EMG

Overview:

A motor unit is defined as one motor neuron and all of the muscle fibers it innervates. During muscle contraction, a motor unit fires, an impulse or action potential is carried down the motor neuron to the muscle. The area where the nerve contacts the muscle is called the neuromuscular junction. After the action potential is transmitted across the neuromuscular junction, an action potential is generated in all of the innervated muscle fibers of that particular motor unit. The sum of all this electrical activity is known as a motor unit action potential (MUAP). This electrophysiologic activity from multiple motor units can be detected via technique called electromyography.




The recording or electromyogram (EMG) represents the combined electrical activity produced by multiple action potentials that are generated by contracting muscle fibers. The composition of the motor unit, the number of muscle fibers per motor unit, the metabolic type of muscle fibers and many other factors affect the shape of the motor unit potentials in the EMG. The EMG is not a regular series of waves like the ECG but a chaotic burst of overlapping spike-like signals that are recorded using invasive or non-invasive electrodes.

Method:

The EMG bioelectrical signals are typically very small in amplitude (mV) and an amplifier is required to accurately record, display and analyze the EMG. Depending on the hardware and software used, the biological amplifier serves not only to amplify the signal but also to apply a range of filtering options for the removal of unwanted signal artifacts.

EMG can be measured from any muscle parts of the body. For guidelines and techniques for measuring human facial EMG the following paper is recommended Fridlund A.J. and Cacioppo J. T. Guidelines for human electromyographic research. 1986 Psychophysiology 23(5):567-589.

Evoked EMG Recordings
Researchers and teachers are often interested in the effects of nerve and muscle stimulation in relation to normal physiological function. Evoked EMG recordings are particularly useful for determining stimulation thresholds, conduction velocities and refractory periods.

For mild electrical stimulation of human subjects with a fully-isolated unit, the ML180 Stimulus Isolator or ML408 Dual Bio Amp/Stim are ideal. These units may be used with the Stimulating MLADDF30 Bar Electrode, the MLA265 Stimulator Rod with Cable or the MLA260 Stimulator Cable (4 mm shrouded plug to Alligator Clip).

For nerve and muscle stimulation of animal tissue preparations ONLY, the MLA0320 Animal Nerve Stimulating Electrode or MLA250 Stimulator Cable (BNC to Alligator Clip) may be connected directly to the analog output of any PowerLab. The analog outputs are NOT to be used on human subjects. Stimulus waveform type, duration and intensity are software controlled.

EMG signals can be recorded using either surface or needle electrodes (intramuscular EMG).

  • Surface EMG recordings provide information about many fibers in superficial muscles.
  • Intramuscular EMG allows the monitoring of potentials generated by fibers that may belong to a few different Motor Units (MUs).


Surface EMG
Surface EMG recordings have amplitudes of a few mV with most of the power between 10 Hz and 300-400 Hz. Surface EMG is useful when intramuscular EMG may be considered too invasive or too specific in some cases. The general picture of muscle activation obtained from a surface electrode may be more useful that recording the activity of only a few fibres using a needle. For example, muscle activation is monitored using surface EMG and subjects have an auditory or visual stimulus (biofeedback) to help them know when they are activating the muscle.

Intramuscular EMG
This recording provides local information with good morphological details that allow identification and separation of the contributions due to different MUs, as well as the recognition of MU Action Potential shapes. Recordings using needle electrodes also have amplitudes of a few mV with most of the power comprised between 10 Hz and 1 kHz.

Muscle tissue at rest is normally electrically inactive. Usually after the electrical activity caused by the irritation of needle insertion subsides, no abnormal spontaneous activity should be detected except the area of the neuromuscular junction, which is normally electrically very spontaneously active. Action potentials begin to appear when the muscle contracts and this number of action potentials increases with increased contraction. A disorderly group of action potentials of varying rates and amplitudes (a complete recruitment and interference pattern) is recorded when the muscle is fully contracted.

Note: The ADInstruments Bio Amps (ML132, ML135, ML136, ML138 and ML408) are not suitable for any biopotential recordings that require a driven guard circuit.
 

Software:

LabChart
LabChart software (for Windows and Macintosh) combines the familiar simplicity of a traditional strip chart recorder with the powerful analysis features of a digital acquisition system. LabChart software and a PowerLab data acquisition unit provide data integrity, easy selection of hardware settings, powerful online and offline analysis, procedure automation, seamless extraction of experimental data and flexible display options. Acquisition and analysis capabilities can be further increased with LabChart Extensions and LabChart Modules. LabChart Modules are available as part of LabChart Pro and LabChart Extensions are free for download from the website for existing LabChart users.

In addition, LabChart software can:

  • Rectify and integrate the raw EMG signal and display the results on a separate channel.
  • Calculate and display the RMS power content of the signal.
  • Display the power spectrum (FFT) of a selected region of data using Spectrum command


Scope
Scope software, supplied with PowerLab systems, provides powerful display, recording and analysis features to transform your computer into a two-channel storage oscilloscope, XY plotter or Power Spectrum (FFT) analyser. Scope is used commonly to measure any high-frequency signal that is time-locked to a stimulus such as action potentials and evoked responses. For analyzing Evoked EMG Potentials, the Scope Software is recommended because it provides signal averaging functions that are necessary to extract the evoked response from background noise.

This software:

  • Provides the ability to record, display and analyze any high frequency signal that is time-locked to a stimulus
  • Synchronizes sweeps with recorded or built-in stimulation patterns
  • Provides a range of real-time and offline analysis features
  • Generate stimuli of differing intensities and waveform structures (i.e. single-pulse, multiple pulse, simple ramps) and control an external stimulator using the analog output on the front of the PowerLab.


GLP and 21 CFR Part 11
For those researchers working within a laboratory requiring GLP and 21 CFR Part 11 compliance the GLP Client and GLP Server are available for use with LabChart (Windows only) and PowerLab data acquisition systems. For more information, visit the Good Laboratory Practice application page or contact your nearest ADInstruments representative.

Hardware:

Bio Amplifiers

The biopotentials are typically very small in amplitude (mV). Therefore accurate recording, display and analysis of an EMG require a suitable bioamplifier. ADInstruments offer a range of Bio Amplifiers, when connected a PowerLab data acquisition unit and, are certified safe for use with humans or used with animals. These bioamplifiers are fully software-controlled using LabChart or Scope. The following ADInstruments' biological amplifiers are fully isolated for connection to human or animal subjects:

Human EMG

The following ADInstruments' biological amplifiers are externally certified to comply with international safety standards and are fully isolated for connection to human subjects:

ML132 Bio Amp

ML135 Dual Bio Amp


ML408 Dual Bio Amp/Stimulator


The Bio Amp, Dual Bio Amp and Dual Bio Amp/Stimulator are manufactured for use with PowerLab data acquisition systems and are fully software-controlled by LabChart or Scope. These Bio Amps cannot be used for recording 3 or more biopotentials on a single subject; however, they may be used for multiple subjects that have separate grounding leads. For recording more than 3 biopotentials from a single subject see below:

Multiple-Channel EMG Recordings

ADInstruments also provides multiple channel bioamplifiers that connect directly to PowerLab data acquisition systems. These units are also fully-isolated, and independently certified, for connection to human subjects. The multi-channel bioamplifiers are particularly suited for exercise physiologists that wish to record multiple channels of EMG (Quadricep, Calf, Bicep, Tricep etc.) while a subject exercises.

ML138 Octal Bio Amp

  • A differential amplifier that consists of eight electrically isolated differential input AC amplifiers
  • A shared ground connection across all eight inputs.
  • Supplied with two packets of MLA0310 Lead Wires (1.8 m, 10 snap on)


Note: The ADInstruments Bio Amps (ML132, ML135, ML136, ML138 and ML408) are not suitable for any biopotential recordings that require a driven guard circuit.

GT201/F 16 Channel Bio Amp

Animal EMG

The following ADInstruments' biological amplifiers for use with animals (i.e. pithed toad, or anaesthetized rat/mouse) only:

ML136 Animal Bio Amp


Multiple channel EEG may be recorded using the ML138 Octal Bio Amp or GT201/F 16 Channel Bio Amps that connect directly to the MLA1505 Lead Wires that terminate in alligator clips or the MLA1203 Needle Electrodes.

Accessories

Bio Amp cables:


Leads compatible with both shielded (MLA2340 & MLA2540) and unshielded (MLA1340 & MLA1540) cables:


Leads compatible with shielded cables (MLA2340 & MLA2540) only:


Leads that directly connect to the Dual Bio Amp


Leads that directly connect to the Animal Bio Amp


Electrodes:


Consumables:

 

Matching different levels of isometric torque in elbow flexor muscles after eccentric exercise
N Weerakkody, P Percival, D L Morgan, J E Gregory and U Proske, Experimental Brain Research, 141-150, 2003

Focal loss of the glutamate transporter EAAT2 in a transgenic rat model of SOD1 mutant-mediated amyotrophic lateral sclerosis (ALS)
D S Howland, J Liu, Y She, B Goad, N J Maragakas, B Kim, J Erickson, J Kulik, L DeVito, G Psaltis, G J DeGennaro, D W Cleveland and J Rothstein, Proceedings of the National Academy of Sciences, 1604-1609, 2002

Citations Database


The material on this page is provided in good faith and believed accurate at the time of writing. No responsibility will be taken, or liability accepted, for damages arising from the use of information herein. Readers are urged to check with respective manufacturers the accuracy of all product related information.

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