A high intensity training stress must be imposed to stimulate muscle growth. The stimulus for growth is only achieved when a stress sufficient to challenge the body's existing ability is imposed. The body will adapt to the demands placed upon it so long as it has the sufficient resources. If the stresses placed upon it can be handled by current capacity then there is nothing to adapt to. Meaning that the muscle must be pushed to the point where it's current capacity to perform is exhausted and the body must tap it's reserve capacity. It is only then that the growth process in put into motion to adapt to the imposed stress to protect it from future damage.

Intensity and volume of work are inversely proportional. Meaning the more intense the work the less of it in volume you can perform and the more volume of work you perform the less intense it will be. Thus, if a high intensity training stress must be imposed to produce muscular growth then logic dictates that it must be very intense and very brief.

Recuperation is equally important to achieving muscular growth. The stimulus for growth is achieved while training, but the actual growth occurs while resting. It must have time to repair muscle damage and add new muscle.

Third, progressive resistance must be applied. When the body has adapted to the previous stress a greater stress must be imposed to achieve new muscle growth.