Let’s look at an example. Performing bench presses with 200 pounds of weight for ten repetitions requires exactly same amount of work, regardless of how you perform movement. If you cheat by using momentum to complete repetitions however, only a very small number of muscle fibers will have been stimulated. The same amount of energy and time was used, but muscles being worked were not fully involved. The only thing you have dramatically increased with this technique, is chance of injury.

I’m not implying that such “cheat” methods should never be used. They can be used at end of a given set of repetitions, if they can be done safely. In short, all repetition should be performed in perfect form, until which point it becomes impossible to perform any more without slight use of momentum. At this point in set, only enough momentum to complete an additional one to two repetitions should be used. And only if it can be done so safely.

You see, muscles are made up of ‘muscle fibers’. Muscles themselves work by contracting and reducing their length. In order for muscles to contract, they must move. For a muscle to produce movement, and therefore power to move a given weight, it must do so by lengthening and contracting.

In other words, a muscle performs exercise by contracting, and by doing this it generates force and power. While a muscle uses some of it’s fibers to perform a given exercise, it almost never uses all of them at same time.

For a muscle to contract every one of its available fibers at same time, it must be in a totally contracted position. To increase your muscle mass in shortest time possible, maximum number of muscle fibers possible must be “stimulated”. The easiest way to achieve this is through multiple repetitions of a particular exercise. More often than not it is impossible to contract all of muscle fibers in a specific part of body, using only one repetition, without risking injury. Multiple repetitions however, allow it to be done safely.

As a general rule, it’s almost impossible to perform an exercise in a way that contracts all of muscle fibers of body parts involved. However, if exercise is performed with intensity, many more muscle fibers will be stimulated, than there would have been otherwise.

As an example, when you perform a basic exercise like barbell curl (standing upright with a barbell and curling your arms from resting against your thighs up towards your chest), you are using fibers of biceps muscle of upper arm. At beginning of exercise, when performing first repetition of a set of ten, your biceps muscles are at their strongest and most rested.

However, during first repetition you can only involve a minimal number of muscle fibers available. Most of fibers are unable to contract unless in a totally contracted position. The bicep itself will only use minimal number of fibers needed to perform that one repetition. Muscle fibers will only perform at full capacity, and are only “recruited” by a specific body part as they are needed.

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