By increasing your speed of movement you can dramatically increase the number of muscle fibers involved. However, in many cases this is extremely dangerous, leading to the muscle tearing loose from its attachment. Not fun or desirable.  As well as the risk of severe injury, increasing the speed of movement will often involve extra momentum.  By using overall body motion to “cheat”, the intensity shifts away from the muscles and body parts you are trying to stimulate.

So keep this in mind. In the case of the barbell curl example, the first repetition should be performed in perfect form, but at a pace that is considerably slower than is actually possible.  A pace that will allow you to perform each repetition as fast as possible without risking injury.

The bottom line is this. Regardless of how you perform the first repetition of the barbell curl, you will still only be involving a very small percentage of the muscle fibers available.  This is due to the following three reasons.  (1) When the muscle is not in a fully contracted position, only a limited number of muscle fibers are involved.  (2) During the first repetition the biceps muscle fibers are at their strongest and most rested.  (3) The majority of the exercise equipment on the market provides practically no resistance to a muscle in a fully contracted position.

Zero resistance can become an issue during almost all basic exercises.  For example, during the bench press, a totally contracted position is reached when the arms are completely extended.  Because your arms also reach a point of total “lockout” of the elbow joints at this point, there is basically zero resistance in this position.  The way this problem can be resolved is by performing all of your exercises in a series of repetitions. You can then make sure that you achieve maximum stimulation of the muscles involved and stimulate fast muscle gains.