Electromyography Flashcards
Motor units include
- Cell body
- Axon
- Dendrite
- All intervated muscle fibers
Motor Units are based on the ____ principle
all or nothing
Draw what a neurons signal to a muscle fiber looks like early and late in transit
Steps of how a motor unit sends signal to muscle
1) Nerve Action potential approaches Junction
2) Spike reaches junction and triggers release of ACh
3) ACh acts on muscle fiber to produce muscle action potential
4) Muscle contracts
Motor Unit Action Potentials start at ____ and propagate along the muscle fiber
- motor end plate
- Signal travels in two directions along muscle fiber, proximal and distal.
- 2-6 m/s
What wave pattern are motor unit action potentials?
- Triphasic
- One electric side is subtracted from the other.
- Positive or Negative does not matter. It is just based on how the electrodes are positioned.
One motor unit innervates ____ muscle fibers
- Several hundred
Motor Unit Action Potential (MUAP) = ____
summed electrical activity of all muscle fibers activated within the motor unit
How does muscle force increase?
- Recruitment of more motor units
- Increase Rate Coding
Explain this image
Spikes are individual nerve signals being sent.
Summation of all triphasic bursts. Rate coding (firing rates) and addition of motor units to grade the amount of force needed. Can quickly change these forces, likes to pulse, hard to maintain one level.
Voltage signal (in blue on right) has both positive and negative voltages (positives spike will cancel negatives when we take a mean for raw EMG signals) we cannot just take the raw data or we would get nothing.
What is an EMG signal?
- Reflects summed electrical field generated by all muscle fiber action potentials near detection site
- We only get a glimpse of the signal which is passed through extracellular fluid up to the surface. Superficial meaures
EMG Equipment
- Electrodes
– Surface, fine wire, needle - Amplifier
– Gain, frequency response, differential, CMRR - Recorders
– Strip chart (paper), UV light & film, tape, PC
Surface EMG uses ____ electrodes per muscle
- 3
- 2 active
- 1 reference/ground (placed on bony prominence away from muscle)
Each active electrode detects activity from motor units beneath the electrodes and compares this signal to the ____ electrode
reference
What is a differential amplifier?
- One signal (active electrodes) is subtracted from the other (ground electrode) and the difference is amplified (gain) to see it better
- Gets rid of common noise between the two (60 Hz)
- Ability to reduce signal common to both recording sites = Common Mode Rejection Ratio (CMRR)
Electrode Pickup Region
- Surface electrodes pick up signals several inches deep; Good for superficial muscles
- Senstiive to direction of electrode placement
Electrode Placement - Why is it important?
- Depending on placement results in different muscle signals
- Middle of the muscle belly is best
- Highest signals near the axon innervation
The amplitude of the EMG (biological signal) is determined by:
- Distance of MU from electrodes (how far/deep)
- Angle between MUAP propagation & electrode surface (muscle orient to angle)
- Tissue impedance
– Impedance - resistance to current flow (low is good)
– Changes with skin preparation and time - MU size
– Fiber type (Fast or Slow Twitch) - Recruitment level
– Number of motor units (High force, needs a lot, low force, needs less)
Surface Electrode (sEMG)
Quiz Question
range = 10-1,000 Hz
Ag/AgCl
Advantages
* Convenient, painless, gross data
Disadvantages
* Limited to surface muscles, cross talk, loss of high frequency component
Fire Wire
Quiz Question
20-2,000 Hz
Advantages
* Detect single MUAP; better diagnostic for neural information
* Great for deep muscles
Disadvantages
* “hope & poke”, migration?
* Cannot perform large dynamic movements
Needle
Quiz Question
- 20-2,000 Hz
Advantages
* Detect single MUAP
* Can reposition
Disadvantage
* Very uncomfortable
Properties of a Good Amplifier
Quiz Question
- Gain
- – Multiplier, adjustable gain for small signals)
- Frequency Response
– Amplify all EMG frequency components equally (response rangeg 10-10,000 Hz) - Differential Amplifier
– Taking the difference between electrodes automatically reduces the majority of noisde common to both electrodes - Common Mode Rejection Ratio
– Want this to be high (at least 10,000:1 = 80 dB reduction
EMG Recorders
- Sampling Rate 1,000 and above
- Bulk of EMG is approximately 50-150 Hz
– Slow twitch MU: 75-125 Hz
– Fast twitch MU: 125-250 Hz
How to prepare subject for EMG readings
- Correct placement of electrodes is critical
- Electrode body oriented parallel to muscle
– Metal contacts down length of muscle fiber
– Wires taped down to reduce artifact - Prepare skin by…
– Shaving
– Abrading
– Cleaning with alcohol wipe - Ground Electrode (on bony segment)
Can we eliminate the ground electrode?
- In some new sensors yes! Greater than 2 active electodes
- 4 contacts measure MU and factor out noise going left to right acting same as ground function
This is an example of:
Red: Baseline Noise
Blue: EMG crosstalk
Yellow: Movement artifact
What is filtering?
- A way to limit the influence of noise and artifacts
- Often performed by an amplifier hardware to retain signal between 10-500 Hz “bandpass” filter
What are the 4 types of filters?
- Low Pass
- High Pass
- Band stop
- Band pass
Low pass filter
- Reduces the signal above the specified cutoff frequency
- Gets rid of high frequency radio noise
High Pass
- Reduces the signal below the specified cutoff frequency
- Removes signals associated with cable and movement artifacts
Band stop
- Filter specific to narrow data to exclude 60 Hz noise
- Note: Muscle contractions also occur at this level so if you get rid of 60 Hz you get rid of noise AND real data; therefore not always recommended.
Band pass
- Filters out less than 10 Hz and greater than 500 Hz
How do we get a full wave rectified EMG?
- Reflect all the negative signals on the positive side
- This eliminates directionality (proximal and distal)
4 ways to process EMG
- RMS: Root Mean Square:
– Raw EMG data squared, from a window period range 20-100 ms. - Averaged Rectified Value:
– First rectify signal
– Moving avergage: sum of signal/time interval
– Simple average: sum of signal/total time - Linear Envelope
– Low pass filter (3-50 Hz) applied to full rectfied signal
– Much smoother graph - integrated EMG
– Sums the activity over a period of time (area under the curve)
What is the only EMG processing technique that you do not have to rectify the signal first?
Root Mean Square
What is an event marker/time stamp? Why is it important for EMGs?
- It is an additional piece of informaation to be able to line up the signal in time. Noticeable time event.
- Ex: Use of a heel switch to create a voltage to indicate when an event is occuring.
How do we know what marker is the most important?
- We know that the third mark is important because we can see the GRF for heel strike, our event marker, which then tells us lthe activity below shows we are getting preactivation for GMED.
For amplitude measures, ____ almost always required
____ against a known reference
- normalization
- Calibration
Why is normalization important?
- It is important because it will be different that the next time it is placed.
- Factors that can effect it:
– hydration
– electroyltes
– skin conduction
– electrode conduction
– electrode placement
– individual nerve conduction differences
Types of normalization options
- Maximum voluntary contraction (MVC)
– Functional activity (Ex: max leg press)
– Isometric activity (MVIC) - Unresisted normal activity
- Sub-maximum contraction
Factors that affect muscle force
- Muscle length
- Contraction Velocity
- Fatigue
EMG signal and muscle force are affected differently by these factors (and others)
- Not a perfect linear relationship (fiber position, pennation angle, length-tensioin relationship, type of muscle activity (con,iso,ecc), velocity, fiber types, electrode placement
- Isometric will result in a more stable measure
- EMG amplitude does not perfectly explain muscle force
- Muscle force is better measured through force plate/capture systems
- EMG can be good for nerve conduction
What should you report in your EMG report for literature?
- Equipment Used
- Recorder
- Electrode type, size, & distance
- Muscles recorded
- Filter settings
- Gain settings
- Normalization Procedures
- Operational definition of discrete variables
EMG Uses
- Kinesiology study (describe movement)
- diagnostic (needle at level of nerve or muscle)
– neuropathy or myopathy - Fatigue
- Biofeedback (Re-education, relaxation)
How do EMG signals change with fatigue and why?
- Changes:
– Increased amplitude (Due to greater numbers of MU’s being recruited because the CNS can’t chose what MU’s to activate because so many are fatigued. Choses to fire all of them at once (All on, all off) resulting in pulsing and shakiness.
– Decrease in mean or median frequency (fast twitch will fatigue early resulting in slow twitch to be left over which fire at a lower frequency. Over time continues to get lower and lower do to only slow twitch firing.
Bigger amplitude with time, lower frequency with time
Example of Discrete EMG Signals
- Activation onset time (ms)
– Relative to initial contact - Activation duration (ms)
– Onset to end of signal - Peak activation (%MVIC)
– From onset to end of signal - Average activation (%MVIC)
– Mean amplitude over signal duration
How do you know the type of information you need from an EMG?
- It depends on your question
