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1 2 . e l e c t r O t H e r a P y t H e O ry
98 EN WIRELESS PROFESSIONAL
The chronaxy corresponds to the time value when
𝑄
= 0
i.e.
𝑄 = 𝑞 + 𝑖𝑡
if
𝑄
= 0 therefore
𝑞 + 𝑖𝑡
= 0 therefore
𝑖𝑡
=
𝑞
and
𝑡 = 𝑞 /𝑖
Fig. 4
12.1.2 Summary
Electrical stimulation, i.e. reducing resting potential to the stimulation threshold using an electric current, is
a phenomenon that fulfils a fundamental physiological law.
This shows us that:
1 The factor determining stimulation is the quantity of electrical
charges created by the current.
Stimulation must be considered in terms of the amount of current, which is the
product (
𝐼 𝑥 𝑡
) of the intensity (
𝐼
) times the duration of the pulse (
𝑡
).
2 This amount of current fulfils a fundamental formula:
𝑄 = 𝑞 + 𝑖𝑡
where
𝑄
is a linear function of time.
Lapicque expresses this formula in another way by the “intensity - pulse duration”
ratio:
𝐼 = 𝑞 /𝑡 + 𝑖
and he deduced that
a) the rheobase (
𝑅ℎ
): minimum intensity that must be reached in order to produce
stimulation using an infinite pulse duration
𝑅ℎ
=
𝑖
b) the chronaxy (
𝑡𝑐ℎ
): minimum time in which a current with double the intensity of
that of the rheobase must be applied in order to produce stimulation
𝑡𝑐ℎ
=
𝑞 /𝑖
We can note that the chronaxy can be calculated mathematically from Weiss’ fundamental formula as
shown in Figure 4.