Weekend warriors and people starting a post-holiday fitness program too hard and too fast after time off are being warned that extreme exercise risks damaging the vital kidneys and muscles.

Dr. Mark Tarnopolsky, director of the neuromuscular and neurometabolic clinic at McMaster University Medical Centre in Hamilton, Ont., is bracing for an increase in cases of a disorder called rhabdomyolysis — rapid breakdown of muscle fibres that leads to the release of chemicals in the blood that can damage the kidneys.

When Jay Armitage had rhabdomylosis after overexerting herself she said the muscle soreness was totally different from what she'd felt in the past. When Jay Armitage had rhabdomylosis after overexerting herself she said the muscle soreness was totally different from what she'd felt in the past. (CBC)

Rhabdomyolysis affects the muscles we use to run, jump and lift.

"We're probably going to see a big rise in this over the next months with the New Year's resolution person," said Tarnopolsky.

He gives the example of someone in their mid 30s who used to be an athlete in high school, hasn't been physically active since, and then decides it's time to lose 15 pounds.

Rhabdomylosis and genetic disorders

Besides exercise cases, Dr. Mark Tarnopolsky sees people with rare genetic disorders, such as McArdle's disease that occurs in about one in 100,000 people who can't break down sugar in the muscle and develop rhabdomylosis.

Less commonly, he treats people have CPT II deficiency, which makes it difficult to break down fat after longer-duration activity. He said it occurs in about one in 200,000 people.

It's normal to have a few aches and pains in the first 48 hours after exercising, but people should watch for more serious signs of trouble, Tarnopolsky says.

"I think where people should be concerned and seek medical attention is if their urine goes dark, if their urine goes brown or red or tea colour. Those are the people who definitely need to go to an emergency department, they will likely need an intravenous, and they'll need lots of hydration to protect the kidneys and ride them through that period."

Tarnopolsky also advises people to seek medical attention if the muscle soreness worsens after two or three days.

Jay Armitage, 33, of Toronto, had rhabdomylosis in May. Armitage said she hadn't exercised for three months during a busy period at work and was getting dehydrated while flying. Then she did an intense, 60-minute class of kettle bells and squats. Her legs were shaking as she walked out.

"When I woke up in the morning, I was in incredible pain. I couldn't raise my arms above my shoulders," Armitage recalled.

When someone's urine goes brown, red or tea colour, they could need IV fluids.When someone's urine goes brown, red or tea colour, they could need IV fluids.

As a kidney donor, she had a home blood pressure machine that showed a high reading. After blood tests, a doctor called her at home and told her to get to an emergency as soon as possible for treatment, reassuring her that she'd be fine.

"The end goal is to be active," said Armitage, who is now seeing Tarnopolsky for advice on getting back into exercise. "It's just a matter of slow and steady wins the race."

The other problem that Tarnopolsky said needs medical attention is significant muscle swelling after exercise, especially in the lower legs, which can push blood vessels and nerves leading to feelings of numbness or tingling or foot discoloration.

With files from CBC's Kelly Crowe and Laura MacNaughton