Can Worm Killers Eventually Lift the Ball?

A while back on Twitter, Razzball’s Rudy Gamble made the following comment in a discussion of Vladimir Guerrero Jr. and his high groundball rate.

Vlad’s career groundball rate in just 900 PA is 50.5% which would rank as the 5th highest of 75 hitters. Over the same sample, his 118.9 mph Max Exit Velocity is the highest. It’s like he’s been blasting holes in the infield. So coming back to Rudy’s comment, will a power hitter, like Vlad, eventually get the ball in the air. While the answer is some, it’s likely not enough to make a difference.

Just comparing 2020 and 2021, it seems like Vlad is taking a major step forward with his groundball rate dropping from 55% in 2020 to 46%. The data in both cases was two months or less and could contain a lot of noise. Here is his how his monthly groundball rate has changed.

While April GB% was low for Vlad Jr., it wasn’t out of the norm. So getting back to the original question, will it ever stay down.

The first key is to determine the statistical constraints. I compared 25-year-old or younger hitters with a minimum 100 PA in matched seasons who had at least a Max Exit Velocity of 110 mph and a 50% GB%. I could have used 1000’s of different stats and benchmarks, but this limit contains players on both sides of Vlad’s 2020 season (55% GB%, 116 MaxEV). Another issue I ran into is that StatCast data only goes back to 2015 for MaxEV. In the end, I was then able to see how these power groundballs performed the next season, in two seasons, and in three without the sample size shrinking too much. Additionally, I calculated their change in OPS and ISO.

Average Change in GB% for Hard Hitters
Start Final Change
Next Season 48 113.0 53.5% .144 .746 49.8% .172 .774 -3.7% .028 .028
In 2 seasons 43 113.1 53.6% .151 .750 48.7% .181 .772 -4.9% .030 .021
in 3 seasons 27 113.0 54.3% .156 .782 47.6% .182 .780 -6.7% .025 -.002


Median Change in GB% for Hard Hitters
Start Final Change
Next Season 48 112.9 52.9% .150 .762 48.9% .166 .786 -4.0% .016 .023
In 2 seasons 43 113.2 52.9% .156 .757 48.9% .179 .758 -4.0% .022 .002
in 3 seasons 27 112.8 53.3% .167 .786 47.9% .163 .763 -5.4% -.005 -.023


On average, these hitters lowed their groundball rate from around 4% to 7% depending on the time frame. Their ISO’s stayed about even to going up around 30 points. If just knowing Vlad’s 2020 55% GB%, we would project it to drop to around 51%.

Sadly, the results aren’t exactly game-changing and could just be explained by regression to the mean. The league average groundball rate is 43% so most groundball rates are expected to regress to that total. Using the same data, but removing the 110 MaxEV restriction, I found the average amount groundball rates are regressed to the mean from year to year.

Average GB% Regression
GB% Avg 1Y Chg
>60% -7.3%
55%-60% -4.8%
50%-55% -3.2%
45%-50% -1.1%
43%-45% -0.6%
40%-43% -0.3%
35%-40% 1.4%
30%-35% 3.1%
<39% 5.0%

The groundball rate for Vlad again was 55%, so he fits right between -3.2% and -4.8% or -4.0% or the same amount found from the hard hitters. Nothing from the past point to hard-hitters airing out the ball more than the average hitter. The simple answer is that those batters who hit a ton of groundballs will regress to the mean no matter if they hit the ball hard or soft.

Jeff, one of the authors of the fantasy baseball guide,The Process, writes for RotoGraphs, The Hardball Times, Rotowire, Baseball America, and BaseballHQ. He has been nominated for two SABR Analytics Research Award for Contemporary Analysis and won it in 2013 in tandem with Bill Petti. He has won three FSWA Awards including on for his MASH series. In his first two seasons in Tout Wars, he's won the H2H league and mixed auction league. Follow him on Twitter @jeffwzimmerman.

newest oldest most voted

Excellent research, Jeff. Thank you!

To cut to the chase, so you have some doubt as to this being the ‘new Vlad’?

Pirates Hurdles
Pirates Hurdles

I dont think he should be saying that at all, Vlad isnt on either extreme end where big regression is expected. (unless you cherry pick small sample 2020 data) The data say big regression is over 55% or below 35%. He is not far off (45.6) from his 2019 where he was at 49.6. Oh and he just turned 22.