EMG Flashcards
What does electro mean
Electrical
What does myo mean
muscle
What does graphy mean
representation writing description
what is EMG
A visual or numeric representation of the electrical actitivity associated with muscle contraction
What is EMG used for
To record, analyze and interpret these biological signals to understand how the nervous system controls the muscles of the body in pathological and healthy conditions
A muscle fibre is made up from a lot of ____
fascicles
A fascicle is made up of multiple ___
muscle fibres
Fast twitch muscles are fast to…
contract and fatigue
____ fibres are recruited first
slow
In imaging slow fibres are ____ because of ____
darker because of high levels of myoglobin
The amount of fast and slow twitch fibres is different depending on…
muscle
Each Muscle is made up of many units of contraction called
motor units
What is a motor unit
a motor nerve and all the muscle fibres that it innervates
A descending motor command that excites the motor nerve will cause…
a contraction of all the muscle fibres in the motor unit synchronously (all of none response, causes all the muscle fibres that it innervates to contract)
Motor nerves attach to…
muscle fibres
a motor nerve and all the fibres that it innervates are called a
Motor unit
Are the muscle fibres in a given motor unit the same or different type (type 1 vs Type 2)
all the same!!
Motor units can be classified as ___ or ____
size! small or large
Most large motor units are _____
fast twitch
Most small motor units are )____
slow twitch
Which motor units get recruited first to produce force
small slow twitch motor units
If you want smooth contraction you want ____ fibres first
slow twitch - avoids jerky movements
Because of the neural organization of muscles the nervous system can vary the contraction by
selectively altering the number of motor units recruited and how often those motor units fire
What is the recruitement of motor units controlled by
the rate of action potentials that are sent to the spinal cord from the primary motor cortex
Neural signals travel along the ______ and disperse through the ____
motor nerves axon and
muscle fibres it innervates
How does the neural signal travelling to a muscle fibre propogate
Through the opening of voltage-gated sodium channels in the neurolemma and sarcolemma
The mechanism of communication along nerves to control muscles is through…
Electrochemical reactions that result in an action potential
Movement of the basic structural components of muscle is produced through …
the propagation of AP within the muscle through a process called excitation-contraction coupling
We know that there is a ____ associated with the contraction
electrical signal - this is what we collect
what are the two commonly used methods of recording EMG
- In-dwelling Electrodes
2. Surface electrodes
What are In-dwelling Electrodes
Small little fine wire with needle electrodes made of fine wire hook onto and record the electrical signals from individual muscle fibres
What are the advantages to In-dwelling electrodes
- Record activity from a single motor unit
- Access to deep musculature
- Little cross talk
What are the disadvantages of in-dwelling electrodes
- Can be painful to participants
- single motor unit may not be representative of entire muscle
- Limitations with recording dynamic muscl activity
How do surface electrodes work
Surface EMG electrodes work in pairs by recording the difference in the electrical signals between each electrodes of the pair
What is the EMG recorded with surface electrodes
The algebraic sum of all motor unit firings near the electrode at that point in time
What are advantages to surface electrodes
- Easy to record
- Minimal discomfort to participants
- provides indication of global muscle activity
What are the disadvantages of surface electrodes
- Can be used only for superficial muscles
- Signal contaminated by cross-talk
- May affect movement patterns of subject
What are we actually listening to in the muscle
As action potentials travel along the muscle fibres EMG electrodes measure the change in voltage that accompany the AP travel. You should be able to hear this anywhere along the electrode (not just at EMJ)
What occurs as the AP passes the electrode
its amplitude is read as a change in voltage by the sensor
What is a problem with a single or un polar electrode
the EMG signal is very small and the measurement is very sensitive to electrical charge. The noise is much larger than signal
What is a problem with a single or un polar electrode
the EMG signal is very small and the measurement is very sensitive to electrical charge. The noise is much larger than signal
What is a bipolar electrode
Two nearby electrodes that independently listen for AP passing. since they are spaced they here the AP at slightly different times
In bipolar electrodes do both electrodes hear the signal and the noise?
yes
What is used to subtract the two signals from a bipolar electrode system
differential amplifier
In a bipolar electrode, what are you left with when you subtract the two recordings
just the signal, no noise!!
Does the AP following subtraction in bipolar electrode measurement represent the true AP
No it is the motor unit action potential, contains same information in terms of amplitude and timing
What occurs when you have multiple motor units firing (which is almost always)
Have to sum all motor units together and get slightly noisy signal
What is the new signal (subtracted signal from bipolar electrodes) called
The motor unit action potential
Each motor unit has its own characteristic ____
MUAP
What are MUAPs based off of
the properties of the muscle fibres within the motor unit (number of fibres, fibre type)
What is the surface EMG signal the sum of?
all the MUAP’s which gives us a gross approximation of the activity of the whole muscle based on certain characteristics
What are the characteristics included a MUAP
timing, rate of force development, fatigue
What is the orderly recruitment of motor units from small to large called
Henneman Size principle
What does Henneman Size principle allow for
smooth, non jerky production of force and preferentially relies upon the non-fatiguable motor units. allows the whole muscle to resist fatigue
how should electrodes be placed on the muscle
Orientated so that the MUAP pass parallel to the shortest distance between electrodes, perpendicular to the travel of the AP and placed on the muscle belly yet not on the innervation zone (motor end plate)
placement of the electrodes at different points on the muscle will result in
a different measurement of the muscle activity. This is based on the MUAP in proximity to the electrode
Why wont electrodes placed on a tendon collect a signal
because the MUAP will not travel along the tendon
Why dont you want to place electrodes on the innervation zone
This will result in a cancellation of the signal
What are 4 key points for electrode placement
- Muscle belly
- Avoid tendon and lateral edge of muscle
- Align electrodes parallel with muscle fibers
- Proximity to motor point or innervation zone (place electrode on midline of muscle belly midway between tendon and nearest innervation zone )
EMG can be used as a indirect measure of ____
muscle force
Why is EMG used as an indirect measure of muscle force
because EME is a measure of the electrical event associated with muscle contraction
Why can EMG tell us about important neuron information
Because it also contains information about the motor units that are firing and when they fire
What can EMG signal tell us about
- Timing of muscle activity
- Force
- muscle fatigue
The raw EMG can be analyzed without processing to tell us information about ____
timing (onset/offset of the signal )
The most common way to process the EMG is to ..
produce an average value associated with its magnitude
What are the 2 simple steps to process the EMG is to produce an average value associated with its magnitude.
- Take the absolute value of the signal (this is called full-wave rectification)
- Take an average value over a certain time period
What is full-wave rectification
The absolute value of the signal
___ is a common first step in EMG Processing
Full-wave rectification
What are two ways to take an average magnitude value of the EMG signal.
- Taking the root mean square
2. Take an average value is to use “filters” to take a more instantaneous average of the signal
What is the resulting tracing following taking a average value over a certain time period called
A linear envelope - this is any filtering that gives you a smooth line
By processing an EMG signal through two steps it can help us…
to quantify and visualize muscle activity during tasks with respect to timing and force of muscle activity
DO the first two steps of EMG processing give us force?
No they give us electrical activity
How do we turn EMG signal electrical activity and determine force
Calibrate it (using voltage output of EMG and weight being lifted)
Does calibration of EMG work well?
Kinda.. mostly just in static contractions, not in dynamic contractions
What is important consideration when attempting to relate EMG to muscle force or movement
the electromechanical delay and the non-linear relation of EMG to muscle force
What is the electromechanical delay
The delay between neural activiaation of a muscle and the development of muscle tension
How long is the electromechanical delay ? what does its length depend on
Approximately 30-50 ms. The muscle and the joint
What is seen by EMG when fatigue occurs
A single MUAP decrease in amplitude with fatigue
Surface EMG of a submaximal contraction ____ in amplitude with ____ muscle fatigue
Increases
Increasing
What do we see an increase in surface EMG amplitude with increasing muscle fatigue?
More motor units are recruited to produce the same force output
The frequency of MUAP firing relates to …
The type of muscle fibres active (
Fast fibres have a ____ median frequency
higher
slow fibres have a ____ median frequency
lower
During fatiguing contractions there is a tendency for frequency spectrum of EMG to
decrease MF and decrease dispersion (thin)
EMG signal can be analyzed for what
different parameters?
Fatigue
Coordination
Force
What is seen with fatigue on EMG signals
- Decrease in single MUAP
- Increase in surface EMG during fatiguing contraction
- Change in frequency component
What are some considerations for EMG set up
- Must ensure that the site that the EMG electrode is placed is properly cleaned and prepared
• tissue (skin, subcutaneous fat) acts as a filter
• Clean skin with alcohol (definitely)
• Shave hair (only excess)
• Lightly abrade skin (? P with sandpaper)
• Use electrode electrolyte (gel, paste, cream)
What are some considerations for EMG analysis
• EMG must be normalized (become a relative measure) to
allow comparison between subjects and condition
• Norm EMG = EMG / (max EMG); MVC, MVIC
