The electrical impulse has a series of characteristics which define it, in a way that changing any of the parameters below will result in one type of current or the other. In this way, we can decide whether we want to mostly recruit fast or slow muscular fibres, or a mix of both; or whether we want to achieve either a relaxing or a stimulating effect, or whether we want to develop resistance, speed, or to improve vascularisation.

1.-Frequency of impulse. This is one of the most important parameters since it is what allows us to regulate which fibres are to be activated. The frequency indicates to us the number of times per second an impulse will be produced. This rhythmic cadence will stimulate slow fibres in our muscular system at a low frequency and the quick fibres at a high frequency. If we want to really improve our muscular resistance we will have to work with a range of frequency approximately between two and thirty hertz; between 30 and 70 Hz a combination of both fast and slow fibres are activated; and between 80 and 120 Hz the quick muscle fibres are stimulated intensely. As we increase the frequency and intensity, we come close to a tetanising contraction. The stimulation is produced very quickly and doesn’t cause relaxation; consecutively, more muscle fibres are recruited which allows for very powerful contractions.

2.-Impulse time or width.

This indicates the time during which the stimulus will be applied. The time of the impulse is measured in microseconds, as established by Weiss’ Law which relates the extent of the impulse with the intensity applied, and the duration of the same. The concept of chronaxy comes from the Greek word ‘cronos’: time and axis; the chronaxy indicates to us the quantity of time necessary for the current to act on the muscle and the stimulus to produce a contraction with a amplitude twice that of the rheobase, which is the minimum amplitude of the current which, upon being applied for a sufficient amount of time, will produce a contraction.

Not all of the nerves, nor all of the muscles, have the same chronaxy. This means that each muscle will need a specific length of impulse so as to obtain a contraction. As the impulse increases the chronoxy more and more, the contraction will be all the more powerful.

3.-Intensity and amplitude of the impulse.

The amplitude is measured in amps which is the basic unit of intensity of the electrical current. The units which are used to measure the levels of amplitude which provoke muscle contraction are milliamperes.

When referring to the usage of an electro-stimulator it’s very important to know the sensations which are produced; thresholds are one series of sensations involved. The sensitive threshold would be that point at which we begin to perceive the current, the motor threshold is when we begin to notice the first contractions, and the pain threshold is the point which we want to avoid. We have to remember always that is is essential to avoid any painful sensation which can accompany the usage of the electro-stimulator.

It is clear that the aim is to program the work between the motor and pain threshold, always avoiding the pain threshold. However, it also becomes evident that working only in the motor threshold barely recruits the fibres, and that raising the amplitude of the impulse along with the adequate frequency will recruit more fibres and cause more intense adaptation phenomenon.

4.-Parameters of the duration of the stimulus and rest.

The duration of the contraction is measured in milliseconds. Milliseconds are also used to configure the amount of time the stimulus is to be maintained to cause the muscular contraption. There are factors which are directly related to this, such as the frequency, which is measured in hertz, the period of rest and the amplitude level. By definition, the rest period is the time between one contraction and the next. The rest period used will be directly related to the objective which we have initially established. If we are pursuing an improvement in resistance, the rest period will be greater than if we are training anaerobically. As we will see further on, higher intensities, higher frequencies and brief rest periods generate muscular strength and will demand a notable load adaptation from our body. However, low frequencies, between two and eight hertz will have a relaxing affect and will also ease the muscular recovery after the training.