Lecture 16, Electromyography Flashcards
A Shokingly Fruitful Accident
meet Luigi Galvani (1737-98), an Italian Physicist - in 1771, he was doing research on static electricity
- his assistant accidently touched a frog muscle hanging from a hook with an iron scalpel
- the muscle (moved) twitched
- “Animal Electricity”
- the muscle is like a battery where there is all this electrical energy within it that needs to be released (when the lab tech hit the frog leg it created a circuit and allowed the energy to be released) - the brain is also involved in this in that it provides the stimulation and releasing the stimulation
- the connection between central nervous system, peripheral nervous system and the muscle
The Motor Unit
- a motor unit is made up of a motor neuron and the skeletal muscle fibers innervated by that motor neuron’s axonal terminals
- groups of motor units often work together to coordinate the contractions of a single muscle
- the button in the cell that sits inside the spinal cord and then there is wire that goes from cell out to muscle
- a nerve cell that sits inside the spinal cord (alpha motor neuron) and it sends its axon out where it connects to a bunch of fibres in the muscle
- one nerve cells (in spinal cord) makes multiple fibres go on (branches out in muscle)
- if there is a problem with a muscle and the connection with the nerve, or cell, or wire - none of this will work (all of it needs to be intact working together - connection called motor unit)
The Motor Unit (2)
- the fundamental unit of the neuromuscular system is the motor unit
◦ a single motor neuron and all the muscle fibers with which it synapses
‣ nerve cell that sends signals from the central nervous system to muscles
‣ the number of muscle cells innervated ranges from < 20 to 1000 < - ratio of muscle fibers to motor neuron indicates the degree of control
◦ the smaller the innervation ratio the more precise control of the muscle
‣ small motor units are not very strong but provide dexterity
‣ give us the precision like in the hand - very precise of where you look at someone (eyes)
‣ control and precision makes us rely on small motor units (lower amounts of force)
◦ the larger the innervation ratio the coarser the control of the muscle
‣ larger motor units aren’t as precise but generate more force
‣ to generate huge amounts of force we rely on bigger motor units (larger muscle cells with larger alpha motor neurons connecting to more musce fibre) - more power, less control
my notes:
* everything needs to work together to get contraction - nervous system and muscle needs to work together as nervous system excites/stimulates muscle and then muscle contracts
* a motor unit is nerve cell that sits inside the spinal cord called alpha motor neuron which sends nerve fibre or axon out and turns on a bunch of cells inside the muscle
* alpha motor neurons differ in size - small alpha motor neurons: connect to a few muscle cells and that tends to be slow twitch - the bigger motor units are made up of bigger alpha neurons which control more muscle fibres which are fast twitch muscles (small motor units or large motor units)
* so important to our ability to move and general force
Electromyography
- electromyography (EMG) is a diagnostic procedure to assess the health of muscles and the nerve cells that control them
- motor neurons transmit electrical signals that cause muscles to contract
- EMG translates these signals into graphs, sounds, or numerical values that a specialist interprets
my notes:
- to measure a connection between nervous system and the muscle to see what is being activated and how it contributes to movements (message to goes from brain to spinal cord and then spinal cord to the muscle and when the muscle is activated by nerve cell we generate an electrical signal)
- electrical signal that is created within the signal - on a computer screen to see how much activity there is where you can hear the sound of the electrical energy through a speaker or you can jsut have a # value on a screen that corresponds which how much electrical activity there is
- use this as feedback to guide you through this
- look at coordination within a muscle, how fast a muscle is being activated and use it as diagnostic, performance tool
The EMG signal
- EMG is the electrical activity from all active motor units close to the electrodes
- measuring how much noise there is going into a muscle - spikes can give us important piece of information
- EMG is the electrical activity from all active motor units close to the electrodes
- you can measure different spikes where they reflect when one motor unit is turned on versus when another motor units is turned on (can give important information)
- instead of listening to general activity you can be very specific about what you hear
Recording Electrical Signals (2 types)
- the EMG signal is recorded using an electrode
- sample electrical signals in the muscle: muscle action potential
- an electrode is a device that listens to a muscle and measures something (measuring device that measures electricity)
- surface electrodes is a sticker placed on the skin whereas indwelling is a needle inserted into the muscle (needles are quite thick; so person does not feel it) - take up muscle action potentials (electrical change that takes place in a muscle
Recording Electrical Signals (surface - advantage and disadvantage)
the EMG signal is recorded using an electrode
surface electrodes: stickers placed on the skin
advantages
- quick, easy to apply - non invasive or scary and you feel where the muscle is and put the sticker on top of muscle
- minimal discomfort
disadvantages
- mainly used only for superficial muscles
- hard to record dynamic actions
- may affect movement patterns of subject - if you put too many stickers it can be restrictive
my notes:
- muscles move - when you go to contract the muscle tightens up and roles - muscle can roll away from electrode which changes ability to record information
- do not work well for movement but works well for isometric movement and surface muscles - hard to get information for muscles that are located deeper
Recording Electrical Signals (indwelling - advantage and disadvantage)
the EMG signal is recorded using an electrode
indwelling electrodes: needles inserted into the muscle
advantages
- extremely sensitive
- record single muscle activity
- access to deep musculature
- get right within a muscle and can look at one motor unit or look at the one next door - very detailed (get information from all over the body)
- some needles have hooks and so when you move the needles do not shift
disadvantages
- extremely sensitive
- requires trained professionals
- detection area may not be representative of entire muscle
- if you get too specific you might activity going on somewhere else in the body meaning it is getting way too detailed
- usually do surface recordings as well
Electrode Placement
two electrodes put on muscle and one on bony landmark
- bipolar arrangement: 2 electrodes placed over the muscle
- reference electrode: typically placed on a bony prominence
- creating an electrical circuit within the body to try and measure as much of the bodies electrical activity and not pay attention to what is going on next door because there is so much outside electrical activity that needs to be kept out
- it does matter where you put the electrodes
Electrode Placement (optimal and what should it be close to?)
- the EMG signal will change based on the position of the electrodes
electrodes should be placed close together - the actual spacing depends upon the size of the muscle
- the smaller the muscle, the smaller the space should be between electrodes
electrodes should be placed close to the belly of the muscle - the quality of the signal will decrease as the electrodes move away from the midline or towards the attachment sites
my notes:
- for a given muscle action you see different squigly lines where there is only one position where you get detail and so if you electrodes close to tendon you do not get much and putting it off to side you do not get as much
- need to be in middle of muscle belly
- as long as you keep circuit contained to the body and electrodes in the heart of the belly you should be able to get some detailed information
The Amplifier
- wires connected to the electrodes send the signal to an amplifier
- the amplifier records the difference between the two signals (differential signal)
- muscle action potential is very small (1-10 mV)
- signal is amplified around 1,000-5,000 times creating a signal of at least 1 V - signal getting magnified
- makes the signal bigger and more clear
- the signal is typically very small so you usually need an amplifier
- when electrical signal comes by it will get picked up in one electrode at one point in time and in another electrode a little later on
- can get electrical interference but the advantage is you get it into the two recordings a the exact same time so the amplifier not only makes the signal bigger but it also subtracts what is happening in one place from the other (the signal you end up up with is removing all the noise that happens at the exact same time)
- differential signal - not measuring specifically what the electrode sees but the difference between what one electrode sees and the other (just to clean up the signal and makes things smoother)
- making signal bigger and cleaning it up to remove any unwanted noise
- where the axon is activating is right in the middle of the muscle cell so that is where you want to be
- if the electrodes are further away from where the electrical signal is being produced it is going to show smaller signal
The Analog-to-Digital Converter
- electrodes detect the electrical signal
- the amplifier cleans up and amplifies the electrical signal but these are electrical, or analog, signals - computers, which are used to view the information, read digital signal
- when you record an electrical signal is there is an analog signal where computers record digital signal
- so we have to convert analog to digital - when we record we are not recording exactly what is happening (as we are recording magnified signal)
- you cannot convert everything as you would have way too much data (tuning stuff out and picking up the important stuff - paying attention sometimes and not other times)
- want to be able to see it on the computer screen - measure certain points in time only to go through (evenly spaced
The Analog-to-Digital Converter (2)
the analog signal is too large to convert the entire signal
- the analog-to-digital converter samples (convert) a portion of the electrical signal
- sample frequency increases as:
◦ the frequency of the event increases
◦ the detail of the signal increases
Sampling Frequency
- this event happens in 1s but what happens if you only sample at 8Hz
- may miss important details if sampling frequency is too low
- EMG usually sampled at 500 to 2000Hz
- the signal is converted 500 to 2000 times every second
my notes:
- if you are not aware of what is going on you are going to miss certain key details - really important that you know what the event looks like and how many times to sample
- if the signal is more complex and detailed - more precise for things you need more precision for (higher sampling frequency)
- how much they amplify a signal and how many times they have converted the signal
The EMG Signal (raw EMG data)
raw EMG data - a signal that has been processed / altered
- muscles are always. alittle it active so the line will never be zero
- timing can be measured - differences between one burst and another
my notes:
- underneath the final signal is often the raw signal which means what the computer spits out and then you add some filters and fix it out (very detailed information with the raw signal - can look at amplitude - more amplitude more muscle - can look at when activity goes quiet but will never see a flat line because your muscles are always a little active (tone) to just give general amount of health - when there is a flat line that means you have severed to the connection between the nervous system and muscle)
- can measure the timing - differences between when one burst comes on and when another burst comes on - how much downtime and activity there is within the muscle - can look at this in between different muscles
- positive and negative changes
