1.MYTH: Crunches are the key to flat abs.

FACT: They may be the most iconic abdominal exercise around, but doing crunches is not actually the best way to slim your midsection.

2. MYTH: The more you sweat, the more you burn.
FACT: Sweat is a biological response that cools your skin and regulates internal body temperature,” Matthews says. It's just as apt to be the result of an overheated studio, the weather or your personal physiology as it is a grueling gym session

3. MYTH: Stretching helps your body recover faster
FACT: Keep doing it if it feels good to you, but a recent University of Milan study on the effects of postworkout recovery methods found no significant changes in blood lactate levels (a measure of how fatigued your muscles are) in folks who stretch after exercise.

info ref: Health.com