Muscling past fitness myths

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.

Here are other fitness myths:

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-La Crosse. If you stop lifting weights 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, said Alex Hutchinson, author of "Which Comes First, Cardio or Weights?" Long stretches of sitting are associated with cardiovascular disease, independent of how much exercise you get.

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. But if you experience pain while running, stop and talk to your doctor.

Try this: Run on a variety of surfaces and incorporate 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 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, Hutchinson said. 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.

Myth: Static stretching improves performance and decreases injury rates.

Truth: The latest thinking shows that old-fashioned static stretches -- standing on one leg, grabbing your foot and pulling the heel to your butt -- before a workout don't necessarily help and can cause injuries.