If you've been doing traditional sit-ups and getting faster and faster, let's face it, it doesn't do any good because it strengthens the wrong abdominal muscles. Luckily, a fitness scientist from San Diego State University in the United States tested the effectiveness of the thirteen most popular abdominal exercises and finally concluded on the four most effective "ab flab killers." To achieve the best results, these four exercises should be done in three sets every day, with each set lasting fifteen minutes.