When your personal trainer says jump, you say how high, right? Well, according to experts interviewed by Fitness magazine’s Peg Rosen, the advice you hear at the gym might not always be correct.

Perhaps you’ve been told that heat and vigorous exercise can help you sweat out toxins. Rachel Vreeman, a pediatrician at Indiana University School of Medicine, argues that the body does a fine job of ridding itself of toxins through digestion and that the only role of sweat is to keep us cool.

Maybe you’ve heard that the more flexible you are, the better. This is not just false; it’s a dangerous theory, says Jo A. Hannafin, orthopedic director at the Hospital for Special Surgery in New York.

“There is no health benefit in having a hyperflexible body or being able to twist yourself into a pretzel just for the sake of doing so. In fact, pushing your body into extreme stretches can cause injury,” Hannafin says.

Should you replace your sneakers every six months? Will the pounding of running break down collagen and give you wrinkles? Can performing yoga twists purify your organs? Those are common claims that have no hard evidence to back them up, Rosen says.