Since the early 1900's there have been so many myths perpetuated about exercise. Everything from if it doesn't hurt it is not doing any good, to stretching before, during or after a workout.
During my exercise class, a member of the group announced that she was told never to stretch before a workout. What she failed to realize is that our class warms up before stretching, she just never realized that using the treadmill for 5 minutes prior to the beginning of workout class was the warm up for the class.
Another myth is that you don't need to exercise if you use diet pills or eat correctly. Even the best diet pills in the world and the most balanced diet will not help your heart or lungs stay in shape for exertion. Exercise is not just for losing weight or looking good it is for staying out of the hospital with a heart attack or artery problems.
The one phase I hate the most as an exercise myth is "no pain, no gain," that saying alone will stop people from even wanting to exercise. Pain is your body's way of saying "stop" before you get badly injured. Listen to you body and don't go over the point of pain but just to that point and no further.