KINESIOLOGICAL ELECTROMYOGRAPHY - Biomechanics and Motor Control of Human Movement - page 253

the depolarization, and a sink of current, − I , representing the repolarization,

both separated by a distance b . The potential , recorded at a point electrode
at a distance r from the current source, is given by:

I
4 II σ ·

1
r

= (10.1)

where σ is the conductivity of the medium, which is assumed to be isotropic
(uniform in all spatial directions).
The net potential recorded at the point electrode from both source and sink
currents is:

I
4 II σ ·

1
r 1 −

I
4 II σ ·

1
r 2

=

1
r 1 −

I
4 II σ

1
r 2

= (10.2)

where r 1 and r 2 are the distances to the source and sink currents.
The time history of the action potential then depends on r 1 and r 2 , which
vary with time as the wave propagates along the muscle fiber. At t 1 ,asthe
wave approaches the electrode (r 1 < r 2 ) , the potential will thus be positive
and will increase. It will reach a maximum at t 2 ; then as r 1 becomes nearly
equal to r 2 , the amplitude suddenly decreases and passes through zero at t 3
when the dipole is directly beneath the electrode (r 1 = r 2 ) . Then it becomes
negative as the dipole propagates away from the electrode (r 1 > r 2 ) . Thus, a
biphasic wave is recorded by a single electrode.
A number of recording and biological factors affect the magnitude and
shape of the biphasic signal. The duration of each phase is a function of
the propagation velocity, the distance b (which varies from 0.5 to 2.0 mm)
between source and sink, the depth of the fiber, and the electrode surface area.
Equation (10.2) assumes a point electrode. Typical surface electrodes are not
point electrodes but have a finite surface area, and each point on the surface
can be considered an area of point sources; the potential on the surface is the
average of all point-source potentials. Figure 10.2 is presented from a study by
Fuglevand et al. (1992) to demonstrate the influence of the electrode size and
shape on the electrode potential. Two different orientations of strip electrodes
are shown: A — along an axis parallel to the muscle fiber — and D — at right
angles to the muscle fiber. B and E show the location of a strip of point-source
electrodes to represent these two strip electrodes, each 10 mm long with 10
point sources; C and F show the action potentials at each point source. For the
strip electrode in A, the distance from the points on the electrode to the fiber,
r f , is constant; thus, each action potential has the same amplitude and shape.
However, there is a phase difference between each of the point-source action
potentials such that the average action potential is lower and longer than the
individual action potentials. For the strip electrode in D, the distance r f is