How Ronald Acuna Changed His Swing
The Braves’ Ronald Acuna entered the season as one of the top prospects in baseball. The 20-year-old demolished minor league pitching in 2017 and the Braves moved incumbent Matt Kemp during the offseason to open a spot in their outfield for their young phenom. Acuna was good, but not great in the season’s first half. His .742 first-half OPS is good for a 20-year-old against the world’s best competition, but his second-half numbers have led the Braves to a four-game lead in the NL East. Those second-half numbers are impressive for a seasoned veteran: .346 AVG, .411 OBP, .722 SLG% with 16 HRs and 9 SBs in 42 games. He has more homers since August 9th than the Pirates. Many have cited the move of manager Brian Snitker to place the youngster in the leadoff spot as the spark that unleashed the Acuna fire on the league, but hitting coach Kevin Seitzer made a less-public move at the same time that’s likely more deserving of the credit.
We’ll investigate those changes, but it often helps to know what type of change we’re looking for. The Athletic’s David O’Brien picked up the following notes from Seitzer:
I love where he’s at with his setup now, his hand position. He’s a little wider in his base, he’s in his legs, where (before) when he was kind of in a narrow stance and slightly open, and his hands were kind of by his face, and then the big leg kick and the windup — I knew it was a matter of time before that wasn’t going to work….Oh yeah, it’s [his high leg kick] gone. I mean, he’s in an athletic position with his legs. I just told him, ‘I just want you to set up different. You can do everything the same way (with your swing), just get in this position to set up.’
It’s common for hitters to change their swing. It’s especially common as young hitters mature and figure out what works best in the Majors. The guys at Tomahawk Take noticed that Acuna was swinging and missing at fastballs. Maybe that’s the motivation that Seitzer leveraged to get Acuna to make subtle swing changes, but the motivation is far less interesting than the process and outcome. We know the outcome: Acuna has been one of baseball’s best players since the change. Let’s figure out what he did.
We have three videos:
- One from October 2017, when Acuna was dominating the Arizona Fall League
- One from May 3, 2018, when Acuna hit a ball 451 feet that nearly knocked over Citi Field
- One from Sunday, September 2, when Acuna tied the Braves record for leadoff homers after just 41 games in the leadoff spot.
Arizona Fall League
This is from batting practice and the outcome is unknown, other than to say that it was an opposite field fly ball. Keep this baseline in mind as we work through the maturation of Acuna. Note how verticle he gets in his load, how his front knee nearly crosses over his base. Also, note how his hands go straight back right before his swing. You can see this between the third and fourth image. We can also see how his legs and hips are leaking towards third base a bit, even as he tries to go to the opposite field.
These are all small critiques that Acuna overcame due to superior athleticism and natural talent.
May, 2018
Acuna has taken largely the same swing to the Majors. His legs look a little larger, thanks to an offseason of squats. Note how his hands are still high and near his right ear. He’s slightly open in his stance and standing quite tall.
We have the same image from a different angle.
We get into Acuna’s leg kick here. As compared with his AFL swing, the leg kick is at least a bit less rotational (his front knee isn’t going past his back leg anymore), but still quite pronounced. 
Look at all this action from the last freeze frame. Acuna has taken a stride that spans multiple feet towards the pitcher. His hands have moved back relative to his body, but his body and head have moved a lot. If we compare the background of the previous freezeframe, we also notice that his head has moved significantly. In the leg kick shot, Acuna is fully behind the guys standing up. In this shot, he’s nearly in front of that guy in the gold shirt.
The rest is great. It’s why Acuna made it to the Major Leagues at 20 years old. His hips come through, his hands work quickly, and he extends through the ball to hit it 451 feet. 
September 2
I’ve highlighted some of the major differences here. Note how Acuna’s hands have moved away from his right ear. He’s closed up a bit, and he’s lower into a squat. Things look dramatically different.
This is the same view, and since we lose depth perception as to how far from his head the hands are, things don’t look that different as compared with the May swing. But he’s significantly wider for sure, more evenly distributing his weight between both legs, and minimizing previous change in angle of vision that resulted from striding from such a tall initial position.
We still have a leg kick – we still have a pretty big leg kick, even. Seitzer’s claim that “the big leg kick is gone” doesn’t look to be completely true, but a few things are notably different. First, the front leg isn’t nearly as far back as it was in May (nearly crossing the back knee then), and his body is at a different angle. If we look at the angle created between his back leg and his upper body, we notice that he’s leaning forward, rather than standing upright.
Remember earlier when Acuna’s head moved across multiple people in the background? Here it moves just about half a person, and most importantly, he keeps his eye level almost exactly flat from pre-pitch, through load (leg kick), and on to contact (next frame). That helps him see the baseball from a steady perspective. His hands also move back, just as before, but they don’t move as far, as late in his load. Also, note that his base somehow ends up more narrow than before. So he started wider and ends narrower than in May. There’s a lot of benefit to simplifying in this way. It’s easier to repeat, and his timing is simpler; he’s more prepared for different pitch types.
Again, the hips come through, his hands extend, and he hits a home run. He’s really good.
Ronald Acuna was capable of hitting home runs before his revamped swing. But he made a few changes, adjusted to competition at the highest level, and now looks like an MVP-candidate. It’s a scary thought for the rest of the league, but the 20-year-old keeps getting better.
-Sean Morash
