Myth #1: "Lifting heavy weights will cause too much bulk."
Women especially seem to be worried about weight training for fear of their muscles bulging. The reality is that when women become stronger, their muscle actually becomes tighter and more compact. Muscle is very dense. When first beginning weight training, it is common for people to notice girth measurements going down, and clothes fitting looser. Although you are getting smaller, it may not show on the scale at first since your muscle is becoming more dense. The more muscle a person has, the higher their metabolism will be, and the better able they are to burn fat. Hence, weight training helps women burn fat and become more toned and defined. Men, on the other hand, will be able to build mass with proper weight training and nutrition. One common mistake that men make when trying to increase muscle size is that they do not also increase healthy calorie consumption.
Myth #2: "Muscle will turn to fat when workouts subside."
Muscle tissue and adipose tissue (fat) are two different things. When a person has more muscle on their body, their metabolism works harder to burn more fat for them. If a person then stops weight training, they will lose muscle which in turn causes the body to store more fat due to their slowed metabolism.
Myth #3: "If you aren't sore after a workout, you didn't work hard enough."
DOMS (Delayed Onset Muscle Soreness) is what some people experience following a workout. The onset and how long the soreness lasts is different for everyone. DOMS is caused by tiny tearing in the muscle that happens during the eccentric phase of an exercise. When the muscle rebuilds to repair these tears, it becomes stronger. Just because you do not experience DOMS does not mean your workout was not effective. The only way to know if your workout was effective is whether you increased the intensity from the last time you did it, and went to your full maximum with proper form. If you did those things, that is all that matters. Each person has a different body so each person's recovery and experience with DOMS will be different.
Myth #4: "Cardio is the best way to lose fat."
Cardio does burn calories, but because cardio does not build muscle as effectively as weight training, you will not experience further calorie burn after the workout like you get from weight training. The best approach is to use a combination of weight training and different types of cardio, plus proper nutrition.
Myth #5: "Ab exercises will give you a flat stomach."
There is no such thing as "spot reduction." The more muscle that you have on your whole body, the higher your metabolism will be, and the better able you are to lose fat on your whole body. When you do exercises that target specific areas, that does help to tone and define, but you won't be able to see that muscle if there is too much fat covering it up. The best approach is to do compound exercises that use large muscle groups. Don't waste your time doing a half an hour "ab workout," rather, aim for a full body conditioning program.