Armin Allahverdy 1935
Electromyography • Defined as the preparation, study of, and interpretation of electromyograms Electromyogram: • Defined as the electrical activity associated with the contraction of a muscle Greek derivative: • elektron, amber, + mys, muscle, + gramma, something written
Dendrite Soma (body) Axon
A motor unit is composed of a motor neuron and all of the muscle fibers it innervates
each muscle has many motor units (m.u.) # of fibers in a m.u. is dependent on the precision of movement required of that
muscle (average: 100-200 fibers per m.u.)
more precision is obtained with more neurons 100 to 2000 motor neurons per muscle
# of m.u.’s in a muscle decreases in the elderly
Motor neurons release acetylcholine. This has an excitatory effect on muscle fibres. A single action potential in the motor nerve is enough to
trigger an action potential in the muscle fibre. Depolarisation triggers the release of calcium ions, which trigger contraction. The potential (voltage) generated by one fibre is small (<100mV).
Clinical applications Invasive recording used in assessment. Biofeedback Can aid rehabilitation after injury or stroke May have other applications e.g. forehead EMG biofeedback may reduce tension headache. EMG and covert behaviour e.g. subvocalisation while reading Facial EMG and emotion
Uses: • Provides an indication of how much a muscle is being used during particular types of activity • Is a muscle on or off? • Is a muscle fatigued?
By placing electrodes over the muscle
we can record the signal generated by muscle contraction. Voltage is displayed continuously and recorded for analysis. The signal includes positive and negative waves and varies rapidly: we also analyse integrated EMG (averages over 20 samples, ignoring the sign)
Length of electrodes # of included fibers vs. increased noise*** Delsys – 1 cm Noraxon - ? Distance between electrodes Increased amplitude vs. misaligning electrodes, Multiple motor unit action potentials (MUAP) Muscle fibers of motor units are distributed evenly, thus large muscle coverage is not necessary (De Luca). Delsys – 1 cm Noraxon – 2 cm?
Away from motor point
MUAP traveling in opposite directions Simultaneous (+) & (-) AP’s
Resultant increased frequency components More jagged signal
Middle of muscle belly is generally accepted
Away from tendon Fewer, thinner muscle fibers Closer to other muscle origins, insertions More susceptible to cross-talk Away from outer edge of muscle Closer to other musculature Orientation parallel to muscle fibers More accurate conduction velocity Increased probability of detecting same signal
As far away as possible from recording electrodes Electrically neutral tissue Bony prominence
Good electrical contact Larger size
Good adhesive properties
Muscle contraction is graded by varying The number of active motor units within the muscle. The frequency of firing of each motor unit. Skeletal muscles are normally slightly contracted, known as tonus. At any given time a small proportion of the motor units will be active. Exertion can increase the size of individual fibres and increase energy stores: Hypertrophy.
Muscle contraction Excitation-contraction coupling • Process of nerve-muscle excitation leading to Ca++ release and force generation
Muscle contraction
1) Nerve stimulation • motor nerve 2) Electrochemical transmission • saltatory conduction 3) Terminal nerve (axon) activity • Ca++ influx • Vesicle docking • Ach release into synaptic cleft
Muscle contraction
4) End-plate potential • Activation of motor end plate • Ach-receptor binding leading to Na+ influx (depolarization) 5) Membrane propagation • muscle fiber membrane excitation spreads over the fiber • THIS IS THE EMG SIGNAL!
Electromyogram (EMG)
• Algebraic summation of all motor unit action potentials propagated along a muscle at a point in time • MUAPs - Motor unit action potentials • Recording all the MFAPs of all the motor units activated by the CNS in a given period of time
Electromyography (EMG)
Generation of the EMG signal • “Wave” of depolarization and the subsequent “wave” of repolarization are detected by recording electrodes
Electromyography (EMG)
Generation of the EMG signal
Recording electrode Muscle surface Repolarization
Depolarization
Electromyography (EMG)
2 types of recording electrodes: 1) Surface • most commonly used • small recording surface • placed over the muscle belly • requires adequate skin preparation: removal of hair, oil, dirt, skin, etc.
Electromyography (EMG)
2) Indwelling • inserted directly into the muscle via a hypodermic needle • used for deep muscles • used to identify different motor units recruited or how many motor units recruited during an activity
Electromyography (EMG)
Characteristics: 1) Muscle primarily involved during a particular movement • no direct indication of muscle force • concentric or eccentric contractions • related to muscle force
Electromyography (EMG)
Characteristics: 2) Measures antagonist muscle activity Example: • hamstring activity at terminal phase of knee extension
Electromyography (EMG)
Characteristics: 3) Assessment of muscle fatigue and recruitment of different motor units and fiber types • Analyzing the frequency of the EMG signals • related to type of motor unit activated and conduction velocity
Electromyography (EMG)
Factors affecting the EMG 1) Depth of the muscle fiber • affecting distance to recording electrode • if distance increases, signal amplitude decreases • muscle contraction: low to high intensities move from deep to surface of muscle
Electromyography (EMG)
Factors affecting the EMG 2) Muscle fiber type Fast twitch fibers: • high conduction velocity • high frequency and higher amplitude
Electromyography (EMG)
Factors affecting the EMG 3) Motor unit size • “all or none” response • a larger motor unit will result in a larger EMG recording
Electromyography (EMG)
Factors affecting the EMG 4) Muscle temperature • increasing temperature increases conduction velocity • negligible increases in body temperature during exercise
Electromyography (EMG)
Factors affecting the EMG 5) Amount of tissue between muscles and electrodes • skin and subcutaneous fat • acts as a “low pass” filter • decreases detection of EMG signals with higher frequencies
Raw EMG signal
• Can be assessed qualitatively • visually observing the size and density of the EMG signal
Assessing amplitude
• Method of evaluating the “amount” of EMG activity collected Different methods: • rectification • integration • linear envelope • root mean square (RMS)
Assessing amplitude
• Raw EMG signal is a bi-polar signal • positive and negative phases Rectification • Method of translating the raw EMG signal to a single polarity Half-wave rectification • eliminating one polarity of the signal (Ex. Deleting the negative phase)
Rectification Full-wave rectification • Inverting the negative polarity onto the positive polarity • Preferred and most often used method • Preserves all the energy in the raw EMG signal
Rectific ation Raw signal
Full-wave rectified signal
Rectification • Once rectified, now can perform calculations on the signal Why not perform calculations on the raw signal itself? • Bi-polar signal (positive and negative phases) • positive + negative = ZERO What do you do now with the rectified signal?
Integration • Mathematical operation of calculating the area underneath the rectified curve • This measure is a function of time • The longer the EMG is collected, the more integrated EMG will appear
Integr • ation Area = length × width
• Units for integrated EMG = mV sec • depends upon the amplitude, duration and frequency of action potentials mV
seconds
Linear envelope • Method of “smoothing” the rectified EMG signal • Used most often in gait studies (cyclical muscle contractions) • Use of a moving average to smooth the signal
Linear • envelope Calculating the average EMG amplitude (mV) over a fixed
window length • moving the window across the whole EMG signal at fixed intervals Rectified signal mV
Linear envelope
seconds
Root mean square • • • •
Different method of processing the raw EMG (AC) signal also gives a moving average over time (smoothing effect) is a measure of signal power converts the raw signal to a single polarity
Root mean square • The value of the raw EMG signal (amplitude) is squared (V2) • the raw signal is the square root of the resultant, or processed, signal • units = mV.
EMG frequency analysis
• Method of assessing the frequency domain of the EMG signal • Amplitude measures (integration, RMS, etc.) are in the time domain • Frequency analysis: assessing the “quality” of the EMG signal • Reflection of the conduction velocity of the recorded MFAPs (muscle fiber type specific)
EMG frequency analysis
• The EMG signal is a composite measure of many simple sinusoid waves • Alternating waves • the EMG signal is non-periodic (i.e. - they do not repeat any any regular interval) • Transformation of the EMG signal from the time domain to the frequency domain
Frequency spectrum