Chalene Johnson, Irvine-based fitness instructor and video and fitness infomercial host: "The No. 1 thing is too much momentum with the weights. The speed of the repetition is way too fast. People are literally swinging the weights and lifting them so fast that the velocity builds momentum but decreases the overload to the muscle, which makes the exercise less effective. The slower the speed, the more effective.
"The quicker you can reach overload by going slower, the more effective the muscle contraction and the results. You're trying to make the exercise as difficult as possible to break the muscle fiber down. Overload creates a momentary failure so that you can't do another rep, and that process breaks the muscle fiber down. That's what causes the muscle to repair itself, leading to increased muscle mass, a higher metabolism and decreased body fat.