The little-known but fatal condition every exercise lover needs to know about.

Late last year, US paralympian Amy Purdy was well into her training for the northern-hemisphere snowboard season. She was pushing herself, hard. Too hard.

“It seemed to happen so innocently, I did a series of pull-ups and simply pushed too hard to complete the set,” the now 37-year-old wrote on Instagram. “My muscles were a bit sore for a day, nothing bad… I almost didn’t come to the hospital when I did because my symptoms were so mild.”

Yet there she stayed for the next eight days.

Purdy was diagnosed with rhabdomyolysis, a rare but serious condition that occurs when muscles are damaged to the point that the fibres break down and release cell contents into the bloodstream. Among those compounds is a protein called myoglobin, which in excess levels can cause damage to the kidneys.

It’s been documented among athletes and soldiers (US research indicated that some 400 members of the Army were diagnosed annually). But according to research conducted by the University of Queensland, with the popularity of high-intensity workouts, doctors are observing rhabdo occurring in increasing numbers among the general population.

Thank you for all your prayers and kind messages! I didn’t think I could be more grateful to be alive then I already was but here I am beyond grateful for my life and health. I almost didn’t come to the hospital when I did because my symptoms were so mild, had I of decided to sleep it off l most likely would have had major kidney failure or even gone into shock by the next morning. Intuition, knowing my body and knowledge about this condition saved my kidney and my life. The scary part about Rhabdo is there isn’t anything the Dr’s can do to stop it once it begins and they don’t know how bad it will get. They can only support your body with constant IV fluids to try to buffer the effects, but your muscles continue to swell and breakdown and your kidneys are still forced to filter the massive amount of toxins. Luckily my kidney has held up like a champ! Thanks Dad! And my numbers have dropped significantly today which is a good thing. This condition is so scary, please pay attention to your body. If you have overworked your muscles, if you are sore and you can see some swelling even the slightest amount like I had, don’t hesitate to go to the ER, it can save your life. I’m so grateful for my husband @dang_ale who’s been by my side, cooking meals at home and bringing them to me each day, for my family and all of your love and support! ❤️️????✨????✨????✨????✨????

A post shared by Amy Purdy (@amypurdygurl) on Oct 27, 2016 at 9:38am PDT

In a review of data from the Princess Alexandra Hospital emergency department, the number of exertion rhabdo cases has increased each year since 2005. In fact, there was a 20-fold increase in the past five years compared to the previous five.

Dr Rob Eley, the hospital’s academic research manager, reported that the top causes observed were gym workouts and long-distance running.

“One form of high-intensity resistance training, CrossFit, has a particularly strong association with exertional rhabdo,” he said.

Is crossfit for kids safe? Post continues below.

There is some evidence, Dr Eley reports, that factors including amphetamines and alcohol, extreme temperatures, dehydration and infections can make someone more predisposed to rhabdo. But it generally occurs when people fail to give their muscles adequate time to adjust to an aggressive new exercise, such as a spin class or weightlifting.

The three major symptoms, he reported, are pain, weakness and tea-coloured urine, while nausea and lethargy may also occur.

Most patients will recover with intravenous fluids and rest, but Dr Eley noted that in rare, extreme cases (especially those involving preexisting kidney disease) it can be fatal.

Amy Purdy believes she could have have been one of those cases, as she underwent a kidney transplant in 1999 after contracting Meningococcal Meningitis.

“Had I of decided to sleep it off I most likely would have had major kidney failure or even gone into shock by the next morning,” she wrote on Instagram.

“Intuition, knowing my body and knowledge about this condition saved my kidney and my life.”

Dr Eley’s advice echoes this: “An appreciation of the condition, knowledge of one’s limits, avoidance of the [contributing] factors, and using common sense when performing high-intensity resistance training will go a long way towards reducing your chance of seeing us in emergency.”

Read Dr Rob Eley’s report on rhabdo and CrossFit here.