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Neighbors of a Park Slope gym that specializes in a worldwide fitness craze say that fitness buffs are making their lives a living hell by tossing big barbells to the ground and grunting loudly throughout the early morning hours.

Residents of a Fifth Avenue apartment building next door to a CrossFit gym near 22nd Street say that they can hear the guttural cries from their bedrooms and that the up-to-300-pound weights land so hard that both buildings shake. The pre-dawn cacophony wakes neighbors and has some saying that if the fitness center does not end the noise, they might pack up and leave.

“I put my pillow over my head and hope that it stops, but it doesn’t,” Jena Battaglia, who lives on the third floor of the brick row-house that shares a wall with the health club. “I love my apartment so much, but if this doesn’t stop I will have to move out.”

The fitness center is one of a purported 7,000 fitness clubs to push the brutal, full-body CrossFit routine, which has gym rats run, jump, row on machines, throw medicine balls, flip tractor-trailer tires, climb ropes, tote thick chains, and lift big weights. The Slope location offers private training for $100 an hour and, for those parents looking to pump up their progeny, kids sessions for $300 a pop. The workout program is so extreme and so popular worldwide that some critics have called it a cult.

The Fifth Avenue outlet CrossFit 718 began its stay in the neighborhood on a sour note when it opened two years ago in a one-story industrial building that was illegally converted from a storage facility, for which the landlord still owes the city $10,000 for failing to file permits, according to public records.

The rec center has been an unwanted alarm clock ever since, say neighbors. And once the racket begins, sometimes as early as 6 am on weekdays and 8 am on weekends, it only gets worse, according to neighbors.

“You feel these vibrations every time they slam the weights down and then babies start crying and dogs start barking,” Battaglia said.

SLEEPLESS IN SOUTH SLOPE: Fifth Avenue resident Jena Battaglia says that weightlifters at the CrossFit gym next door are harshing her Zs. Photo by Elizabeth Graham

Residents also complain about the thud of semi-truck wheels tumbling over on the rubber-lined floor and the shouts of trainers leading cardio exercises on the sidewalk out front, but their gripes are not limited to noise. The harried apartment-dwellers say that the dropped barbells fall with such force that they are afraid it will crack the walls.

“It’s been a nightmare,” said 27-year Fifth Avenue resident Edwin Torres. “It’s so bad the walls shake. It might eventually start causing structural issues.”

Neighbors say that they have repeatedly complained to the city, police, and the owner of the CrossFit outpost, but that their gripes continue to fall upon deaf ears.

A patron of the hated health club cops to letting weights fly but said that sometimes dead-lifting gets so strenuous that he has no choice but to let go.

“At the end of a high intensity workout you just don’t have anything else left in the tank,” said CrossFit regular Mike Soffer, who pumps iron as heavy as 300 pounds. “You shouldn’t drop [the weights], but a lot of the times you can’t help it.”

CrossFit 718 owner Israel Gonzalez did not return repeated calls for comment, but a Williamsburg gym owner facing remarkably similar complaints last year said that, beyond padding the floors, which it appears that Gonzalez has, there is not much else that can be done.

Reach reporter Natalie Musumeci at nmusumeci@cnglocal.com or by calling (718) 260-4505. Follow her at twitter.com/souleddout.