1 2 . e l e c t r O t H e r a P y t H e O ry
102
EN
WIRELESS PROFESSIONAL
12.2.2.4 duration of rectangular electrical pulse
First of all, it must be specified that this is in a specific pulse duration phase. Weiss’ law is used for
stimulation pulse durations close to the excitation constants
𝑘
.
In the case of motor neurons, this means a time period ranging from 100 to 3,000 microseconds.
k = Chronaxy / In
2
= Chronaxy / 0,693
The third electrical factor, which should be minimised in order to produce the most comfortable possible
stimulation, is electrical energy
𝑊
.
We know that electrical energy is given by the formula
𝑊
=
𝐼
2 .
𝑡
.
𝑅
, where:
𝐼
: is the current intensity
𝑡
: its pulse duration
𝑅
: the skin resistance
The Weiss or Lapicque relationship states
𝐼 = 𝑞 /𝑡 + 𝑖
and we can replace I by its value in the energy equation.
We get
𝑊 = (𝑞 /𝑡 + 𝑖) 𝑡.𝑅.
by developing:
𝑊 = (𝑞
2
/𝑡
2
+ 2 𝑖 𝑞 /𝑡 + 𝑖
2
) 𝑡.𝑅. = (𝑞
2
/𝑡+ 2 𝑞 𝑖 + 𝑖
2
𝑡) 𝑅.
When
𝑡
→ 0,
𝑊
→ ∞
When
𝑡
→ ∞,
𝑊
→ ∞
The shape of this curve is given in Figure 4.
Fig. 4
Fig. 5
Relationship between energy and
the pulse duration
W minimum
Variations in electrical energy depending
on skin resistance
𝑊 = (𝑞
2
/𝑡+ 2 𝑞 𝑖 + 𝑖
2
𝑡) 𝑅𝑛
where
𝑅1
›
𝑅2
›
𝑅3