Seen At 11: 'Fatal Fitness' – When Extreme Exercise Turns Deadly

NEW YORK (CBSNewYork) -- We've all heard the saying, 'no pain, no gain,' but what happens when you do too much of a good thing?

"I would go on my lunch break, I would go after work, people would see me and say, 'wow, you look incredible," Katherine Schreiber told CBS2's Dick Brennan.

Her extreme exercise was actually killing her, and she's not alone. A growing number of people are dealing with severe side effects after high intensity workouts.

"I never thought something like that would happen to me," Niki Kushner said.

The week before getting married, Kushner ended up in the hospital, she was hooked up to IVs -- something she claims was the result of an especially challenging spin class.

"It got to the point where I may not be going to the wedding," she said.

Kushner's extreme exercising caused a condition known as rhabdomyolysis. Her muscles broke down, released toxins into her body, and lead to kidney failure.

"People do crossfit, and cycling, and classes, and more intensive forms of exercise, and I think we have to be more aware of it, and help people understand that this is a consequence," Dr. Ed Cikowski said.

It's a consequence that left Kushner barely able to walk -- still she was determined to make it down the aisle.

"It's a once in a lifetime thing that someone dreams about their whole life, and you can't even dance," she said.

Sean Kelleher is a world class power lifter, and owner of Edge Gyms.

"People think you have to train harder. No, you have to back off," he said, "You see it with people on the bikes and on the elliptical -- I'm going to do 90 minutes, and then I'm going to do two hours."

He said the all or nothing mentality is actually wrong.

"Your muscles have to recover, if you don't you're not going to get results," he said.

Jodi Rubin with 'Destructively Fit' counsels people who over-exercise and said it's often connected to low self-esteem.

"It's very, very obsessive," she said.

Schreiber's breaking point came when wearing an orthopedic boot on her foot, she still showed up at the gym.

"I said to myself, I need to get help," she said.

Now, she hopes to help others with a book she wrote about her ordeal.

"I think we have this misconception about exercise that no matter what, it's always good and nothing is like that, even exercise needs to be done in moderation," she said.

Experts said it's best to give your muscles 48 hours to rest between high intensity workouts.