In order to make a tissue pass from the phase of rest to the one of excitation by means of an induced
electrical stimulus (impulse of the electrostimulator), some factors have to be considered:
- the stimulation intensity
- the duration of the stimulation that must be adapted to the body district that is wanted to be
stimulated.
Indeed, to excite a tissue, not only the pick intensity value is particularly important, but also the
relationship between the duration and the intensity of the stimulus.
This relationship varies according to the muscular districts.
Chronaxy and reobase
The graphical course that represents the relationship between the intensity of a stimulus (l) and its
duration (t) is not linear, as the studies carried out by Lapique demonstrate. Observing img. 1,
which shows the relationship between intensity and duration of an electrical stimulus provoked to
excite a target tissue, it turns out to be obvious that, an increase in the duration of the stimulus
corresponds to a decrease in its intensity.
Image 1
However, it does not spoil its effectiveness. On the other hand, when a stimulus of short duration is
wanted to be used, its intensity will have to increase for being able to reach the excitation threshold.
Lapique's important discover consists in having understood that for being able to obtain an
electrical stimulus, qualitatively valid, it is not sufficient to establish a parameter and consequently
derive the other. That depends on the intrinsic features of the organic tissues, which try to get
accustomed to a constantly repeated stimulus. This phenomenon, called "tissue accommodation",
involves the realization of a stimulus that does not depend on it.
The two parameters individuated by Lapique, necessary in order to obviate the problem, are:
REOBASE and CHRONAXY.
REOBASE: it is the value of minimum intensity in order to excite the tissue independently from its
duration.
CHRONAXY: it is the duration of the stimulus, with a double intensity compared to the reobase, it
is necessary in order to excite correctly the target tissue.
Once the chronaxy has been identified, a suitable stimulus is automatically produced to excite
correctly the target muscle group. In this way it is also possible to avoid all those typical
annoyances of an extended electrical stimulation such as a burning sensation and formication.
During the researches to define the electrostimulation programs, therefore, it is indispensable to
consider the chronaxy, which varies according to the muscular group that you want to stimulate.
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