On September 19, 2017, Alex Gordon hit the 5,694th home run of the Major League Baseball season, breaking the record for most home runs in a single season set in 2000. En route to the new record, MLB players also broke the record for home runs in a month with 1,101 in June. When the dust had settled on the season, the new record stood at 6,015 home runs.

One of the biggest reasons for the home run barrage is the current trend of uppercut swings. Justin Turner might be the poster child for this trend. A journeyman at best since breaking into the big leagues in 2009, Turner changed his swing in an effort to hit more fly balls in September of 2013. Turner has hit more fly balls than ground balls in each of his last three seasons. He has also slugged .491 or better in each of those seasons.

While Turner may be the best example of the uppercut revolution, he certainly isn’t the only one. The list of players who have turned their careers around after adopting uppercut swings reached new heights in 2017, and the trend will almost certainly continue in 2018.

To properly evaluate the uppercut trend and the corresponding home run explosion, there are two essential statistics fantasy players must understand: GB/FB and HR/FB. We will begin with GB/FB rate.

As you may have surmised, GB/FB is the ratio of ground balls to fly balls. It is essential for understanding the type of contact hitters make and pitchers give up. When someone refers to Marcus Stroman as a ground ball pitcher or Marco Estrada as a fly ball pitcher, GB/FB is one of the stats informing that assertion.

If a pitcher allows exactly as many ground balls as fly balls, his GB/FB rate would be 1.00. Three pitchers finished with a GB/FB rate of 1.00 in 2017: Jason Vargas, Robbie Ray and Julio Teheran. Interestingly, they finished 12th, 13th and 14th out of 58 qualified starters in lowest GB/FB. The overwhelming majority of pitchers in 2017 allowed more ground balls than fly balls. So even though Vargas, Ray and Teheran may not technically be fly ball pitchers by a strict definition, they can probably be considered fly ball pitchers for the purposes of evaluating them for fantasy.

GB/FB rate is extremely important as it relates to BABIP as well as home runs. While you can’t hit a home run on a ground ball, you can hit it into the hole, hit it away from the shift, or outrun the throw for an infield single. As we noted in our Draft Guide entry on Understanding BABIP, ground balls go for more hits than fly balls. For that reason, players with a high GB/FB rate tend to hit for a higher BABIP than players with a low GB/FB rate. None of the top 20 players in GB/FB rate had a BABIP under .299. At the same time, none of the six pitchers with the lowest GB/FB rate allowed a BABIP over .295.

Another important note from Understanding BABIP is that pitchers don’t have a whole lot of control over their BABIP allowed. Pitchers can’t control what their defense does, or the ballpark they are pitching in. One of the few things pitchers do have some control over is how often they allow ground balls compared to fly balls.

Like many other batted ball stats, GB/FB shows some year-to-year fluctuations but tends to be pretty stable for most players over multiple seasons. Marco Estrada hasn’t had a GB/FB over 0.93 since his rookie season, and his GB/FB has been under 0.70 in each of his last four seasons.

On the other extreme, Marcus Stroman has led MLB in GB/FB in each of the last two seasons. Stroman allowed 21 home runs in each of those seasons, and if he ever stops being such an extreme ground ball pitcher, he could be in a world of trouble.

In between, we have Jake Arrieta and Dallas Keuchel. Arrieta finished fourth in GB/FB in 2015 when he won the NL Cy Young award. Dallas Keuchel, the AL Cy Young winner that year, finished second. Both pitchers have been inconsistent since then, and their GB/FB numbers help to illustrate why.

In 2015, Arrieta had a 1.77 ERA, 2.35 FIP and 2.47 GB/FB. In 2016, he had a 3.10 ERA, 3.52 FIP and 1.89 GB/FB. In 2017, those numbers were 3.53, 4.16 and 1.31. The Cy Young version of Jake Arrieta may never come back, which may be why it has taken him so long to find a team as a free agent. That being said, if he gets back to throwing more ground balls it could go a long way towards being an ace pitcher again.

Dallas Keuchel did return to being an ace following a down 2016 season. His GB/FB fell from 3.14 in 2015 to 2.33 in 2016. His ERA rose from 2.48 in 2016 to 4.55 in 2016. In 2017 he had a 3.77 GB/FB, which was the highest in baseball among pitchers with at least 100 innings. It probably isn’t a coincidence his ERA fell back down to 2.90 in 2017.

While pitchers control their GB/FB profile mostly by the types and locations of the pitches they throw, hitters can influence their GB/FB ratio by changing their swing. As we mentioned above, Justin Turner is just one of several hitters to adopt an uppercut swing in an attempt to hit more home runs. Similarly, fast players who don’t have a lot of power will try to hit more ground balls so they can take advantage of their speed to leg out hits. 2017 saw several notable GB/FB performances that bear discussion.

Of the top 10 players in home runs last season, only Giancarlo Stanton hit more ground balls than fly balls. One of the most surprising names on that list is Logan Morrison. Morrison hit 31 home runs combined in 2015 and 2016, and had never topped 23 in any season in his seven-year career. He had also never had a GB/FB rate under 1.01. That changed in 2017. Morrison had a 0.77 GB/FB rate, and he is a strong candidate to hit 30+ home runs again in 2018 so long as he keeps the same swing.

Justin Smoak also hit 38 home runs last season. In his case it marked the continuation of a trend few people noticed in 2016. Smoak had a 0.72 GB/FB in 299 AB in 2016 and a 0.77 GB/FB in 2017. This wasn’t completely new for Smoak, as he also had a 0.76 GB/FB in 2013. Smoak will presumably try to continue to loft the ball in 2018, but he might be a bit more risky than other players we discuss here.

Yonder Alonso hit 28 home runs in 521 plate appearances in 2017 after hitting 39 home runs in his previous 2,343 PAs. He finished the season with a 0.78 GB/FB rate, the only time in his eight-year career is was under 1.11.

The Dodgers had the sixth-lowest GB/FB in baseball last season, likely due to Justin Turner’s influence on teammates like Chris Taylor and Cody Bellinger. Bellinger hit 39 home runs with a 0.75 GB/FB as a rookie last season, while Taylor batted .288/.354/.496 after spending the previous three seasons as a part-time player. Taylor actually had a 1.16 GB/FB despite overhauling his swing prior to the 2017 season. He might actually be a 30 home run candidate if he continues to use an uppercut swing.

Of course, not every player who suddenly displays a lot of power owes it to hitting more fly balls. Zack Cozart has had a GB/FB under 1.00 for each of his last three seasons, but 2017 was the first time he hit more than 16 home runs. Cozart’s career-high 24 homers in 2017 were not a result of hitting more fly balls, but simply hitting more fly balls over the fence.

Cozart had a 15.6% HR/FB in 2017, his highest since his rookie season.  Interestingly, Cozart’s hard and medium contact rates were nearly identical to 2016, when he had a 10.5% HR/FB. Cozart is a perfect example of how hitting home runs can sometimes come down to luck, and how that luck is reflected in HR/FB.

Home run to fly ball rate, or HR/FB, is simply the number of home runs divided by the number of fly balls. It is usually expressed as a percentage. As we know from experience, as well as from studying ballparks, sometimes players hit a fly ball that would have been a home run in other parks, but is simply an out in a larger park or when the wind is blowing in or if the outfielder catches a ball that would have cleared the fence. These things tend to even out over time, but they can make a big difference from year-to-year.

In Zack Cozart’s case, he hit eight more home runs in 2017 than in 2016 mostly due to luck. When we project Cozart’s 2018 home run totals, we have to take that into account. That being said, context matters as well. Cozart’s career HR/FB is 9.5%, and his HR/FB was actually much lower than that from 2012 to 2014. Of course, as we mentioned above, Cozart hit a lot fewer fly balls back then. So long as he continues to get the ball in the air like he has the last three seasons, his HR/FB will probably remain higher. Cozart had a 12.9% HR/FB in 2014. If I am projecting Cozart going forward, I would peg his HR/FB at around 13.0%, perhaps a bit lower since he is leaving the Reds for a less favorable home ballpark. For fantasy, this means we should probably expect 18-20 home runs from Cozart in 2018, not the 24 he hit in 2017.

HR/FB rate is a great tool for looking at players who may have gotten lucky or unlucky with home runs. We can compare HR/FB to recent seasons or to career numbers to try to spot some regression. For instance, Giancarlo Stanton hit a career-high 59 home runs last season despite a 1.13 GB/FB rate, which matched his highest since 2011. That being said, Stanton’s 34.3% HR/FB was his career high, and was topped only by Aaron Judge. While both players are candidates to lead the league in home runs again in 2018, we should probably project them both for fewer than 50 home runs.

Of course, with Stanton as with Zack Cozart, we have to consider that other things factor into HR/FB besides luck. Stanton is also switching teams, in his case for a more favorable park, so while I still think his HR/FB will be below 30 percent next season, it may not be much below that.

HR/FB rate is also extremely useful for identifying pitchers who may have gotten lucky or unlucky in 2017. Masahiro Tanaka allowed a league-high 21.2 HR/FB in 2017. Even after that borderline disastrous luck, his career mark still sits at 16.1%. He probably isn’t as good as he looked in 2016, when he had a 3.07 ERA and allowed a 12.0% HR/FB, but he feels like a safe bet to bounce back in 2017.

Of course, Tanaka could allow more home runs in 2018 even if his luck turns around if he allows more fly balls. He had a 1.51 GB/FB in 2017, matching his career average, but as we saw with Jake Arrieta and Dallas Keuchel, those numbers don’t necessarily have to remain stable from year-to-year.

The bottom line is that when we evaluate hitters and pitchers, it is important to look at both the rate of fly balls compared to ground balls as well as the rate at which those fly balls clear the fence. As with all of the advanced stats discussed in the Jim Bowden’s MLB Draft Guide, GB/FB and HR/FB help us get a better understanding of how a player achieved his numbers in 2017 so we can have a leg up in predicting his 2018 numbers.