The idea that a hitter's swing will be ruined by participating in the home run derby is a myth but that doesn't mean it has never happened.
Cases where such a drop has occurred have been the exception more than the rule. Research by the Hardball Times' Derek Carty in 2009 showed that, while there has been an aggregate post-derby dip in the participants' power numbers, much of that dip can be explained as the players in question returning to their established levels of performance after playing over their heads in the first half. Just as frequently, injuries are to blame for a post-derby power decline.
Below is a look at the five biggest post-derby decreases and increases in home run rate by participants in the game's annual home-run-hitting contest. Bobby Abreu is not on the list, even though his drop in power after setting the standing derby record with 41 home runs in the 2005 contest is perhaps the most frequently cited example of the illusory Home Run Derby Effect. Also not included is Hubie Brooks, the Expos' shortstop in 1986, who played just five games after the All-Star break that season before torn ligaments in his thumb ended his season. Brooks, who hit just one ball out of the Astrodome in the second ever home run derby, remains the only hitter to go homerless for the rest of the season after participating in a derby.
In 1986, babyfaced Angels rookie Wally Joyner had the greatest post-derby drop in home-run frequency in the contest's 27-year history, due to a double-whammy of injury and regression to expected levels of performance.
Joyner, who had never hit more than 12 home runs in a single minor league season, established a 37-homer pace prior to tying Daryl Strawberry with a derby-best four home runs at the cavernous Astrodome. However, Joyner's health had started to become an issue in June, when he hurt his shoulder. He later had a bout of food poisoning and, eventually, a staph infection, and hit his last home run of the year on August 5.
In 1987, Joyner joined the league-wide home-run barrage with 34 round-trippers, but thereafter he never again surpassed his rookie total of 22, and only once hit more than 16 in his remaining 14 years in the big leagues.
As a perennial All-Star in the late '70s, Garvey's hit at least 20 home runs six times, topping out at 33 in 1977, but he was not really a home-run hitter. By '85, he was 36 and had averaged just 12 per season, with a high of 16, in the four years leading up to his first-half outburst. He hit just two home runs in the inaugural derby at the Metrodome in Minneapolis, then hit just four more in the second half to finish with 17. Despite the lack of homers, he was far more valuable to defending National League champion Padres after the derby than before; he struck out less frequently and walked, singled, doubled, and tripled more often. Still, even the more productive, second-half version of Garvey wasn't much more than a league-average first baseman at the plate, though his hair remained far above replacement level.
Ripken won the 1991 derby without derailing what proved to be his last great season, which culminated in a richly deserved second AL MVP award. His home run rate dropped by just 3.1 PA/HR in the second half that year, which would seem to be, at worst, a natural sign of fatigue from a 30-year-old shortstop who never took a day off. By comparison, 1992 was by far Ripken's worst season to that point. Even his pre-derby isolated slugging (slugging percentage minus batting average) would have stood as the worst of his first 11 full seasons in the major leagues. Don't blame the effort he expended to hit four home runs at Jack Murphy Stadium in that year's derby. Blame the pitch he took off his elbow in April and the one that hit him in his back in July, or the cumulative effects of age and his consecutive-games-played streak, which together prevented him from ever recapturing the greatness of his first ten seasons.
In an effort to promote the inaugural World Baseball Classic, which would take place the following March, Major League Baseball made the 2005 derby at Comerica Park in Detroit an international affair, selecting the eight contestants so as to represent eight different nations, each of which would be participating in the WBC. After Japan's Hideki Matsui and Ichiro Suzuki declined, MLB tapped Choi, a 26-year-old South Korean, to represent the Asian continent, despite the fact that he, like Matsui, was not an All-Star that season. Choi had many fans among the Moneyball set due to his power and patience at the plate, but he struggled to win over his field managers. Indeed, Dodgers skipper Jim Tracy limited Choi to pinch-hitting and spot starts at first base in the second half of the 2005 season, which proved to be the end of Choi's major league career. In 2007 he opted to return to his home country.
The slugging third baseman of the 1987 World Series champions averaged 31 home runs from 1986 to 1988, and maintained that pace through the first half in '89, but he was the only one of that year's eight derby participants not to hit a ball out of Anaheim Stadium and managed just three home runs in the second half. The main culprit was an abdominal strain that Gaetti suffered in June and attempted to play through before finally succumbing to the disabled list in late August after going 75 plate appearances without a home run. Likely due to the cumulative effects of his injuries, Gaetti was never the same save for a lone fluke season with the Royals at age 36 in 1995. Even with that season included, he averaged just 18 home runs over his final nine seasons as a major league regular.
Anderson finished third behind Barry Bonds and Mark McGwire in the 1996 derby without slowing his 50-home-run pace in what proved to be one of the game's most memorable fluke seasons. Anderson, who had only once hit more than 20 home runs before '96, reverted to form in '97, finishing with 18. He did show more pop after hitting four home runs in the derby at Jacobs Field, though, nearly halving his first-half homer pace of 54 PA/HR, though still going deep only half as often as he had the previous season, when he had homered once every 13.74 plate appearances. With that, Anderson found a new level and hovered around 30 PA/HR for the next three years before age finally overcame him.
A quick scan of Pudge's annual home run totals reveals a natural arc that peaks with his career-best 35 home runs at age 27 in 1999. By 2005, he was 33, six years removed from that peak and four years removed from his last 20-homer season, thanks in part to his leaving the friendly Ballpark in Arlington for spacious yards in Miami and Detroit. However, he seemed to solve Comerica Park as the Puerto Rican representative in the WBC-themed 2005 derby, hitting 20 home runs across the derby's three rounds, more than he has hit in any full season since 2001, only to lose in the finals. Rodriguez's increase from six home runs before the derby to eight after may not seem drastic, but given that he had 95 fewer plate appearances after the break, he actually came close to homering twice as often in the second half (26.9 PA/HR to 51.7 PA/HR in the first half). It's also worth noting that the two players who saw the largest post-derby increase in their home-run rates set very low first-half baselines that were relatively easy to improve upon. That is not the case for the next three men on this list.
Using the derby as the dividing line in Belle's 1995 season actually masks just how insanely hot he was over the season's final two months. On the morning of August 1, Belle was hitting .295/.374/.562 with 19 home runs in 374 plate appearances (19.7 PA/HR). He then hit three home runs in his first 10 plate appearances of August, kicking off a stretch run in which he hit a whopping .350/.439/.885 with 31 home runs in 257 PA, or one every 8.3 times he stepped to the plate. Put another way, Belle averaged more than one home run every other game for two months. He led the majors with 50 home runs in that strike-shortened season. He also hit the most home runs, 16, in the derby in Arlington but lost the finals to Frank Thomas.
Holliday, like Belle, has been a slightly better hitter after the All-Star break than before over the course of his career. Also like Belle, his post-derby increase in home run rate was simply the result of an elite player getting blazingly hot down the stretch. The Rockies won 14 of their last 15 games in '07 to surge into the playoffs, and Holliday's .442/.532/.846 performance over that span was a huge reason why. He hit 13 home runs in the derby at AT&T Park in San Francisco, but at one point in September he seemed to be having his own, one-man derby as he went deep 11 times in just 12 actual games, failing to homer in just three of those dozen contests.
In 1997, Garciaparra didn't just win the Rookie of the Year award, he won it unanimously, finished eighth in the AL MVP voting, won the Silver Slugger, made the All-Star team, led the AL in hits (209) and triples (11), stole 22 bases, smacked 44 doubles, batted .306 and hit 30 home runs. The only thing he didn't do in '97 was hit a home run in the derby at Jacobs Field.