The answer, it turns out, is that there are no cutting-edge scientific studies.

I don’t mean that exercise physiologists don’t conduct brilliant research. They do. I mean that they rarely research the practical questions you and I want answered, like which workout routine is best.

“A lot of physiologists come into the discipline because they fundamentally like exercise,” Martin Gibala, an exercise physiologist at McMaster University in Ontario, told me. “But you learn very quickly that there’s not a lot of research money out there to fund applied studies.” On matters as simple as how many sets and reps best promote muscle growth, Mr. Gibala explained, “We can’t nail down the answer.”

Even if the funding were there, Mr. Gibala says, “That’s not state-of-the-art research that you’re going to publish in the best journals and advance your career.” Instead, he says, physiologists study questions of basic science, “like the molecular signaling proteins that regulate skeletal muscle adaptation.”

You know, those.

Of course, Mr. Gibala and his peers are not the problem. The problem is that everybody in the fitness industry grabs onto this basic science — plus the occasional underfunded applied study with a handful of student subjects — and then twists the results to come up with something that sounds like a science-backed recommendation for whatever they’re selling. Most gym owners, for example, want you to walk in the door on Jan. 2 and think, Hey, this looks easy. I can do this. So they buy stationary cardio and strength machines that anybody can use without hurting themselves, often bearing brand names like Sci-Fit (Scientific Solutions for Fitness), which might more accurately be described as scientific solutions for liability management.

As for personal trainers, I’ve known great ones. But the business model is akin to babysitting: There’s no percentage in teaching clients independence by showing them basic barbell lifts and telling them to add weight each time. Better to invent super-fun, high-intensity routines that entertain and bewilder clients, so they’ll never leave you. The science of muscle confusion, in other words, looks a lot like the marketing tradecraft of client confusion.

THEN there’s the matter of our collective cravings. From cable news to the nation’s great newspapers, there is a tacit understanding that in fitness stories you and I want to hear variations on exactly one theme: that a just-published research paper in a scientific journal identifies a revolutionary new three-and-a-half minute workout routine guaranteed to give you the body of an underwear model. So powerful is this yearning — this burning ache to look good naked and have great sex and live forever — that even the best-intentioned of fitness journalists scour every little academic study for anything that might justify telling you that same sweet story, one more time.

Steven Devor, an exercise physiologist at Ohio State University, says that people in his profession have become painfully aware of this problem. “A lot of my colleagues would rather poke themselves in the eye than talk to the media,” he says.