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)
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
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)
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.
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
