The incident occurred about 5:30 a.m. Tuesday at 24-Hour Fitness near Highway 183 and MacArthur Boulevard.
Witnesses told police the man was working out and then for some reason just attacked the woman. They held the suspect, 32-year-old Phillip Roberts, down until police arrived.
"He grabbed her, pulled her off and threw her to the ground," said Justin Harper, a reserve police officer who was there working out at the time. "She never knew it was coming."
The woman was taken to the hospital and suffered broken ribs and stab wounds to her back, neck and arm.
According to a probable cause affidavit, Roberts said he did it because the woman was "playing inside his brain." The two did not know each other.
Police couldn't find an address for Roberts and believe he may be homeless.
One woman, a longtime gym member who was there, became suspicious after she said Roberts circled and paced around the gym, never exercising
"He pulls his suitcase from in back of the back of an ab machine, walks into the basketball court, comes out with a trench coat on," the gym member said.
She called 911 and Irving police came to the front desk.
"The staff told them they know of him, he has a membership there, he's not a concern to them and they're familiar to them. So it wasn't suspicious in their minds and they deemed him OK," said Officer James McLellan with the Irving Police Department.
The woman who called 911 wished those officers would've talked to her before leaving.
She returned to the gym Tuesday afternoon to cancel her membership.
"I think the cops should've went in and spoke to the man and asked him questions because I think it would've prevented [the victim] from getting hurt," the woman said.