To cause an isometric contraction or a voluntary movement, the brain generates an impulse which travels through the central nervous system to the motor nerve. It produces a discharge in the muscular motor plate and creates a regular contraction or movement. It’s a very complex process; at the same time neurons very quickly replace others, producing polarisation and depolarisation pheromones until finally the desired objective is achieved: a contraction. The intensity of this will be more or less directly related to the intensity of the initial stimulus, such that a threshold must be reached to induce the minimum contraction. Below this threshold a contraction will not be produced, and as the stimulus raises, so does the intensity of the contraction or of the movement.
Neuromuscular electro-stimulation causes an electrical discharge in the skin which travels to the motor nerve and induces a discharge in the motor plate, thus producing a contraction. It is not the brain so much as the electro-stimulator which is in charge of generating the electrical impulse which, travelling through the nerve, has the desired affect. Whether as a voluntary contraction or as a contraction caused by electrical equipment, the result is the same; a muscular contraction.