Starting a post-holiday season fitness program too hard and too fast after a hiatus can do real damage to your kidneys and muscle fibres, a McMaster doctor warns.

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

Rhabdomyolysis affects the muscles we use to run, jump and lift. Historically, the condition was most common during basic training in the military, where bodies are pushed to their physical limits. According to Ontario's Ministry of Health, it can also pop up as a result of West Nile virus.

However, with extreme workouts and exercise boot camps growing in popularity and New Year's resolutions of better physical fitness in full swing, the condition is popping up in the general public.

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

'Seek medical attention if urine goes dark brown or red or tea colour'

He says it could be especially prevalent in the kind of person who's in their mid-30s and used to be an athlete in high school, but hasn't been physically active for 15 years and decides it's time to drop 15 pounds.

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

Matthew Green, president and owner of Hamilton's Freestyle fitness, says overexertion and wanting "too much, too fast" is something he's always talking about with his clients. He says they need to focus on partial, incremental change at first.

"The process of gaining strength is a slow one," Green said. "Muscle mass growth happens during the recovery process."

'High intensity workouts are fine for developed and experienced individuals'

He says high-stress, high-intensity workouts like Crossfit are just fine for "highly developed and experienced individuals who know when to stop."

But for the average person? Not so much, he says.

"If you're an office clerk who picked up an exercise program off the internet to do a Tough Mudder and then end up in the park doing parcour, you're going to get hurt."

Green says anyone embarking on a new exercise plan needs to watch out for sharp, instant pain — not plain old muscle fatigue. That kind of pain signifies damage to muscle tissues or a ligament.

Tarnopolsky says people should also watch out for incredible muscle swelling following exercise, especially in the lower legs, which can push blood vessels and nerves leading to feelings of numbness or tingling or foot discolouration.