NEW YORK • Three years ago, Christina D'Ambrosio went to her first spin class, pedalling fast on a stationary bike to the rhythms of popular music as an instructor shouted motivation.

But D'Ambrosio, who exercises regularly, found the hour-long class was harder than she anticipated. By the end her legs were sore and wobbly.

"I thought my body just wasn't used to that kind of muscle ache because it was my first class," said the kindergarten teacher from Pleasantville, New York.

Over the next two days, her legs throbbed with excruciating pain, her urine turned a dark shade of brown, and she felt nauseated.

Eventually she went to a hospital, where she was told she had rhabdomyolysis, a rare but life-threatening condition often caused by extreme exercise. It occurs when overworked muscles begin to die and leak their contents into the bloodstream, straining the kidneys and causing severe pain.

After a two-week hospital stay, D'Ambrosio was released and has since recovered. Her case was highlighted in April in The American Journal of Medicine, along with two other cases of spinning-induced rhabdomyolysis treated by the same doctors.

The report noted that at least 46 other cases of people developing the condition after a spin class were documented in the medical literature, 42 of them in people taking their first class.

The report cautioned that the condition was very rare, and not a reason to avoid high-intensity exercise. But the authors said their goal was to raise public awareness so that people who begin a tough new workout programme will ease into it to lower their risk of injury.

Said Alan Coffino, the chairman of medicine at Northern Westchester Hospital and a co-author of the new study: "Spin class is a great exercise. But it's not an activity where you start off at full speed. And it's important for the public to realise this and for trainers to realise this."

Rhabdo, as many experts call it, has long been documented among soldiers, firefighters and others whose professions can be physically demanding. But doctors say they are now seeing more of it among weekend warriors driven in part by the popularity of high-intensity workouts such as spinning.

A study found that, between 2010 and 2014, there were 29 emergency room visits for exercise-induced rhabdo at the New York-Presbyterian Hospital alone.

Dr Todd Cutler, an internist at the hospital and lead author of the study, said the patients all fit a similar profile: "These are people who are not unfit. They are being pushed too hard, and they're not trained to do this, and so they get really bad muscle trauma."

In general, rhabdo occurs when people simply do not give their muscles time to adjust to an aggressive new exercise, experts say.

A little damage to muscles is a good thing because that stimulates them to grow and adapt to stress. But when the stress is too great, fibres are destroyed. When that happens, they break apart and release compounds that can be harmful to the liver, such as a protein called myoglobin, which causes brown or tea-coloured urine, a classic symptom of rhabdo.

While almost any intense activity can cause rhabdo, it almost always strikes people who are doing something new. That is why people should always progress from light to moderate and then vigorous intensity when doing a new exercise, said Eric Rawson, chair of the department of health, nutrition and exercise science at Messiah College in Pennsylvania.

"You can be fit, and I can come up with a workout that you are unaccustomed to, and that could be what causes rhabdo," he said.

Even elite athletes are not immune. Amy Purdy, a bronze-medallist Paralympic snowboarder and Dancing With the Stars contestant, went to an exercise class last year after taking three weeks off from her training regimen. The class consisted of a circuit of challenging exercises, she said, including dozens of pull-ups.

"About halfway through I realised my arms were completely fatigued," she said.

The next morning she could not straighten her left arm. Then it became sore, stiff and swollen, prompting her to go to a hospital. She remained there for eight days as doctors flushed her kidneys with water, she said.

She was diagnosed with rhabdo, and when she wrote about the experience on social media she was inundated with responses.

"Thousands of people have reached out to me on my Instagram page who have had it as well," she said. "Almost everyone was fit before, got it from pull-ups and is trying to figure out the way to get back into fitness without risking a recurrence."

Two things can help avoid rhabdo, said Joe Cannon, an exercise physiologist. Before starting a new programme, do a less intense version of it first. That means riding a stationary bike at a moderate pace before starting a spin class, or doing just one set of a weightlifting exercise rather than multiple sets.

But the most important advice is to know your limits: Don't be afraid to leave a class or to say no to a trainer if you are struggling.

"One thing I've noticed when people tell me they've gotten rhabdo in the gym is that they gave up their personal power," said Cannon.

"They kept doing what the instructor told them to do because they did not want to look weak."

NYTIMES