The feat of checking a baseball swing is truly one that requires much physical power. The majority of people compliment the batter's keen eyesight when he stops his bat mid swing, when in reality they should be complimenting his strength. Thinking about it from a physics perspective, it is simple to see how much strength is required to stop a swing. 250 milliseconds after the ball is released, the batter starts his swing, generating all the force he can out of muscles in his arms, legs, hips, shoulders and abdomen. If he were to commit to it, the next 150 milliseconds would be spent taking a complete swing at the ball. If he changes his mind, he must do so within 50 milliseconds of the start of his swing for a very important reason: he must slow the bat back down to rest using the muscles in only his upper body, taking his hips and legs out of the equation. Compared to the arms and shoulders, the lower body generates a force considerably larger. This means, if applied to kinematics, the roughly 50 milliseconds of full force, full body swing, could only be stopped with roughly 100 milliseconds of full force from the batters upper body in the exact opposite direction. With MLB swings clocking in at over 80 mph, it is a true physical marvel that these players can stop their swing in such a short period of time.
To take this a step further, we can even estimate the force a batter needs to apply to the bat to get it to stop within that 100 ms time frame. Given that the swing is 80 mph and the batter has exactly 100 ms to stop his bat, we can use to determine the the acceleration of the bat when the opposing force from the arms is applied. By plugging in the values converted to m/s and s, we can find that: . This means that using his upper body, the batter is decelerating his bat at -357 meters per second squared. Plug this into the force equation and assume the league- standard 32 oz (.91kg) bat is being used and you get: . Here we can roughly estimate that an average MLB player applies a 325 Newton force to his bat when he checks his swing. This is just as impressing as it is eye opening... just because they only run 90 ft at a time doesn't mean pro ball players aren't very powerful athletes!
Also, enjoy this video of Yasiel Puig, one of the strongest guys in the league actually break his bat because of how fast he decelerated his hands. Enjoy!