If you still think sit-ups will reduce your belly flab, we’ve got some depressing news: You’ve fallen for one of the all-time great exercise myths.

Fitness misconceptions are rampant, in part due to misleading infomercials, but also because scientific results are mixed on some commonly held beliefs. Meanwhile, what works for one person doesn’t necessarily benefit another.

Some fitness myths are harmless half-truths. Others, however, scare people away from working out or lead to injury. The spot-reduction myth — which holds that you can, for example, flatten your stomach by doing crunches — wastes time, effort and can even add size to your middle.

Ab exercises can strengthen muscles. But they don’t remove fat because from a metabolic standpoint, fat isn’t connected to the muscle it covers. That means working certain muscles might make them bigger, but it doesn’t necessarily burn calories from the fat covering them.

The problem with fitness science is that while we want simple answers, "humans are really complicated," said Alex Hutchinson, the author of the book "Which Comes First, Cardio or Weights?" a comprehensive science-based look at fitness myths. Some things you think you know may be misguided:

Myth: When you stop exercising, your muscle turns to fat.

Truth: Rocks don’t turn into trees. Likewise, muscle won’t morph into fat because they’re different types of cells, said Brian Udermann, a professor in the department of exercise and sport science at the University of Wisconsin at La Crosse. Although you can lose muscle mass, one doesn’t replace the other. When you gain muscle mass, the muscle fibers or individual cells get bigger, Udermann said. If you stop lifting or have your leg in a cast, the muscle fibers don’t go away, they just shrink. The same thing happens when you gain fat; the existing cells get bigger. If you lose weight; the fat cells decrease in size.

Try this: Incorporate two resistance training sessions a week. This could include using body weight, free weights, resistance bands, kettlebells or machines.

Myth: You can sit for long periods if you exercise.

Truth: Unfortunately, you can’t exercise away the effects of sitting for 10 hours at your desk, Hutchinson said. Long stretches of sitting are associated with cardiovascular disease, independent of how much exercise you get. Researchers think being motionless for long periods of time without a break causes changes in the enzyme levels in your muscles, allowing for more fat storage. "The muscle says, ‘I’m not needed!’" Hutchinson said. "So it helps to take short breaks throughout the day."

Try it: Get up at least once every hour; pace around your desk or do five jumping jacks to remind your muscles that you’re not dead. Prompt yourself by setting an email reminder.

Myth: Running is bad for your knees.

Truth: What’s really hard on the knees is extra body weight. Each additional pound of body mass puts 4 additional pounds of stress on the knee, according to the American College of Sports Medicine. Meanwhile, two recent studies have suggested that running protects the knees, Hutchinson said. One of these studies looked at 45 runners and 53 nonrunners over 18 years. The Stanford researchers found that the runners had lower rates of arthritis in the knee than the nonrunners. That said, if you experience pain while running, stop and talk to your doctor.

Try this: Try running on a variety of surfaces and incorporating balance training and knee-strengthening exercises including squats. Elliptical machines offer a no-impact alternative, but they train your legs to move in elliptical patterns, which isn’t very useful in real life.

Myth: To tone muscles without bulking up, lift light weights and don’t push hard.

Truth: There’s actually no such thing as toning, said Hutchinson. If you’re poking a muscle that feels soft even when it’s flexed, that means you’re poking fat, not "untoned" muscle," he said.

Try this: To make your muscles stand out, you either have to lose fat or make your muscles bigger. Light weights won’t help you do either unless you do enough reps to reach or get close to failure (exhaustion).

Myth: Muscles lengthen.

Truth: Muscles have what’s called an origin and an insertion. Both are fixed and attached to bone, said personal trainer Tom Holland. "In order to lengthen it, you’d have to detach it and re-attach it farther down the bone."

Try this: Make your muscles look longer by performing exercises like seated cable rows or bent-over dumbbell rows that pull your shoulders back and help dramatically improve your posture, Holland said.

Myth: Static stretching before an event improves performance and decreases injury rates.

Fact: The latest thinking on stretching (and this changes on a regular basis) shows that old-fashioned static stretches — standing on one leg, grabbing your foot and pulling the heel to your butt — before a workout doesn’t necessarily help and can cause injuries.

Try this: Before a workout, think warm-up rather than stretch, Holland said. Do a light cardio exercise to increase blood flow to the muscles and elevate your core temperature. Save the old-school static stretching for after the workout, when you’re warmed up. Spend 30 to 60 seconds on each muscle after exercise, Holland said.