Biomecanica Angular

SURFACE ELECTROMYOGRAPHY: DETECTION AND RECORDING
Carlo J. De Luca © 2002 by DelSys Incorporated. All rights reserved.

CONTENTS
GENERAL CONCERNS ................................................................................................... 2 CHARACTERISTICS OF THE EMG SIGNAL ....................................................................... 2 CHARACTERISTICS OF THE ELECTRICAL NOISE.............................................................. 3 MAXIMIZING THE FIDELITY OF THE EMG SIGNAL ............................................................ 3 ELECTRODE AND AMPLIFIER DESIGN ............................................................................. 4 ELECTRODE GEOMETRY ............................................................................................... 6THE PARALLEL-BAR ELECTRODE ................................................................................... 7 EMG ELECTRODE PLACEMENT ..................................................................................... 8 REFERENCE ELECTRODE PLACEMENT ........................................................................... 9 ELECTRICAL SAFETY CONCERNS................................................................................... 9 EMG SIGNAL PROCESSING........................................................................................... 9 APPLICATIONS OF THE EMG SIGNAL............................................................................ 10

© DelSys Incorporated

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INTRODUCTION TO SURFACE EMG

GENERAL CONCERNS

GENERAL CONCERNS
When detecting andrecording the EMG signal, there are two main issues of concern that influence the fidelity of the signal. The first is the signal to noise ratio. That is, the ratio of the energy in the EMG signal to the energy in the noise signal. In general, noise is defined as electrical signals that are not part of the wanted EMG signal. The other is the distortion of the signal, meaning that the relative contributionof any frequency component in the EMG signal should not be altered.

CHARACTERISTICS OF THE EMG SIGNAL
It is well established that the amplitude of the EMG signal is stochastic (random) in nature and can be reasonably represented by a Gausian distribution function. The amplitude of the signal can range from 0 to 10 mV (peak-to-peak) or 0 to 1.5 mV (rms). The usable energy of the signal islimited to the 0 to 500 Hz frequency range, with the dominant energy being in the 50-150 Hz range. Usable signals are those with energy above the electrical noise level. An example of the frequency spectrum of the EMG signal is presented in Figure 1.

Figure 1: Frequency spectrum of the EMG signal detected from the Tibialis Anterior muscle during a constant force isometric contraction at 50% ofvoluntary maximum.

© DelSys Incorporated

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INTRODUCTION TO SURFACE EMG

CHARACTERISTICS OF THE ELECTRICAL NOISE

CHARACTERISTICS OF THE ELECTRICAL NOISE
The noise may emanate from various sources such as: • Inherent noise in the electronics components in the detection and recording equipment - All electronics equipment generates electrical noise. This noise has frequency components thatrange from 0 Hz to several thousand Hz. This noise cannot be eliminated; it can only be reduced by using high quality electronic components, intelligent circuit design and construction techniques. Ambient noise - This noise originates from sources of electromagnetic radiation, such as radio and television transmission, electrical-power wires, light bulbs, fluorescent lamps, etc. In fact, anyelectromagnetic device generates and may contribute noise. The surfaces of our bodies are constantly inundated with electric-magnetic radiation and it is virtually impossible to avoid exposure to it on the surface of the earth. The dominant concern for the ambient noise arises from the 60 Hz (or 50 Hz) radiation from power sources. The ambient noise signal may have an amplitude that is one to three...