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What is Electromyography?

Taken from Dr. John Keener's UMD PEP 3040 Electromyography Lab: Background Information; Dr. Vaughan Kippers 1999 University of Queensland AN212 Lecture, "Electromyography (EMG) – Principles and Biological Bases of EMG" at http://www.uq.edu.au/~anvkippe?an212/emg.htm; and clinical application Web sites given below.

Electrical Activity during muscle contraction is produced by movement of Na+ and K+ ions:

Action Potentials, or small currents on membranes which can be measured and recorded.

Recall that a Motor Unit is a single nerve fiber (motor neuron) and all of the muscle fibers it innervates. The axon of the motor neuron may branch as few as five to six six times (e.g., to five to six muscle fibers within extraocular muscles) for fine control or as many as 2000 times (e.g., to 2000 muscle fibers within the gastrocnemius or gluteus maximus) for coarse control. Each muscle fiber has its own terminal axon and neuromuscular junction, but the muscle as a whole is made of multiple motor units which may be "on" at different times. Smooth muscle contraction usually involves overlapping of motor unit firing, and thus asynchronous volleys of impulses traveling down the many axons innervating a single muscle.

Electromyography (EMG) is the study of muscle function through analysis of the electrical signals produced during muscular contractions. EMG involves detection, amplification, recording, analysis, and interpretation of electrical signals. Surface EMG uses bipolar surface electrodes placed on the surface of the skin to detect voltage differences due to asynchronous activation of motor units. Each motor unit’s activation involves a “wave” of depolarization along the neuron and muscle fiber membranes. Each fiber is a different distance from the electrode, and therefore signals take different times to reach the electrode. Therefore, the surface EMG actually provides indirect information about levels of motor unit activity.

Bipolar Surface Electrodes: Small signals detected by the active pair of electrodes on the surface of the skin over the muscle are compared to the signal detected by a reference electrode ("ground") placed over connective tissues. The typical signal is small, up to a few hundred microvolts, so it requires amplification.

Bipolar Fine-wire Electrodes are implanted to detect activity of deep muscles. After skin is anesthetized, a hypodermic needle is used to inject two insulated fine wires with bare tips.

What is measured by EMG signals? It is assumed that the greater the muscle activation, the more action potentials, and the more action potentials, the stronger the EMG signal.

Sequence of Electrical and Mechanical Responses:

1. Neuromuscular Junction (release of chemical neurotransmitter)
2. Endplate Potential
3. Propagated Action Potential ("all or none")
4. Motor unit Action Potential Train (asynchronous activation of different motor units, and traveling along muscle fibers)
5. AP affects T-tubules
6. Ca+2 ions released from sarcoplasmic reticulum into sarcoplasm and myofibrils
7. Formation of crossbridges
8. Crossbridges produce mechanical force leading to stretch of connective tissue
9. Application of contractile force (physiological advantage) to bone
10. Production of joint movement (mechanical advantage)

Biological variables which may affect the EMG signal include:

1. Force of muscular contraction, in turn related to number of motor units firing,
2. Size of muscle,
3. Position of muscle (superficial vs. deep), and
4. Thickness of subcutaneous fat, an electrical insulator

Technical variables which may affect the signal include:

1. Skin preparation, which determines skin impedance
2. Distance between electrodes
3. Position of electrode relative to muscle and
4. Orientation of electrodes, with respect to muscle fiber direction

Quantification of EMG:

Methods are still being developed. "On" and "off" times for muscle activity are difficult to specify. The complex signal can be modified by converting all negative voltages to positive and then integrating the signal by calculating the area under the curve during a certain period. A plot of mean amplitudes can be used to describe the change in activation levels over time. The time constant (period during which the mean is calculated) can be varied and will affect the smoothness of the signal.

Our computer software does not allow this type of integration, but will provide a statistical analysis of a segment of the recording. The standard deviation from this analysis will be used as a measure of the electrical activity associated with muscle contraction, and therefore as our indirect measure of motor unit activity during muscle contraction.

Relationship between EMG and Force:

In 1952, Inman and colleagues published a paper supporting a direct relationship between the quantified EMG signal and joint movement for isometric contractions. However, they noted that when joint angle changed (i.e. muscle length was altered), that relationship changed.

Force and Length:

As a muscle is lengthened beyond resting length, connective tissue is stretched and tensile force is produced, but connective tissue does not produce an electrical signal. Also, when a muscle is very short, it cannot produce much force, even when maximally activated. Patla and colleagues showed that as muscle is lengthened, the slope of the isometric force/EMG relationship decreases. For dynamic contractions, the relationship between EMG and work is dependent on the type of contraction.

Any recorded EMG is a relative measure only, and cannot be compared between muscles or between people. It is best compared between different conditions for the same muscle. Changes in length for isometric or dynamic contractions are confounding variables which makes interpretation difficult.

Clinical Uses of EMG:

Tension-Type Headache Patients: Consult the website http://www.drjimboyd.com/emg-data.htm and summarize procedure and results.

Diagnosis of neuromuscular diseases includes thorough medical examination and EMG, including single-fiber, macro EMG, and motor-unit estimation. Additional testing may include lumbar puncture and muscle and nerve biopsies. Consult the Web site:

http://www.clevelandclinic.org/neurology/treat/neuromuscular.htm

Then, list four major types of neuromuscular disease which require these diagnostic tools.

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