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6 Applications and Methods in Biosignal Processing

external stimulus is applied to a sensory organ and the EEG response is recorded. The

stimulus may be, for example, visual, auditory, or olfactory⁶. Evoked potentials usu-

ally result in only a very small deflection in the EEG. The detection is done by repeat-

ing the stimulation at specific time points. The EEG sequence following each stimulus

consists of a stimulus-independent part and an evoked potential sequence charac-

teristic of the stimulus. The stimulus-independent EEG component can be stochastic

signal. If many EEG sequences are added up, the stimulus-independent parts aver-

age out, and the evoked signal components of the individual EEG sequences with the

same course are emphasized in the sum. The larger the number of stimuli and thus

of EEG sequences, the more clearly stands out the sum of the evoked potentials from

the stimulus-independent part. The evoked potentials method is used in neurology to

examine the functionality of sensory organs and nerve tracts.

6.2 Signals of the Muscles and Motions

The first measurements by electromyography (EMG) date back to H. Piper in the year

1912 [59]. Since then, a large number of research groups have studied the measure-

ment methodology and the evaluation in relation to muscle physiology and the basic

processes. The fields of application of electromyography are manifold. EMG provides

direct access to the action potentials of the involved muscle cells and their propaga-

tion along the fibers. Thus, myopathies such as myasthenia, amyloidosis or multiple

sclerosis, but also disorders in the neuronal stimulation of muscle cells can be in-

vestigated. In sports science, EMG measurement offers the possibility to monitor the

training condition, to follow the muscle build-up after injury or to analyze and op-

timize the course of movement. Active orthopedic prostheses are controlled by EMG

signals, among other things. In ergonomics, stress conditions can be detected.

The force and speed of movement resulting from muscle contraction depend both

on the number of stimulated muscle cells and on the frequency with which the action

potential of the cells is triggered. It is therefore obvious to be able to draw conclusions

about physiological processes via the EMG measurement. With increasing force, both

the EMG amplitude and the frequency of stimulations increase. The axon of a neuron

branches out to up to 500 muscle cells in the limbs. In a healthy muscle fiber, the

action potential propagates from the coupling point of the electrical excitation with

a velocity of about 4 m/s over the elongated muscle fiber, whereas the propagation

velocity along the axon is approximately 100 m/s.

Individual motor units have dimensions on the order of 10 mm. The purpose of

needle EMG is to measure the action potential of single to multiple muscle fibers in

order to study their physiology or pathology. Pathologies may involve the muscle fibers

6 Lat. olfacere = to smell