interested in getting as much lumber as possible on the ball. This is the “great compromise” for hitters. They must find the right combination of a quick swing versus putting “good wood on the ball” that will maximize their paycheck. Batters who tend toward the quick swing sometimes choke up, while power hitters focus on the big stick.
If batters want to get their bats into the hitting zone more quickly, a physicist would tell them to reduce the rotational inertia of the bat, making it easier to rotate the bat more quickly. That is, the batter must get a greater fraction of the bat closer to the axis of rotation or equivalently have less of the bat further away from the axis.
The sketch below illustrates two ways to accomplish this goal. In the left-most image, there is a bat swung around the red dot near the handle end. Notice the fraction of the bat outside the red circle. It might be around 80 percent of the bat’s total mass.
The middle picture shows the bat rotating about a red dot further up the handle as if the batter is choking up. Now, only about 60 percent of the bat is outside the circle. Since there is less of the bat further from the axis, the rotational inertia is smaller and the bat can be sped up more rapidly.
The right-most image shows a smaller bat being swung around the red dot back near the handle end. If you look carefully, you’ll notice the red circle goes a bit farther up the bat than the red circle in the left-most image. So, swinging a shorter bat is somewhat equivalent to choking up.
Why would a batter choose to choke up instead of just using a shorter bat? Great question — I’m so glad you asked. A shorter bat probably weighs less, making it somewhat less effective. On the other hand, the average speed of a choked-up bat is smaller. More on this in a bit.
Both reasons result in somewhat less effective ball-bat contact. So, I suspect the reason that hitters choke up instead of going with less bat has to do with situational hitting. With the count in your favor late in a close game you might risk not choking up to put a serious charge into a ball. On the other extreme, if you are behind in the count, choking up might be the way to go, just to put the ball in play. The point is, it is bad form to call timeout to change bats.
Perhaps you have already realized that you can make the bat easier to swing and quicker to the hitting zone by simply choosing a lighter bat of the same length or, dare I mention it Sammy, hollow out the center of the barrel. These methods do make the bat faster, but less effective because it is now lighter.
So, is any of this real? A 2010 research paper in The Sport Journal reports the following results comparing the swings of batters with and without choking up. The authors state:
- The choke-up grip had significantly less swing time and stride time than the normal grip.
- The choke-up grip had significantly greater bat tip velocity than the normal grip.
- No significant difference between choke-up and normal grips in bat-ball accuracy.
Items one and two make sense because the lower rotational inertia of the bat allows the hitter to speed the bat up more easily. Item three is a bit disturbing. After all, the purpose of choking up is to increase the likelihood of solid contact. I’m not going to worry too much about it because the batters were hitting off a pitching machine with speeds around 75 miles per hour – not near major league game conditions.
Now that I think about it, item two is also kind of funny. If choking up results in faster bat tip speeds, why do hitters who choke up sacrifice power for contact? I suspect this is due to the distinction between the speed of the tip of the bat and the average speed of the entire bat.
The physics of the ball-bat collision tells us that the effectiveness of the bat depends on the mass of the bat and the average speed of the entire bat (usually called the “center-of-mass speed”). The motion of a bat moving to the hitting zone is complex, but it can be thought of as a combination of the forward motion of the bat toward the pitcher plus the rotational motion of the bat we mentioned earlier.
The complex motion of the hands as a whole is a portion of the motion of the bat. In addition, as batters go through their swing their dominant hand (right hand for a righty) starts farther from the pitcher than their non-dominant hand (left for a righty). As the bat reaches the hitting zones the two hands are about equidistant from the pitcher. During the follow-through the dominant hand is now closer to the pitcher.
My point is, a portion of the bat’s motion involves rotation about a point between the two hands during the swing. Looking at a normal grip, the batter’s hands are going through their complicated motion while at the same time the bat is in rotation about a point near the knob end.
For choked-up grips, the hands are also moving, but the bat is now rotating about a point further up the bat. Just looking at this rotational motion, the knob end of the bat is actually rotating away from the pitcher. This results in the average forward speed of the bat being lower as mentioned earlier. Below is a sketch designed to help understand this difference in average total speed.
The bat on the left has a normal grip near the knob, while the bat on the right is choked up. Considering only the rotation of the bats, each is broken up into five sections of roughly equal mass by the black dashed lines. For each section the speed of the bat is shown.
These are round numbers to illustrate the issue and not to be taken too seriously. I picked them so averaging would be relatively easy, not because they are correct for some particular batter’s swing. It should also be noted that, in addition to the rotation of the bat, the hitter’s hands are also moving forward toward the pitcher. I have ignored this piece of the motion for clarity and because it is very similar for both grips.
The choked-up bat has a higher tip speed than the normal grip. The rest of the speeds were estimated by using the “merry-go-round rule.” The horse near the outside edge is going twice as fast as the horse only half-way to the outside edge. The speed of each portion of the bat is linearly proportional to the distance from axis of rotation — the red dot — roughly the spot between the batter’s hands.
According to the data we looked at, the tip speed of the choked-up bat is higher that the tip speed for the normal grip. Note the speed of the bat just due to the rotation must be zero where the hands grip the bat. These two conditions require the speeds to drop faster for the choked-up grip compared to the normal method. Also notice the knob of the choked-up bat is indeed moving backwards as mentioned earlier.
For each grip, we just need to average to five speeds on each bat to get the average speed. For the normal grip the result is 50 mph, while the choked-up bat has an average speed of 45 mph. So, we see that the speed of the tip of a choked-up bat can be higher and the average speed be lower. Since the average speed of the bat determines the exit speed of the ball, choking up indeed sacrifices power hopefully in exchange for a higher probability of contact.
I hope this explains some of the physics of choking up. If not, I guess I’m the one that choked up.
References & Resources
- Coop DeRenne, Charles F. Morgan, Rafael F. Escamilla & Glenn S. Fleisig, The Sport Journal, “A Choke-Up Grip Facilitates Faster Swing and Stride Times Without Compromising Bat Velocity and Bat Control”