KINESIOLOGICAL ELECTROMYOGRAPHY - Biomechanics and Motor Control of Human Movement - page 256

10.1.4 Duration of the Motor Unit Action Potential
As indicated in the previous section, the larger the surface area, the longer
the duration of the m.u.a.p. Thus, surface electrodes automatically record
longer duration m.u.a.p.'s than indwelling electrodes (Kadefors, 1973; Bas-
majian, 1973). Needle electrodes record durations of 3 - 20 ms, while surface
electrodes record durations about twice that long. However, for a given set
of electrodes, the duration of the m.u.a.p.'s is a function of the velocity of
the propagating wave front and the depth of the motor unit below the elec-
trode's surface. The velocity of propagation of the m.u.a.p. in normals has
been found to be about 4 m/s (Buchthal et al., 1955). The faster the velocity,
the shorter the duration of the m.u.a.p. Such a relationship has been used to
a limited extent in the detection of velocity changes. In fatigue and in cer-
tain myopathies (muscle pathologies), the average velocity of the m.u.a.p.'s
that are recruited is reduced; thus, the duration of the m.u.a.p.'s increases
(Johansson et al., 1970; Gersten et al., 1965; Kadefors, 1973). Because the
peak amplitude of each phase of the m.u.a.p. remains the same, the area under
each phase will increase. Thus, when we measure the average amplitude of
the EMG (from the full-wave rectified waveform), it will appear to increase
(Fuglevand, et al., 1992). During voluntary contractions, it has been possi-
ble, under special laboratory conditions (Milner-Brown and Stein, 1975), to
detect the duration and amplitude of muscle action potentials directly from
the EMG. However, during unconstrained movements, a computer analysis of
the total EMG would be necessary to detect a shift in the frequency spectrum
(Kwatny et al., 1970), or an autocorrelation analysis can yield the average
m.u.a.p. duration (Person and Mishin, 1964).
The distance between the motor unit and the surface of the electrode
severely influences the amplitude of the action potential, as is predicted
in Equation (10.2), and also influences the duration of the action poten-
tial. Figure 10.4, taken from Fuglevand et al. (1992), predicts the dura-
tion and amplitude of the action potential from a 50-fiber motor unit with
electrode-unit distances out to 20 mm. It is evident that the duration of the
action potential increases as the amplitude decreases with distance. Thus,
the frequency content of the m.u.a.p.'s from the more distant sites decreases.
Fuglevand et al. (1992) showed that the mean power frequency of the m.u.a.p.
would decrease from 160 Hz at an electrode-motor unit distance of 1 mm to
25 Hz at an electrode-motor unit distance of 20 mm.
10.1.5 Detection of Motor Unit Action Potentials from
Electromyogram during Graded Contractions
Using recordings from multiple indwelling electrodes, DeLuca et al. (1982)
pioneered techniques to identify individual m.u.a.p.'s during a low-level
graded contraction. More recent techniques use a quadrifilar needle electrode
recording of three channels followed by decomposition algorithms involving