KINESIOLOGICAL ELECTROMYOGRAPHY - Biomechanics and Motor Control of Human Movement - page 251

10.1.1 Motor End Plate
For a given muscle, there can be a variable number of motor units, each
controlled by a motor neuron through special synaptic junctions called motor
end plates . An action potential transmitted down the motoneuron (sometimes
called the final common pathway ) arrives at the motor end plate and trig-
gers a sequence of electrochemical events. A quantum of ACh is released;
it crosses the synaptic gap (200 - 500 A wide) and causes a depolarization
of the postsynaptic membrane. Such a depolarization can be recorded by a
suitable microelectrode and is called an end plate potential (EPP). In normal
circumstances, the EPP is large enough to reach a threshold level, and an
action potential is initiated in the adjacent muscle fiber membrane. In disor-
ders of neuromuscular transmission (e.g., depletion of ACh), there may not
be a one-to-one relationship between each motor nerve action potential and a
m.u.a.p. The end plate block may be complete, or it may occur only at high
stimulation rates or intermittently.
10.1.2 Sequence of Chemical Events Leading to a Twitch
The beginning of the m.u.a.p. starts at the Z line of the contractile element
(see Section 9.0.1 ) by means of an inward spread of the stimulus along the
transverse tubular system. This results in a release of Ca 2 + in the sarcoplasmic
reticulum. Ca 2 + rapidly diffuses to the contractile filaments of actin and
myosin, where ATP is hydrolyzed to produce ADP plus heat plus mechanical
energy (tension). The mechanical energy manifests itself as an impulsive
force at the cross-bridges of the contractile element. The time course of the
contractile element's force has been the subject of considerable speculation
and has been modeled mathematically, as described in Section 9.0.5.
10.1.3 Generation of a Muscle Action Potential
The depolarization of the transverse tubular system and the sarcoplasmic
reticulum results in a depolarization “wave” along the direction of the muscle
fibers. It is this depolarization wave front and the subsequent repolarization
wave that are “seen” by the recording electrodes.
Many types of EMG electrodes have developed over the years, but gener-
ally they can be divided into two groups: surface and indwelling (intramus-
cular). Basmajian (1973) gives a detailed review of the use of different types
along with their connectors. Surface electrodes consist of disks of metal, usu-
ally silver/silver chloride, of about 1 cm in diameter. These electrodes detect
the average activity of superficial muscles and give more reproducible results
than do indwelling types (Komi and Buskirk, 1970; Kadaba et al., 1985).
Smaller disks can be used for smaller muscles. Indwelling electrodes are
required, however, for the assessment of fine movements or to record from
deep muscles. A needle electrode is nothing more than a fine hypodermic