In many ways, A’s pitcher Blake Treinen was baseball’s most dominant reliever last season, a closer with a devastating sinker and minuscule ERA, but this year he hasn’t been trusted in high-stakes situations.

Treinen said his featured pitch still feels right coming out of his hand. The difference is the movement. It’s not making hitters feel helpless like last year. It’s not resulting in routine grounders and swings and misses.

“I feel like I’m chasing down a rabbit hole trying to figure out why it’s different,” Treinen said last month. After some self-evaluation and help from teammates and coaches, he came to one conclusion:

“Anyone who watches the games knows the ball has changed,” Treinen said, “and if that’s the case, it’s messing with a lot of people’s careers, a lot of people’s seasons in a way that’s not fair.”

The baseball — a 9-inch, 5-ounce, 108-stitch orb — has been the focus of many a debate, controversy and conspiracy theory in this record-breaking year for home runs, and much of the dialogue is legitimate.

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Major League Baseball says the ball isn’t “juiced.” Say what you want about launch angles, exit velocities and bigger and stronger hitters, but if players are saying the ball is different, that’s the best proof available. Plenty of other evidence, including anecdotal, circumstantial and scientific, supports the players’ claims.

“There’s no denying it,” Giants ace Madison Bumgarner said. “I don’t think anybody at this point is denying the ball is different. It’s definitely different, and it’s affecting a lot of the all-time stats.”

Bumgarner said he noticed a different ball a few years ago and that this year’s ball also is different. It jumps off bats. Not just opposing hitters’ but his own in batting practice.

“I’ve hit in the same stadium for a long time. It’s drastically different,” said Bumgarner, who with 18 career homers leads active pitchers. “I don’t know what all the thinking is. I’ll pitch with whatever ball they give me. I just don’t like it when they change the game so much. This changes it a lot.”

How homer-happy has this season been?

• When A’s shortstop Marcus Semien homered in Houston on Sept. 11, it tied baseball’s record for homers in a season, 6,105. The record was broken moments later and continues to recede by the day.

• More homers were hit in May than any other calendar month in history. In June, more homers were hit than in May. In August, more homers than in June.

• The Yankees set the single-season team record in 2018 with 267 homers, and at least four clubs will surpass that number this year. The Yankees hit 74 homers in August, and no team had ever topped 58 in any month. They set another record by homering in 31 straight games.

• The A’s entered Saturday one of 10 teams to set a franchise record for homers in a season, with 248, breaking their 1996 record of 243. The A’s have three players with 30 homers and a team-record seven with 20.

• On a smaller scale, a Giant hit 20 homers for the first time since 2015. Two, in fact: Kevin Pillar and Mike Yastrzemski.

• Both Triple-A leagues began using the big-league ball this season, and homers increased 57.4%, from 3,652 to 5,749.

“The biggest thing is: Just tell us,” Treinen said. “You don’t want pitchers, especially sinkerballers, chasing something and overpronating and getting injured if you’re not getting the same movement you were last year. Whoever’s responsible: Just be honest. That’s all we want.”

Perhaps underscoring his point, Treinen was placed on the injured list Wednesday, shutting him down for the regular season and potentially the postseason. He said his back started bothering him about three weeks ago, and he has also had shoulder issues.

“As players, we don’t have any say in what the ball is. But just say — if you’re trying something different, tell us,” Treinen said. “But I’m not pointing fingers. The first thing you have to do is look yourself in the mirror, and at the end of the day, I know I’m trying everything I can to be what I was last year. Even years in the past when my numbers weren’t as good, the ball was still moving the same.”

Photo: Tam Duong Jr. / The Chronicle

To explain how the ball has changed, a pitcher showed a Chronicle reporter two baseballs, one from 2014 and one from 2019. The ball from five years ago had noticeably higher seams, and the surface was soft enough that it would give to the pressure of a thumb or finger. This year’s ball had much lower seams, almost as low as the leather surface, with little give. It was a harder ball and tougher to grip.

It’s a tiny sample size, and no two baseballs are exactly alike, but it was clear how a hitter could have a bigger advantage these days.

“The seams are lower, the ball is definitely harder than it’s been before,” A’s pitching coach Scott Emerson said. “It’s a tough ball to throw. … There’s got to be something the league can do to make this better for pitchers.”

What will MLB do? Will it deaden the ball? Will the old ball resurface next year to bring back a balanced game?

In a statement to The Chronicle, MLB said, “In general, we make all decisions on rule and equipment changes in the offseason once we have had a chance to review data from a full season and discuss with all stakeholders. For the ball specifically, that process includes receiving a report from outside scientific experts who are studying all aspects of the ball.”

Last year, MLB studied game-used balls from 2012-17, and a 10-person committee of scientists and data specialists reported in June 2018 that it wasn’t juiced but instead had greater carry because of “changes in the aerodynamic properties” that creates a drag effect, a slower ball path.

For example, according to the report, a ball that’s not aerodynamic at its center and is a bit off-kilter creates drag. The more aerodynamic (and spherically symmetric) the ball, a result of manufacturing advances, the less the drag and the more the lift.

Commissioner Rob Manfred offered a theory during the June owners’ meetings in New York in regard to the pill, the rubber-coated cork in the center of the ball that’s wrapped in several layers of yarn and thread and covered with white leather and red stitches.

“To the extent that the pill is not perfectly centered, the ball wobbles when it’s hit, creates more drag,” Manfred said. “We think one of the things that may be happening is they’re getting better at centering the pill. It creates less drag.”

More than ever, MLB has control over the manufacturing after purchasing the ball maker, Rawlings, last year for $395 million.

It’s a talking point for conspiracy theorists who say MLB intentionally is creating a ball that will fly farther and boost attendance, which is set to drop for a fourth straight year. Manfred said at the All-Star break that MLB has not sought to alter the ball and suggested owners have had no desire to increase home run totals.

Human element remains a factor with the baseballs, which are made and hand-stitched in Costa Rica and rubbed with Delaware River mud in clubhouses by team personnel.

Manfred said MLB is discussing changing the baseball’s specifications and striving for consistency with the way they’re rubbed and stored (and whether to use humidors in all ballparks).

“If we were going to do it,” Manfred said, “we would do it in a way that was transparent to the media and the fans and in advance of that change.”

Giants pitcher Jeff Samardzija said the ball, which he agrees is different, isn’t an issue for him so long as it’s the same for both teams.

“As a player, you adjust,” said Samardzija, whose 3.64 ERA is his best in four seasons with the Giants. “If it’s the ball, figure out why it’s different and how you can make it your own advantage. Belichick it, right?”

Other factors to consider for the homer surge: The flyball revolution and focus on launch angle and exit velocity. Pitchers throw harder; therefore, batters make harder contact. The two-strike approach virtually is extinct — swinging aggressively supersedes making simple contact.

“Regardless of whether the ball has been altered or not, I think you were going to see an influx in flyballs,” said A’s catcher Josh Phegley, who has a career-high 12 homers.

“To me, it’s the perfect storm. I don’t think it’s just the balls. But if Major League Baseball wanted an influx of home runs and power numbers, they could have waited for the different philosophies to catch up with each other. But then they changed the balls, and that’s why you see such a drastic incline of power numbers.”

There’s also performance-enhancing drugs, though with better testing, it’s not considered to be rampant as it was in the 1990s and early 2000s.

Pitchers who throw two-seam, sinking fastballs seem especially affected by the lack of drag. As Emerson said, “They’re spinning it faster and tighter, and the sinkerballers don’t want that, they want the ball to dance and jump around. The sinkerballers are definitely taking a hit on this.”

A’s reliever Lou Trivino, who like Treinen throws a sinker, is struggling as well after succeeding as a rookie setup man in 2018.

“It’s no good if you’re saying, ‘It’s all the ball’s fault,’ and you’re not changing what you’re doing, because you’re clearly at fault,” Trivino said. “At the same time, the ball is clearly different. It’s jumping (off the bat) more.”

Treinen wasn’t making excuses for his 4.91 ERA as much as stating facts. His walk rate was way up. So was his home run rate. When he threw strikes, too often they got hit hard. In the air. And far.

His sinker didn’t have the same effect.

“Look across the league,” Treinen said. “There are some people who have done some great things in the past that rely on movement, and it’s different. … I’m not bitter, but if that’s the case, it would be frustrating if they say nothing’s changed and we’re the problem.

“I guess you just have to reinvent yourself. But I have what I have, that’s really all it comes down to. If I’m attacking guys like I did last year and starting off in a similar spot and having to create more, that’s a problem I have to figure out. I’m not going to cast blame on anybody. It’s just an ugly road to go down.”

John Shea is The San Francisco Chronicle’s national baseball writer. Susan Slusser is a San Francisco Chronicle staff writer. Email: jshea@sfchronicle.com, sslusser@sfchronicle.com Twitter: @JohnSheaHey, @susanslusser