Measurement of Voluntary Muscle Activity Flashcards
Non-sporting Applications of EMG
Ergonomics – workplace assessment e.g. repetitive strain problems
Clinical Diagnostics e.g. Gait irregularities, incontinence, rehabilitation
Helps in decision making both before/after surgery
Product design e.g. orthotics, bracing, taping
Sporting Applications of EMG
See if a muscle is active
When is a muscle active? e.g. Pattern of
activation in skill acquisition
Is a muscle more or less active? e.g. Changes in activity as a result of training
Does the muscle fatigue? e.g. isometric tasks
Is there coactivation?
Physical activity
What are the four components of the motor unit?
- Muscle fibres
- Motor axon
- Motor neuron
- Motor end plate
Explain the process of the EMG signal?
A nerve impulse causes contraction of the muscle fibres. The stimulation of the muscle fibre at the motor end-plate produces a reduction of the electrical potential of the cell (depolarisation) which then propagates over the entire fibre before being followed by a repolarisation wave.
DETECTION of the potential fluctuations (electrodes)
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Signal TRANSMISSION (hard-wire/telemetry/data logger)
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Signal MODIFICATION (amplifier)
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STORAGE of the resulting waveform (computer)
Waveform =
Muscle action potential
Spatio-temporal summation of Muscle action potentials =
Motor unit action potential
Repetitive sequence of Motor unit action potentials =
Motor unit action potential train
What are the Extrinsic types of factors that can affect EMG?
Extrinsic (Controllable)
• Electrode types (Indwelling/Surface electrodes)
• Impedance
(Skin preparation/ gel vs no gel)
• Orientation of electrodes
(Parallel with muscle fibres)
• Location of electrodes
(NOT on outside edges – crosstalk, NOT on motor point (greatest neural density), NOT on tendon – fewer and thinner fibres)
• Determination of innervation zone
(Stimulator/location of highest value of frequency)
• Noise
What are the Intrinsic types of factors that can affect EMG?
Intrinsic (Uncontrollable)
Physiological • Number of active MU’s • MU firing rate & synchronisation • Fibre type & diameter • Blood flow • Metabolic factors
Anatomical
• Fibre diameter
• Depth and location of fibres
• Subcutaneous tissue
Three advantages of Active surface electrodes?
- Early pick up of signal
- Amplification
- Transmitted at low ohm level which is less sensitive to motion artefact
Three disadvantages of Active surface electrodes?
- Bulky – pressure artefacts
- Placement errors
- Cross-talk
Factors to consider when standardising EMG recordings
• Joint Angle
(Influences EMG amplitude because muscle mechanics change with length)
• Range of Motion
• Movement Velocity
(Higher velocity may mean more MU recruitment)
- Load/Resistance
- Duration/Repetitions
• Preliminary status
(e.g. fatigue) – metabolic and CNS conditions and the time of day – same time of day, warm-up subjects
What are three general recommendations when
Standardising EMG recordings?
- Prefer isometric tests
- Use dynamometers if a high standardisation is needed
- Single joint exercises have less variability
Sampling Frequency =
> 1000 hz
What is cross talk?
When the detected signal contains noise from another muscle
- Common in small, close muscles e.g. the forearm
- Range between 3-10%, can be as high as 17%
- Detected using cross-correlation
How can cross talk be reduced?
- Decreasing electrode size and spacing
- Placing the electrode on the muscle belly
- Double differential technique – 3 electrodes
What is noise and how do you reduce it?
Noise is any signals that are not part of the physiological signal
Reduced by -
Differential amplification/CMRR
(The 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. CMRR should be as high as possible)
Double differential
(The signal is detected at three sites and the subtraction procedure is performed twice)
What are the four inherent filters?
- Tissues: low-pass filter
- Electrode to electrolyte interface: high pass filter
- Bipolar configuration: bandpass filter
- Amplifier: bandpass filter
What do AREMG and RMS stand for?
• 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 motor units)
What three reasons cause fatigue results in compression of the frequency spectrum towards lower frequencies?
- Fatigue of higher threshold MU’s
- Decreased conduction velocity – metabolite accumulation
- MU synchronisation
What does Normalisation mean?
Expression of muscle activity in relative terms
MVC normalisation advantages and disadvantages?
• Advantages:
– Allows comparisons with other muscles, other subjects and between studies.
– Eliminates any influence of the detection conditions
• Disadvantages:
– Ability to consistently elicit an MVC varies between individuals
– Poor repeatability of EMG from MVC’s
– Mechanics of movement – MVC at fixed angle
Threshold Analysis of EMG
▪ To accurately assess when the muscles is active / inactive
▪ Need to define a threshold and assess when the EMG signal is above this threshold
▪ Muscle Activity tested at baseline for ~25ms
▪ Activity classified when activity is > baseline by 2 SD for more than 25ms
