Many have claimed that reps should be done slowly. Sometimes people design exercise theory around limitations of their equipment. If pushing a machine fast causes it to break, then you would tell people to do there reps slow when those machines are in use.

Then there is the "Go for the burn" people who will try to convince you that the pain you get from the slow reps is necessary for growth. Here is what I have to say about that "bullshit." Muscles are made and designed for speed. You don't see people walking around at .00001mph. The faster you can move the weight through space, the more stress is applied to the muscle. That is the exact opposite of what slow trainers are saying.

And as for the burning sensation that you get when you do super-slow reps, that is from a buildup of lactic acid in the muscles and not from any increased stress. The two things that stimulate your muscles are the weight and the speed of the muscle contraction. When you do slow reps, you must use less weight than normal and keep yourself from contracting too fast. In essence you are doing the exact opposite of what you should be doing for maximum muscle growth.

The only problem with training at maximum speed is that the momentum created can lead to sloppy from, risking injury. Also, if the speed of the repetition is too fast, it can reduce the stress to the muscle by relying on more momentum.

That is why the best approach to an exercise is to perform each repetition fast as possible while still maintaining proper form and constant control of the weight. This will produce maximum stress while minimizing the chance of injury.