Separating fitness fact from fiction: experts debunk 18 common exercise myths

The fitness industry runs on certainties that feel obvious until you actually look at the evidence. Sit-ups will flatten your stomach. Stretching before a workout prevents injury. Muscle turns to fat when you stop training. These ideas have calcified into common sense, passed from gym to gym, from friend to friend, until they feel like laws of physics rather than assumptions worth questioning.

Daniel Lieberman, a paleoanthropologist at Harvard and author of a book on exercise science, offers a correction on the first one: sit-ups will strengthen your core, but they won't eliminate belly fat. The mechanism is simpler than most people think. When your body begins to lose weight through exercise, belly fat is the first to go—not because of targeted movements, but because humans evolved to store energy there as a short-term reserve. What matters is overall calorie deficit. A personal trainer whose clients have included Jennifer Lopez and Robbie Williams puts it plainly: compound exercises like deadlifts and squats burn far more calories than isolated movements like sit-ups, so they're more effective for fat loss.

The soreness that follows a new workout routine feels like proof of progress, but it's actually just surprise. Delayed onset muscle soreness, or DOMS, typically appears after your first few sessions at the gym, or when you try something completely different, or push longer than usual. Some muscles—the biceps, for instance—rarely get sore at all, yet working them clearly produces results. Soreness and success are not the same thing.

Stretching before exercise has been studied exhaustively. Hundreds of research papers have examined whether it reduces injury risk, and the consensus is that if an effect exists, it's too small to measure reliably. People confuse stretching with warming up. A light jog or gentle movement will raise your body temperature and make muscles more supple. That warm-up doesn't require stretching. The distinction matters because it changes what you actually need to do.

The 10,000-step target originated without any scientific basis—a pedometer company simply chose the number because it seemed auspicious. Since then, studies have validated it as a reasonable measure of daily activity. Benefits increase as step count rises, but they plateau somewhere between 8,000 and 10,000. There's nothing magical about the round number, but it's solid advice. The better target is probably just a little more than whatever you're currently doing.

Muscle and fat are entirely different tissues. One does not transform into the other. If you stop exercising and eat more calories, you gain fat. If you stop stimulating muscles, you lose muscle mass. The confusion arises when highly active people who eat large amounts suddenly stop training due to injury or circumstance. If they maintain their calorie intake while ceasing resistance work, their muscles shrink and their fat increases—but the tissues don't convert. They simply change in opposite directions.

Running damages knees only in theory. If knees were like car shock absorbers, overuse would wear them out. But knees are living tissue that adapt to regular use. Underusing them is actually worse for the cartilage. Researchers tracking runners and non-runners over decades find no difference in knee problems between the groups. Running doesn't worsen your odds, though it doesn't guarantee immunity either.

Lifting weights won't accidentally make you bulky. Building muscle requires not just progressively heavier weights but also eating more calories than you burn—a calorie surplus. It's genuinely difficult to achieve. For people trying to lose weight, lifting in a calorie deficit is efficient: it burns fat while preserving muscle, and stronger muscles mean heavier lifts, which means more calories burned overall. Recovery and rest matter more than most people realize. Three workouts per week with rest days between them is a solid benchmark for results. Training constantly just leaves you exhausted and unable to generate intensity.

Exercise becomes more important as you age, not less. It stresses the body in ways that activate repair and maintenance mechanisms. It produces antioxidants that repair DNA damage, reduces inflammation—the root cause of most diseases—and generates brain-derived neurotrophic factor, which helps prevent Alzheimer's. Study after study shows exercise slows the deterioration associated with aging. The older you get, the more you need it.