Electromyography
Electromyography (EMG) is the measurement of the electrical signal associated with muscle activity. The recorded signal reflects the electrical signal associated with the action potentials of the motor units. The equipment used for EMG data collection is
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Electromyography
Overview Electromyography (EMG) is the measurement of the electrical signal associated with muscle activity. The recorded signal reflects the electrical signal associated with a cascade of events related to the generation of muscle force. The actual recorded signal varies with various aspects of the measurement equipment, as well as with the amount of muscle activity. Compared with many other methods used in biomechanics, there are more variables to consider when appropriately collecting and processing an EMG signal. That the production of muscular force is associated with electrical activity was first observed by the Italian physician Luigi Galvani (1737–1798). He examined the leg muscles of a frog and identified that “animal electricity” was important for the generation of muscular force (Galvani 1791). In 1849 the German physician Emil du Bois-Reymond (1818–1896) was the first to record the electrical signal associated with a voluntary muscular activity, examining the human upper limb (Du BoisReymond 1849). Our ability to measure muscle electricity activity greatly increased in the last century due to the development of microelectronics and the availability of computers; in addition our ability to process and analyze these signals was advanced by developments in signal processing. The content of this chapter requires a working knowledge of signals and signal analysis (see Chaps. 3 and 4).
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What Are the Electrical Events?
If electromyography is measuring the electrical signal associated with the activation of the muscle, what is the source and magnitude of this electrical activity? The functional unit within a muscle responsible for responding to electrical activity and producing force is the motor unit. The motor unit comprises the cell body and dendrites of a motor neuron, the branches of its axon, and muscle fibers it directly innervates. For a single motor neuron the number of muscle fibers it innervates varies between muscles, for example, the platysma has a ratio of 1:25, the first dorsal interosseus 1:340, and the medial gastrocnemius 1:1600 (Feinstein et al. 1955). Similarly the number of motor units varies between muscles: the platysma has ~1000, the first dorsal interosseus ~120, and the medial gastrocnemius ~580 (Feinstein et al. 1955). The fibers associated with a motor unit are not necessarily located adjacent to one another; the fibers from different motor units are intermingled (Bodine et al. 1988). The stimulus from a motor neuron is an action potential (see Fig. 5.1). The resting potential within a muscle fiber is about 90 mV, and the peak during an action potential is 40 mV (see Fig. 5.1 for typical time course). Because of the anatomy of the motor unit when a muscle fiber is stimulated, other muscle fibers will also be stimulated; therefore the recorded EMG signal represents the sum of multiple muscle fiber action potentials and results in the recording of a motor unit action potential. In many cases the EMG signal is the approximate sum of multiple motor unit action potentials (e.g.
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