Why the Mets' Pete Alonso and Brandon Nimmo use a sledgehammer in the on-deck circle before at-bats

Pete Alonso brought the hammer down on the Angels on Wednesday afternoon with a 439-foot three-run homer that capped a four-run third inning and sent the Mets to a 6-3 win that vaulted them into first place in the NL East.

However, a different kind of hammer that Alonso has been swinging this season has raised the eyebrows of fans at Citi Field and around the country.

This past offseason, he tried and adopted the practice of swinging an eight-pound sledgehammer to loosen up before he steps into the batter’s box.

He brought the sledgehammer to spring training and some of the other Mets gave it a whirl, but Brandon Nimmo is the only other player on the team to have adopted the practice. There they are at every game, Nimmo and Alonso, swinging the hammer as they get ready to hit.

“I started swinging and hitting front toss and balls off the tee with it in the offseason,” Alonso told Newsday before Wednesday’s game. “I use it as a tool, just to make sure everything’s sequencing correctly in my swing.”

Barry Bonds was the inspiration for him to pick up the sledgehammer, Alonso said. Bonds started using a sledgehammer as part of his warm-up ritual when he was the leadoff hitter for the Pittsburgh Pirates.

“I learned that was a tool that he used, and we all know about his swing,” Alonso said.

Hitters use a myriad of instruments in the on-deck circle. The most common are weighted doughnuts that slide over their bat and add 16 to 24 ounces, “heavy bats’’ with extra weight in either the bat head or handle, and isometric devices.

Alonso, though, likes the sledgehammer.

“It’s an eight-pound sledgehammer [so] it forces you to move efficiently,” he said. “If your body’s not sequencing correctly, then it’s going to hurt the swing because it’s a eight-pound sledgehammer . . . I’m a big believer in connectivity and everything moving together, because when things aren’t fluid, it’s hard to be consistent . . . Consistency in the box happens best when you’re feeling everything is connected.”

Willie Stargell and Dave Parker — both Hall of Famers — also were known to swing a sledgehammer in the on-deck circle.

Other players have gotten creative with their on-deck routine. Frank Thomas, also a Hall of Famer, used a piece of rebar he found at a site near the White Sox spring training facility where a structure was being demolished. In a 2006 interview with the San Jose Mercury News, when he was with the Athletics, he spoke of the rebar, saying “I like swinging something heavy so that when I get to the plate, my bat will feel light. That way, you don’t have to overswing to catch up to stuff and you feel natural. It’s just a warm-up tool.”

Alonso’s initial impression of holding the sledgehammer was that it would be good for building up the forearms.

“Then I thought, what if I took swings with this?” he said. “I took a couple off a tee and a couple in front toss and I kind of liked it. Other guys saw me do it and tried it. [Nimmo] is a big user of it. He loves it [because] it forces the body to hold the correct positions through the swing and stay connected.”

Alonso, however, is quick to say that the sledgehammer isn’t for everyone. As he puts it, “this isn’t magic fairy dust that will cure everything in your swing.”

He added, “Think about it like an artist who has many different kinds of brushes. They have their tools. We have ours. We have like a trillion different things: heavy bats, different grips, weights, sizes. They are all tools and [it’s] whatever tools a hitter needs to use to produce their work.

“You get the tool you’re most comfortable with and think you can use to produce your best work,” he added. “For me it’s an eight-pound sledgehammer.”

Notes & quotes: The Mets open a three-game series against the Giants on Friday night in San Francisco, and the scheduled starters are, in order, Clay Holmes, David Peterson and Kodai Senga . . . The Mets announced that they’ve signed 18 of their 19 picks from last week’s draft and nine undrafted free agents.