Researchers at the University of Gothenburg's Sahlgrenska Academy have found that a spin class can trigger the same biochemicals in your body that heart attacks do.
When you have a heart attack, your body secretes certain cardiac biomarkers that doctors use to diagnose and treat you. It turns out that an hour-long spin class makes your body secrete the same biomarkers at the same heart-attack levels. (The levels return to normal after 24 hours, which is different than in a heart attack, where biomarkers can be elevated for at least several days.)
"Heart attack" and "spin class" in the same sentence seems scary, but high levels of these biomarkers after exercise doesn't mean you're actually having a heart attack--the study's authors say that the biochemical reaction is normal and healthy.