I think if more people followed these scientifically-based exercises, there'd be no need for classes, gyms, coaches, etc. Article here.
"The exercises should be performed in rapid succession, allowing 30 seconds for each, while, throughout, the intensity hovers at about an 8 on a discomfort scale of 1 to 10, Mr. Jordan says. Those seven minutes should be, in a word, unpleasant. The upside is, after seven minutes, you’re done."