Lecture 16, Electromyography

Question 1

A Shokingly Fruitful Accident

Answer 1

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

Question 2

The Motor Unit

Answer 2

• 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)

Question 3

The Motor Unit (2)

Answer 3

• 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

Question 4

Electromyography

Answer 4

• 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

Question 5

The EMG signal

Answer 5

• 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

Question 6

Recording Electrical Signals (2 types)

Answer 6

• 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

Question 7

Recording Electrical Signals (surface - advantage and disadvantage)

Answer 7

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

Question 8

Recording Electrical Signals (indwelling - advantage and disadvantage)

Answer 8

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

Question 9

Electrode Placement

Answer 9

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

Question 10

Electrode Placement (optimal and what should it be close to?)

Answer 10

• 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

Question 11

The Amplifier

Answer 11

• 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

Question 12

The Analog-to-Digital Converter

Answer 12

• 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

Question 13

The Analog-to-Digital Converter (2)

Answer 13

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

Question 14

Sampling Frequency

Answer 14

• 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

Question 15

The EMG Signal (raw EMG data)

Answer 15

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

Question 16

The EMG Signal (full wave rectification)

Answer 16

full wave rectification: take a peak-to-peak value of the signal (absolute value)
- just looking at an absolute signal - not changing what is happening at the muscle rather how the computer reads that - not changing original photo just making it look prettier
- no positive or negative - can look at where the biggest spikes are and the most amount of activity

Question 17

THE EMG Signal (linear envelope data)

Answer 17

linear envelope data: looking at the average trend of the EMG data
- do not want so many data points and know exactly what is happening rather you want the average score (get rid of any big spikes and just look at mean amount of electrical signal for a given amount of time)
- taking full wave rectified and looking at mean
- not changing what the muscle is doing just how you see it is changing (can make some conclusions based on the data you get)

Question 18

What Does EMG Actually Measure?

Answer 18

• an EMG signal will not necessarily reflect the total amount of force (or torque) a muscle can generate
• the recording electrodes cannot see everything
  ◦ the electrode will ONLY record the active fibers that are nearby
  ◦ # active of fibers recorded will be less than the total # of active fibers
• where the active fibers are in relation to the electrode matters
• active fibers far from the electrode will have no effect on EMG despite an increase in force
• active fibers close to the electrodes will increase EMG amplitude

my notes:
- the electrode only listens to what is nearby and it is very loud when something is too close (electrical signal can be big or small because of where you stand and not necessarily the activity in the muscle)
- correlation does not always equal causation - just because you see a pattern it does not mean it is true

Question 19

The EMG Amplitude vs. Force Produced (linear and non linear)

Answer 19

• there is a linear relationship between EMG and force…for isometric contractions
• there is a non-linear relationship between EMG and force….for isotonic contractions
• during an isometric muscle action, the muscle is not moving (the body is not moving) - the position of muscle and electrode will be fixed so we see a linear relationship
• when we go to see movement, the size of EMG and force is all together off
• the electrodes measure electrical activity within a muscle there are not measuring force being generated because they would be a strain gauge, force platform etc.

Question 20

The EMG Amplitude vs. Force Produced - eccentric muscle action

Answer 20

EMG amplitudes are generally less during eccentric muscle actions than concentric muscle actions - why might this be?
- lots of activity when you shorten and less when you lengthen - but if you measure in force the force production is going to be exact same in each condition
- when it shortens the active component is active and when it lengthens the passive component is active - there is not machinery inside the passive, passive is the elastic band and what surrounds the muscle
- when you lengthen you are generating force from the passive tissue but when you shorten you are generating force from the muscle cells so the electrical activity in the force and force being produced so not correlate to one another

Question 21

Contraction Mechanics

Answer 21

a twitch, is a small, local, muscle contraction and relaxation - what is happening during each phase?
- you are going to stimulate muscle cell at the beginning and nothing happens as the muscle starts contracting a little later on
- during the latent period there is nothing seemingly happening when measuring force but if measure EMG, the EMG starts the moment you stimulate a muscle - no force happening in stimulation but there is electrical stuff happening
- measuring the muscle action potential - signal flowing down muscle cell which causes calcium to be released from SR and then force gets created when calcium gets into muscle

Question 22

The EMG Amplitude vs. Force Produced (electromechanical delay)

Answer 22

there is a delay between the electrical activity in the muscle and the force development by the muscle (electromechanical delay)
- EMG does not equal force, can correlate well but not the exact same

Question 23

So, What Does EMG Actually Measure? (TEST QUESTION)

Answer 23

• the EMG signal is affected by, among other factors:
  ◦ the position of the electrode
  ◦ the type of contraction
  ◦ the phase of the action
• EMG provides information about muscle electrical activity
  ◦ EMG can be reflective of muscle strength (indirect information)
  ◦ but EMG is not a measurement of force production
  ◦ EMG is an indication of muscle activity
• measures the electrical activity in the muscle - all you can conclusively say (not measuring force) - indication of muscle activity
• when is it active and when it goes off, how much activity in the muscle and use that to measure timing with a muscle or coordination between muscles
• so many factors that effect size, shape and integrity of the electrical signal

Question 24

Other Factors That Affect the EMG Signal (intrinisic or physiological factors (4))

Answer 24

muscle fiber diameter, muscle temperature, blood flow to the muscle and muscle fatigue

Question 25

Other Factors That Affect the EMG Signal - Muscle Fiber Diameter

Answer 25

• as muscle cell diameter increases, EMG amplitude increases
• type II fibers tend to have a large diameter than Type I fibers
• bigger fibre bigger signal - not neccesarily the muscle doing more it is just louder generating a larger electrical signal

Question 26

Other Factors That Affect the EMG Signal - Muscle Temperature

Answer 26

• as muscle temperature increases, conduction velocity increases
• this leads to higher frequency and amplitude of the EMG signal
• temperature affects your ability to generate force - shift in line - stronger at given velocity because the system is working stronger generating more electrical signal
• a warmer muscle generates more electrical signal

Question 27

Other Factors That Affect the EMG Signal - Blood Flow to the Muscle

Answer 27

• as muscle blood flow increases, the amplitude of the EMG increases
• for the same level of force more blood flows to a muscle during dynamic exercise than during isometric exercise
• as blood flow increases an EMG signal will increase for the same level of force is there is more blood flow rushing to the muscle the signal will be bigger

Question 28

Other Factors That Affect the EMG Signal - Muscle Fatigue

Answer 28

◦ EMG amplitude decreases during maximal contractions
◦ why does this happen?
‣ all fibers in the muscle are active
‣ as the muscle fatigues, active fibers drop out
◦ EMG amplitude increases during submaximal contraction
◦ why does this happen?
‣ during a submaximal action typically type I fibers are active
‣ as fatigue sets in, type II fibers are activated to prolong activity
* could be a central reason, our ability to activate our muscle decreases as the signal can no longer come down - you stop after a while of sprinting because you cannot remove lactate from the body - fatigue can mean a lot of things
* EMG looks different depending on what tired means and what you are doing
* you tend to rely on small motor units that rely small signals when doing endurance activities - when in slow twitch fibres they can be active for a very long time but as they start to fatigue the amount of oxygen going to them is less
* we have larger muscle cells (fast twitch) that have energy source within the muscle cells so as we fatigue we start to bring on larger muscle cells with that there is more force so you cannot control the same way
◦ the EMG activity gets bigger
* if you are doing a maximal contraction (100m dash) you do not want to waste time you only care about big muscles being active right off the bat but as the energy quickly goes away and you have to rely on small ones in the end - the EMG activity gets smaller
* the strategies for activating the muscles are different even tho force may be similar at some point in time

Question 29

Other Factors That Affect the EMG Signal - extrinsic or technical Factors

Answer 29

electrodes, the study participants themselves, and the equipment

Question 30

Other Factors That Affect the EMG Signal - The Electrodes

Answer 30

the electrodes
- best location for the electrodes is on the belly of the muscle, on the midline
- larger electrodes have a larger recording surface
- electrodes aren’t selective and can pick up activity from nearby muscles (crosstalk)
- electrodes can pick up electrical signals that do not originate from the muscle (ambient noise, electrical activity associated with any given environment)

• a needle, wire or sticker - when you put an electrode overtop a muscle and bone - everytime it becomes active you pick up activity from muscles and so usually you can use wires instead but if you use wires you might miss out on ones you do want from other muscles

Question 31

Other Factors That Affect the EMG Signal - Partcipants Themselves and Equipment

Answer 31

the study participants themselves
- skin and subcutaneous fat acts as a filter
- as the distance between active fibers and the electrodes, the frequency and the amplitude of the EMG signal decreases
- the tissue type can affect what around - in positive way as barrier and negatively in terms of getting information

the equipment
- the connection between the electrode and skin
- the location of the equipment (relative to one another) and the quality of the equipment will affect the quality of the EMG signal
- cost
- general sense of what is happening at the muscle not force