Electromyography 1 - Measuring Neuromuscular Activity

Question 1

state what it is meant by the key term - electromyography (EMG)

Answer 1

EMG is the registration and interpretation of the electrical activity associated with the contraction of skeletal muscle

Question 2

state what it is meant by the key term - electrograph

Answer 2

an electrograph is the instrument used to record EMG

Question 3

state what it is meant by the key term - electrogram

Answer 3

the electrogram is the recording of the electrical signal generated by the neuromuscular activity

Question 4

state what EMG is measured in, and give 2 supporting statements

Answer 4

• EMG signal is measured in Volts (V/mV)
  1) the magnitude of the EMG signal is usually < 5 mV (peak to peak)
  2) that is 1/1000 the voltage of usual batteries

Question 5

state what it is meant by the key term - Motor Unit (MU)

give a supporting statement

Answer 5

the MU is the motor neurone plus all the muscle fibres it innervates

the functional unit of the neuromuscular system

Question 6

state what it is meant by the key term - Innervation Ratio

Answer 6

the Innervation Ratio represents the number of fibres that are activated by the motor neurone

Question 7

give 3 examples of innervation ratios for the following:

1) Rectus Lateralis
2) Opponens Policis
3) Medial Gastrocnemius

Answer 7

Rectus Lateralis –> 5 fibres per motor neurone

Opponens Policis –> 89 fibres per motor neurone

Medial Gastrocnemius –> 1934 fibres per motor neurone

Question 8

use 7 steps to briefly explain the muscle fibre action potential

Answer 8

• axonal AP triggers ACh release
• Na+ channels open in sarcolemma
• depolarisation of the membrane
• generation of the MAP
• propagation along the fibre and down the transverse tubules
• release of Ca2+ by the SR
• interaction of the contractile proteins

Question 9

state the directions and speed of muscle fibre action potential (MAP)

Answer 9

longitudinally –> 2-5 m/s

transversally –> 0.04 m/s

Question 10

will there be a delay in EMG readings?

Answer 10

there is a delay between the AP and the receptor picking up the signal due to velocity of the MAP and the distance between the motor end plate and the electrode

Question 11

what is the magnitude, time interval, and frequency of EMG signals?

Answer 11

Magnitude = (baseline to peak) is called the amplitude

Time Interval = the time between two subsequent EMG recordings

Frequency = the inverse of the time interval

Question 12

what is a ‘bipolar/differential electrode’ ? (3 points)

Answer 12

• two electrodes are placed (one positive and one negative)
• when the MAP travels along the muscle, its picked up by one before the other
• you don’t report both readings, so you minus one from the other to get the bipolar reading (above and below zero)

Question 13

what are the two factors to which MUAP (motor unit action potential) varies with?

Answer 13

1) Muscle fibre type
- amplitude is higher in fast-twitch muscle fibres
- conduction is faster in fast-twitch muscle fibres

2) The measurement sites
- amplitude decreases with distance
- longer time delay with distance

Question 14

state what it is meant by the key term - Motor Unit Action Potential (MUAP)

Answer 14

the MUAP is the sum of the electrical signals generated by all the fibres of a motor unit

Question 15

state what it is meant by the key term - Motor Unit Action Potential Trace

Answer 15

the MUAPT is the sequence of MUAPs resulting from repeated neural stimulation

Question 16

so, what actually is EMG?

Answer 16

the EMG is the space and time information of all the MUAPTs from all the motor units that are activated in a specific amount of time

Question 17

State, and possibly explain, 2 facts about muscle force generation

Answer 17

1) recruitment - recruited in order of inc^ size
2) discharge of firing rate - frequency of MU activation
- the more MU’s that are recruited, and the higher the frequency of the signal, the larger the amplitude of the EMG signal

Question 18

EMG is recorded via electrodes. Explain this

Answer 18

electrodes are conducted elements that can be put inside your muscle (closer to the activation sites) or on the skin’s surface

Question 19

EMG recordings can have monopolar configurations. Use 2 points to explain this

Answer 19

1) you have a detection electrode on the skin or in the muscle, you then compare the recorded potential with a reference electrode (usually on a bony, non-activated site)
2) issue is, when you record with monopolar configuration, you not only record the activation from the muscle, but also associated noise

Question 20

EMG usually has bipolar configuration. Explain this using 4 points

Answer 20

1) electrical difference between the positive and negative electrodes
2) attenuates noise
3) improves signal:noise ration
4) although electrical signal reaches electrode at diff times, the noise hits at the same time. this should then cancel out when you differentiate from one electrode to another

Question 21

state what it is meant by the key term - noise

Answer 21

Noise is unwanted signals detected by the electrodes

e.g. - other biological signals (ECG), external sources (mains electricity, movement aftifacts…)

Question 22

what is the difference between surface electrodes and indwelling electrodes?

Answer 22

Surface electrodes: record on the skin

Indwelling electrodes: (fine wire, needle) record within muscles

Question 23

state what it is meant by the key term - Muscle Action Potential (MAP)

(2 points)

Answer 23

the MAP is the name given to the waveform resulting from the depolarisation of the muscle fibres

depolarisation depolarises along each muscle fibre from the motor end plate, followed by a repolarisation wave

Question 24

what is the cause of a MUAP?

Answer 24

the summation in space and time of MUPs of a given motor unit is termed a Motor Unit Action Potential (MUAP)

Question 25

what is a MUAPT

Answer 25

a sequence of MUAPs resulting from repeated neural stimulation is referred to as a Motor Unit Action Potential Train (MUAPT)

Question 26

so, what is the EMG actually measuring? (2 points)

Answer 26

the physiological EMG signal is the sum, over space and time, of the MUAPT from the various motor units

EMG is the only objective method of assessing whether a muscle is activated or not

Question 27

What information do EMG recordings give you?

Answer 27

provides information on timing & sequencing of the activity of various muscles in sporting environments

EMG can also validate assumptions about muscle activity that are made when calculating the internal forces in the human MSK system

Question 28

what does EMG allow for biomechanics?

Answer 28

allows sports biomechanists to study changes in muscle activity during skill acquisition and as a result of training

Question 29

what can’t EMG do?

Answer 29

EMG cannot necessarily reveal what a muscle is doing, particularly in fast, multi-segment movements that predominate in sport

Question 30

Explain what is it meant by the term ‘Time Series’ (2 points)

Answer 30

EMG signals change over time (‘time series’) which could be a change to a different movement pattern which could be useful to qualitative and quantitative movement analysis

due to signal complexity however, it has rarely been used in this way

Question 31

the EMG signal recorded is not the actual signal produced by the motor units. its characteristics are modified through which the signal passes in reaching the recording electrodes. state 3 supporting statements

Answer 31

1) tissues act as ‘low-pass filters’, rejecting some of the high frequency components of the signal
2) the electrode-electrode interface acts as a ‘high-pass filter’, discarding some of the lower signal frequencies
3) the bipolar configuration (2 electrodes) changes the two wave (depolarisation to repolarisation) into a three-phase one and removes some of the low and high frequency signals –> acts as a ‘band-pass filter’

Question 32

state 2 basic facts about extrinsic factors which can influence the EMG recording

Answer 32

• can be controlled

- the use of equipment with appropriate characteristics is vitally important

Question 33

explain, using two points, how external characteristics which can influence the EMG signal can be controlled

Answer 33

• this includes the location of the electrode in relation to the motor end plate
• the orientation of the electrodes with respect to the muscle fibres and the electrical characteristics of the recording system

Question 34

explain how the use of equipment with appropriate characteristics is vitally important to prevent external characteristics influence the EMG signal

Answer 34

• the requirement is to detect the electrical signals (electrode), modify the signals (amplifies), and store the resulting waveform (recorder)
• all should be done linearly and without distortion

Question 35

are intrinsic factors modifiable?

Answer 35

the electromyographer has little control over these factors

Question 36

state 4 physiological (internal) factors which influence the EMG recording

Answer 36

• the firing rate of motor units
• the type of muscle fibres activated
• the conduction velocity of those muscle fibres
• the detection volume (the volume of muscle from which the electrodes can detect a signal)

Question 37

explain the anatomical (internal) factors which influence the EMG signal (2 points)

Answer 37

• muscle fibre diameters and the positions of the fibres of a motor unit relative to the electrodes
• the seperation of the individual MUAPs becomes increasingly difficult as the distance to the electrode increases

Question 38

when are indwelling electrodes usually used?

Answer 38

during clinical research

Question 39

what is the major benefit of surface electrodes?

Answer 39

they’re safe (as oppose to indwelling electrodes having the be implanted into the muscle)

Question 40

state 2 facts about the ‘use’ of surface electrodes

Answer 40

can be passive, having no electrical power supply; or active, having a power supply

safer, easier to use, more acceptable to the participant, provides quantitatively similar results to indwelling electrodes

Question 41

what are EMG cables?

Answer 41

EMG cables are required o connect the EMG electrodes to the amplifiers (or pre-amplifiers)

Question 42

state, and explain, how EMG cables can cause problems in EMG recording

Answer 42

can cause ‘cable artefacts’ as have frequencies in the range of 0-10 Hz

Question 43

explain how cable artefacts can be reduced (2 points)

Answer 43

use of ‘high-pass filtering’, high quality electrical shielded cables, and carefully taping reduces cable movement and reduces artefacts

artefacts can further be reduces using pre-amplifiers mounted on the skin or near the detection electrodes & can virtually eliminate cable artefacts

Question 44

what do EMG amplifiers do?

Answer 44

EMG amplifiers provide linear amplification over the while frequency and voltage range of EMG signals

Question 45

what are the 4 most common amplifier characteristics?

Answer 45

1) common mode rejection
2) frequency response
3) input independence
4) gain

Question 46

state what is is meant by the amplifier characteristic - Gain

Answer 46

Gain: the ratio of output voltage to input voltage

Question 47

explain ‘gain’ in terms of being an amplifier characteristic (2 points)

Answer 47

• Gain should be large, and ideally in a range of 100 - 10000
• proper gain settings improve the signal to noise ratio, while an improper setting can cause clipping of the H-reflex signal or can increase the noise level

Question 48

explain what it is meant by ‘Input Independence’ in reference to it being a key amplifier characteristic (2 points)

Answer 48

• attaining low skin resistance can be obtained by using an amplifier with a high input independence (high resistance to the EMG signal)
• independence should be 100x skin resistance to avoid attenuation of input signal

Question 49

what are the usual input independencies used in amplifiers? (2 points)

Answer 49

• recommended minimum input independence of an amplifier for the use with passive surface electrodes is 100 MΩ
• input independencies for high performance amplifiers used with active electrodes can be as high as 10 MΩ

Question 50

what is Frequency Response (amplifier characteristic)

Answer 50

Frequency Response: the ability of the amplifier to produce the range of frequencies produced in the signal

Question 51

explain Frequency Response (amplifier characteristic)

Answer 51

• required frequency depends on EMG frequency
• typically EMG signal in in band of 10 - 1000 Hz for surface electrodes
• most EMG amplifiers meet this requirement (use a low-pass filter for higher frequencies)

Question 52

what is Common Mode Rejection? (4 points)

Answer 52

• mains > approx 100 mV EMG signal
• overcome by recording diff in potential (bipolar configuration)
• eliminates ‘hum’ as is picked up by each electrode (subtracted from each other)
• closer electrodes, triphasic wave becomes an approximate to a time derivative of a single electrode wave

Question 53

in practice, is perfect elimination of the ‘hum’ (mains) possible? (3 points)

Answer 53

no, and the success of removal is expressed by the Common Mode Rejection Ratio (CMRR)

should be > 10000

pre-amplifiers reduce noise picked up and minimise any degradation of the CCRR arising from differences between cable resistances

Question 54

explain ‘Recorders’ in EMG recording (3 points)

Answer 54

• A-D recorders and computer processing most commonly used
• an A-D converted with 12-bit or 16-bit resolution is recommended
• high sampling rates needed to reproduce successfully in the signal digital form

Question 55

what is ‘Muscle Redundancy’ or ‘Muscle Indeterminacy’ ? (2 points)

Answer 55

obtaining a predictive relationship between muscle tension and EMG signal could solve a major problem for quantitative sports biomechanists. this problem arises as the equations of motion at a joint cannot be solved as the number of unknown forces exceeds the number of equations available

if a solution to Muscle Redundancy could be found, it would allow the calculation of forces in soft tissue structures and between bones

Question 56

why do researchers expect a relationship between muscle tension and EMG recordings (2 points)

Answer 56

• EMG provides excitation measure –> if force depends on excitation, surely a relationship is there?
• muscle’s tension is regulated by number & firing rate of fibres, & EMG amplitude depends on the same two factors

Question 57

where’s the issue in expecting an EMG-Tension relationship ? (2 points)

Answer 57

discrepancies between studies for isometric contractions –> lead us to not expect a simple relationship for fast, voluntary contractions

for such movement, the relationship between EMG and muscle tension still remains elusive, although the search for it continues to be worthwhile

Question 58

where do FURTHER (different reasons) issues in expecting an EMG-Tension relationship (3 points)

Answer 58

in complex, multi-segment movements, muscles influence other joints as well as the ones that they cross

therefore, EMG tells us when a muscle is active, but not what it’s doing

^ this is a very important limitation to the use of EMG

Question 59

state 3 simple pieces of advise for skin electrode placement

Answer 59

• place both electrodes over the mid-point of the muscle belly
• parallel orientation of electrodes to muscle fibre
• if parallel arrangement not linear or parallel, the line between the two muscles should point to the origin and insertion for consistency

Question 60

state 3 ADDITIONAL pieces of advise for skin electrode placement

Answer 60

• electrodes placed onto shaved skin
• attachment area clean by alcohol wipe (allowed to dry)
• abrasion of site to aid adhesion
• pre-gelled electrodes
• reference electrode in use
• if using older equipment, a quick ohmeter reading of skin resistance should be done and resistance should be < 10 MΩ