The musician told Us Weekly that he was diagnosed with ankylosing spondylitis, or AS.
"I was diagnosed with it many years ago, but the process of it was incredibly difficult because it's a hidden disease so not a lot of people know it. It's not mainstream," he told the magazine.
Reynolds, 31, said that before he was diagnosed with the acute arthritic condition, he was "bouncing around from doctor to doctor for over a year, in a lot of pain." Things like sciatica or slipped discs were thought to be the issue, until a rheumatologist finally determined it to be AS.
He said from there, he was able to develop a treatment plan to manage the inflammation.
"There's really three things — at least, that I found that work for me — and that's balancing out your diet, exercise and a treatment plan," Reynolds said. "There's foods that are trigger foods for people that you remove, things you add that you needed, nutrients, vitamins, exercise; some people use yoga. For me I find a lot of my pain is in my hinge movement, so I do a lot of hinge movement and then also a treatment plan… it's individualized for everybody."
The biggest non-negotiable? "I work out five days a week," Reynolds said. "Just for an hour."
He said his routine is made up of flexibility and strength training – "Things to get blood flowing to different parts of my body. I do a lot of yoga, I think yoga is just, for anybody that has back pain, things like that, just stretching out, getting blood flowing to places that don't typically get the blood when you're sitting in a chair all day."
The dad of three, who is expecting baby number four with wife Aja Volkman, is healthy now, and he has a new goal: "To get people to not have that years process of pain," he said. His organization, Monster Pain in the AS, helps with that.
"It's confusing," he said of the time and emotions leading up to a diagnosis. "The cash adds up. So we want to bring this to the mainstream. Millions of people are suffering from it and not anybody knows AS, so we're trying to get it out there, get the word out there."
Earlier this month, Reynolds told Us Weekly that he and Volkman separated last April, but reconciled this past January — and are now expecting a baby boy together.
"We have a boy on the way," Reynolds said. "Three girls and a boy on the way, due in October."
He added that Volkman is "feeling upset" with him because she doesn't want their son to be as large a baby as he was.
"I was over nine pounds … and she's showing much more than she was with the girls," he said. Because of that, the musician joked that he has been telling her to "eat those potatoes [and] drink a protein shake" for an even bigger baby.
Reynolds and Volkman of the band Nico Vega, married in 2011 and started their family just a year later, welcoming their daughter Arrow, 6, in 2012 and fraternal twin daughters Gia and Coco, 2, five years later.