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Revision > Neuromuscular Biomechanics - EMG > Flashcards

Neuromuscular Biomechanics - EMG Flashcards

1
Q

Sport & Exercise uses for EMG (x4)

A
  • is the muscle active
  • when is the muscle active
  • is a muscle more or less active
  • does the muscle fatigue
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2
Q

How can EMG be used for the assessment of injury risk

A
  • using muscle (concentric) pairs, are they both active/equally during contractions & the EMD of the muscles
  • lower explosive force production of the hamstring relative to the quads could compromise knee joint stability & increase ACL injury risk
  • also want EMD to be similar and forces exerted to be similar to reduce risk of injury
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3
Q

Ricci et al. (2013) example study for EMG injury risk

A
  • quads were 79% stronger than hamstrings, explosive force 480% greater from 25-50 ms after first activation
  • hamstring EMD was 95% greater than quads EMD resulting in 21 ms later onset of force in hamstring
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4
Q

how an EMG signal is generated

A
  • the stimulation of the muscle fibre at the motor end-plate producing a reduction of the electrical potential of the cell (depolarisation)
  • this then propagates over the entire fibre before being followed by a repolarization wave
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5
Q

4 Steps of EMG generation

A
  • detection of the potential fluctuations (electrodes)
  • signal transmission (hardwire/telemetry/data logger)
  • signal modification (amplifier)
  • storage of the resulting waveform (computer)
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6
Q

process of signal amplification

A
  • signal is small, varying from 10 to 5mV, so signal needs to be amplified up to at least a level of 1V
  • differential amplifier usual type of amplifier, which amplify the EMG signal linearly without amplifying noise or error in the signal
  • sources of error in the EMG signal can be from sources other the muscle (e.g. machinery/amplifier itself)
  • amplifier must have high input impedance (resistance) & good frequency response
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7
Q

Causative intrinsic non-controllable factors affecting EMG

A

Physiological
- number of active MU’s; MU firing rate and synchronisation; fibre type and diameter; blood flow; metabolic factors
Anatomical
- fibre diameter/type; depth and location of fibres; subcutaneous tissue; number of muscle fibres

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8
Q

Electrode types are an extrinsic (causative) factor affecting EMG. What types are there and what’re the +ve’s and -ve’s?

A

Indwelling electrodes
- used for deep muscle and isolated MU’s
- (-ve) invasive and difficult to use for dynamic actions
Active Surface Electrodes
- used for superficial muscles/large muscle groups
- (+ve) early pick up of signals; amplification; transmitted at low ohm level which is less sensitive to motion artifact; less skin prep; less sensitive to impedance of the electrode-skin interface
- (-ve) bulky; placement errors; cross-talk

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9
Q

Causative extrinsic factors affecting EMG

A

Impedance
-skin prep (light red colour); electrode impedance tester
Location of electrodes
-NOT on outside edges (cross-talk); NOt on motor point (greatest neural density); NOT on tendon (fewer & thinner fibres); between motor point (innervation zone) and tendon (point where muscle begins to twitch with lowest amount of current); on the muscle belly
Orientation of electrodes
- parallel with muscle fibres

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10
Q

How to determine innervation zone?

A
  • use an electrical stimulator
  • array sensor = the most accurate
  • location of the highest value of frequency
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11
Q

When standardising EMG recordings, what factors do you have to consider?

A

Joint Angle
- influences EMG amplitude because muscle mechanics change with length. The muscle may also migrate below electrodes
Range of Motion
- (same as above) use goniometers, training machines or mirrors
Movement Velocity
- higher velocity may mean more MU recruitment
Load/resistance
- use static resistance or external weights
Duration/repetitions
- strongly determines influence of fatigue (so use fixed durations/reps)
Preliminary status (e.g. fatigue)
- metabolic & CNS conditions & the time of day (e.g. same time of day, standardized warm up)

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12
Q

General EMG recommendations

A
  • prefer isometric tests
  • use dynamometers if a high standardization is needed
  • single joint exercises have less variability
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13
Q

what is Nyquist Sampling Theory

A

A band limited continuous-time signal can be sampled & perfectly reconstructed from its samples if the waveform is sampled over twice as fast as its highest frequency component

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14
Q

Causative extrinsic factors: crosstalk characteristics

A
  • when the detected signal contains noise from another muscle
  • common in small, close muscles (e.g the forearm)
  • range between 3-10%, can be up to 17%
  • detected using cross-correlation
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15
Q

Methods of reducing crosstalk (Causative extrinsic factors of EMG)

A
  • decreasing electrode size & spacing
  • placing the electrode on the muscle belly
  • double differential technique (3 electrodes)
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16
Q

Methods of reducing noise

A

Differential amplification/CMRR
- signal is detected at two sites, the signals are subtracted, and the difference is amplified
- as a result, any signal that is ‘common’ to both detection sites will be removed
- most noise is ‘common’ to both detection sites
Double differential technique
- the signal is detected at three sites & the subtraction procedure is performed twice

17
Q

What is noise as an extrinsic factor affecting EMG signal

A
  • noise = any signals that are not part of the physiological signal
  • there is inherent noise within detection/recording equipment
  • types of noise: ambient noise (50/60 Hz); motion artifact (0-20 Hz); physiological noise e.g. ECG (around 80 Hz); electromechanical noise
  • can be caused by external pressure or change in position of detection site
  • av. baseline noise should not exceed 3-5 microvolts, frequency between 10-250 Hz
  • aim is to maximise signal-to-noise ratio
18
Q

Types of filters used to reduce noise

A 
Low-pass 
- lets lower frequency through
High-pass
- lets higher frequency through
Band-pass
- lets frequencies within certain range
19
Q

Inherent filters used to reduce noise

A 
Tissues
- low-pass filter
Electrode to electrolyte interface
- high-pass filter
Bipolar configuration
- band-pass filter
Amplifier
- band-pass filter
20
Q

what is a time-domain used for when processing an EMG signal?

A
  • used to estimate amplitude
  • Raw EMG data is rectified using…
    AREMG
  • average rectified value over a time period (area under rectified EMG)
    RMS
  • the square-root of the average power of the signal in a given time (measure of the number of recruited MUs)
21
Q

what is a frequency-domain used for when processing an EMG signal?

A
  • used as a fatigue indicator
  • parameters mainly used…
    Median Frequency
  • more sensitive to spectral depression
    Mean Frequency
  • less variable
    Zero crossings
    Time to peak
22
Q

how does frequency analysis of an EMG signal work as a fatigue indicator

A
  • fatigue results in compression of the frequency spectrum towards lower frequencies.
    This is due to…
  • fatigue of higher threshold MU’s
  • decreased conduction velocity (metabolite accumulation)
  • MU synchronisation
23
Q

what is normalisation of EMG signals and what’re the +ve’s and -ve’s

A
  • expression of muscle activity in relative terms
  • main one used is PEAK EMG (isometric contraction)
    (+ve’s)
  • allows comparisons with other muscles, other subjects & between studies
  • eliminates any influence of the detection conditions
    (-ve’s)
  • ability to consistently elicit an MVC varies between between individuals
  • poor repeatability of EMG from MVC’s
  • mechanics of movement but MVC is completed at fixed angle
24
Q

what is the EMG-tension relationship

A
  • as muscle tension & EMG signal are affected by the same factors (number and firing rates of the active muscle fibres), then a relationship should exist between them
  • however the expected relationship only applies to the contractile elements
  • both linear (Lippold, 1952) & non-linear (Vredenbregt & Rau, 1973) EMG-tension relationships have been found
25
Q

possible reasons for contradictory elements (x8) of the EMG-tension relationship

A
  • control properties of EMG signal unstable between 80-100% MVC
  • cross-talk
  • temporal dissociation/EMD
  • muscle type (e.g. plantar flexors vs. biceps brachii)
  • motor unit synchronisation
  • individual characteristics (e.g. fibre type ratios)
  • electrode detection volume
  • CNS control strategy
26
Q

what problems are there when calculating the EMG-tension relationships

A
  • for slow movements (constant speed/constant force), a linear relationship has been demonstrated by some researchers
  • this however causes problems calculating EMG-tension relationship due to short activation periods & changes in muscle length
  • therefore, need for future research
27
Q

what relationship is apparent between sEMG & force during dynamic contractions?

A
  • not linear during dynamic contractions
  • because of change in muscle length, force & moment arm
  • also there is a change of the EMG signal
28
Q

how to use the EMG-tension relationships to predict (neuromuscular) training status

A
  • has been suggested that the EMG-force relationship can be used to determine the neuromuscular training status of a muscle
  • due to ramping isometric protocols, well trained muscles have shown a clear shift to the right whilst atrophic untrained muscles shown a left shift (on graph with EMg activity (y-axis) and force production level (x-axis))
29
Q

other uses of the EMG-tension relationship

A
  • used to assist torque calculations within biomechanical models
  • therefore, one can safely derive that with any EMG increase the torque & compression force around a joint increase in a similar fashion
30
Q

limitations with the EMG-tension relationship

A
  • it is impossible to record a single MAP
  • muscle fibres overlap, so sEMG detect MUAPTs from several MU’s

*MUAPT = motor unit action potential train

31
Q

limitations of EMG

A
  • EMG is a semiquantitative technique as it only gives indirect information regarding the strength of the contraction of muscles
  • difficult to obtain satisfactory recordings of dynamic EMG during movements such as walking/running
  • when not normalised, the EMG is only an indication of muscle activity
32
Q

steps of analysing the EMG signal

A

1) rectify the EMG signal, which involves taking the absolute value of the raw signal & making all signals positive
2) a linear envelope may be determined at this point, which involves filtering out high-frequency content of the signal to produce a smooth pattern

33
Q

what conclusions cannot be drawn from sEMG?

A

Longitudinal outcomes
- strength & hypertrophy
- not supposed in literature to use sEMG to exhibit gains in these
Acute & Mechanistic Variables
- as, muscle dependent, motor units are recruited differently, this will alter sEMG amplitude
- therefore, motor unit recruitment & rate coding conclusions cannot be made

34
Q

what conclusions can be drawn from sEMG for within muscle research? (within subject & between subject)

A

Within subject
- changes in neural drive, associated with changes in motor performance
- low changes in function occur
Between subject
- can inform potential mechanism for differences in function
- can explain differential mechanisms for changes in function

35
Q

what conclusions can be drawn from sEMG for between muscle research? (within subject & between subject)

A

Within subject

  • provide insight into how changes in function may arise from acute interventions
  • which different muscles are recruited during exercise (and to what extent)

Between subject

  • when used with statistical analysis, can provide information about complex neuromuscular pathology
  • can inform mechanisms for differences in function

Both
- can help explain how changes in function arise

36
Q

Considerations for applicability of sEMG (x4)

A

1) comparing sEMG signals between different exercises that don’t control for muscle length, contraction mode & speed of contraction should be avoided
2) even if sEMG signal represents force of a muscle, caution should be exercised when concluding an exercise is better for strength/hypertrophy adaptations due to confounding factors
3) changes in sEMG normalised value over time cannot indicate changes in excitation as these values can be influenced by excitation during both the normalisation & measured exercise contractions
4) within-subject, within-muscle comparisons of the sEMG signal across different exercises may be able to provide insights into muscular force production, provided controls are made

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