While many Americans claim that being in good shape is a priority to them, more than 80 percent of adults do not follow the Department Of Health's guidelines for both aerobic and muscle-strengthening activities.

And for those that are hitting their fitness goals, there are a whole set of other concerns. 

Certain workout trends that seem healthy can be bad for the body. For example: spin classes.

The New York Times reported on the potentially life threatening consequences from spin classes, an exercise fad featuring high intense training on stationary bikes, popularized by brands like Soulcycle.

The story warns of a condition called rhabdomyolysis, which is associated with active duty soldiers, firefighters, and other physically demanding work. Rhabdomyolysis — or rhabdo, as it’s commonly referred to — is marked by a trio of symptoms: muscle pain, frail arms and legs, and browned urine. It is considered serious because, in rare situations, it can lead to kidney failure.

Rhabdo occurs when a person exercises too intensely without proper rest periods and, in relation to exercise fads, has been most commonly observed in spin classes, where participants — usually beginners — over exert themselves. “They are being pushed too hard,” Dr. Todd S. Cutler, lead author of a recent rhabdomyolysis study, explained to the Times. “They’re not trained to do this, and so they get really bad muscle trauma.”

Is this a common phenomenon? 

Still, Dr. Beth Taylor, professor of kinesiology at University Of Connecticut and director of exercise physiology research at Hartford Hospital, doesn’t see rhabdo as being a likely outcome of a typical workout.

“It is unlikely that a casual exerciser would run a high risk of rhabdo with a specific workout unless there are additional precipitating risk factors,” Taylor told ATTN:. She noted that these factors could be environmental (specifically temperature), genetic, metabolic, or related to an infection or illness.

Dr. Yuri Feito, associate professor of exercise science & sport management at Kennesaw State University, said that awareness of issues like rhabdo are important. “I think the casual exerciser should be aware about this condition,” Feito told ATTN:. “But not to a point that it will defer them from participating in the activity...It is a rare condition.”

But spin classes aren’t the only exercise fad which could lead to rhabdo.

CrossFit and similar extreme workouts like P90X and Insanity have become notorious for their association with rhabdo (or, as some call it, “Uncle Rhabdo”).

These workouts can be “cause for concern for greater injury risk, rhabdo, or other structural injuries,” according to Taylor, as the levels of exertion often exceed the recommendations of the American College Of Sport Medicine.

Sports medicine specialist Dr. Caitlyn C. Mooney is similarly concerned. “You’ll be more at high risk when you have an high intensity sport like spin and a trainer or instructor who might push you when you already feel bad,” Mooney explained to ATTN:. “But there aren’t a lot of public numbers on these activities that cause rhabdo."

There are other health threats related to fitness trends and extreme workouts. 

Rigorous workouts done in heated rooms — like yoga, pilates, and spin done in rooms with upwards of 100º F temperatures— pose dehydration and nausea threats. In extreme cases, one could be at risk for heat stroke.

Exceedingly intense workouts can pose risks from decreased libido to long-term digestion problems. And ultra marathoning—featuring runs no shorter than 26.2 miles—have been found to be particularly problematic, as the practice is linked to permanent heart damage.

“High intensity and adventure sports have been increasingly popular in the last decade,” Mooney said, and noted that overtraining and a lack of rest other associated problems people tend to ignore. “Anything that is a one day event might put you at risk for rhabdo or exhaustion,” she added.

The biggest problem is confusing athletic desire for athletic capability: being prepared is key.

These problems can be avoided and may simply be the result of an unprepared person participating in an activity that is very taxing on the body.

“There is a concern any time an unconditioned or even conditioned adult engages in extreme exercise,” Taylor said. “Because post-exercise signs and symptoms of adverse events such as rhabdo, dehydration, heat stroke, and even cardiac fatigue are not easy to diagnose.”

“We see a large number of people who are sedentary and people in their twenties or thirties who didn’t do a sport, come into a sport without a background of training,” Mooney said. “You’re setting yourself up for hurting.”