A Brooklyn social worker described as "the picture of health" was slain at his neighborhood gym in a feud over exercise equipment.

Castello Smith, 77, holds a photo of 'gentle giant' son Douglas, 50, who was fatally knifed and beat with hammer during fight at East Flatbush, Brooklyn, gym.

Douglas Smith, 50, was knifed and clobbered with a hammer by a pair of muscleheads who attacked him as he worked out at his East Flatbush health club.

Smith died Sunday morning at Kings County Hospital, a day after he was jumped inside the Church Avenue Fitness Club, cops said yesterday.

"He was a gentle giant," Smith's heartbroken father, Castello Smith, 77, said last night.

The father said he learned of his son's death from police yesterday afternoon.

"I wish I could have been with him [at the hospital]. At least he would have been able to hear my voice. I would have said, 'Dougie, this is your daddy,'" the elder Smith said.

An argument between Smith, a bodybuilding buff standing more than 6 feet tall, and a younger, scrawnier club member preceded the fatal attack, witnesses said.

Clubgoers said the dispute was over who claimed first dibs on a Nautilus elliptical machine. The club member quarreling with Smith summoned two pals to the gym for backup, witnesses said.

"They came from outside and attacked him with a hammer and a knife," Pat St. Charles, 37, a trainer at the club, told the Daily News.

Witnesses said Smith was exercising on the elliptical machine when the duo ambushed him from behind, stabbing him three times in the back and walloping him in the head with a hammer.

The only signs remaining of the attack yesterday were Smith's workout gloves and Gatorade bottle - left on the machine he had been using.

"How could a person so gentle come to such an end?" asked club member John Grimes, 29, who often worked out with Smith.

In addition to being a social worker for a nonprofit that helps parolees, Smith moonlighted as a bar bouncer and got a master's degree in psychology in the past year, Grimes said. Smith recently reunited with his 19-year-old son after they had been estranged for seven years, he said.

"He's the last to deserve this," Grimes said.

One of the alleged killers, Shawn Hing, 19, was arrested Sunday night and was arraigned in Brooklyn Criminal Court yesterday on a charge of second-degree murder, officials said. Court papers charge Hing, who is being held without bail, hit Smith with the hammer.

A source said Hing, of East Flatbush, immediately lawyered up and refused to speak with detectives.

"He's never been in a fight before," Albert Hing, 94, said of his grandson. "As far as I know, he's been a good boy."

A manhunt was on for Hing's accomplice, and detectives were seeking to question the club member who called the killers to the gym.