Measurement of Voluntary Muscle Activity

Question 1

Non-sporting Applications of EMG

Answer 1

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

Question 2

Sporting Applications of EMG

Answer 2

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

Question 3

What are the four components of the motor unit?

Answer 3

1. Muscle fibres
2. Motor axon
3. Motor neuron
4. Motor end plate

Question 4

Explain the process of the EMG signal?

Answer 4

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)
»
Signal TRANSMISSION (hard-wire/telemetry/data logger)
»
Signal MODIFICATION (amplifier)
»
STORAGE of the resulting waveform (computer)

Question 5

Waveform =

Answer 5

Muscle action potential

Question 6

Spatio-temporal summation of Muscle action potentials =

Answer 6

Motor unit action potential

Question 7

Repetitive sequence of Motor unit action potentials =

Answer 7

Motor unit action potential train

Question 8

What are the Extrinsic types of factors that can affect EMG?

Answer 8

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

Question 9

What are the Intrinsic types of factors that can affect EMG?

Answer 9

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

Question 10

Three advantages of Active surface electrodes?

Answer 10

1. Early pick up of signal
2. Amplification
3. Transmitted at low ohm level which is less sensitive to motion artefact

Question 11

Three disadvantages of Active surface electrodes?

Answer 11

1. Bulky – pressure artefacts
2. Placement errors
3. Cross-talk

Question 12

Factors to consider when standardising EMG recordings

Answer 12

• 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

Question 13

What are three general recommendations when

Standardising EMG recordings?

Answer 13

1. Prefer isometric tests
2. Use dynamometers if a high standardisation is needed
3. Single joint exercises have less variability

Question 14

Sampling Frequency =

Answer 14

> 1000 hz

Question 15

What is cross talk?

Answer 15

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

Question 16

How can cross talk be reduced?

Answer 16

• Decreasing electrode size and spacing
• Placing the electrode on the muscle belly
• Double differential technique – 3 electrodes

Question 17

What is noise and how do you reduce it?

Answer 17

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)

Question 18

What are the four inherent filters?

Answer 18

• Tissues: low-pass filter
• Electrode to electrolyte interface: high pass filter
• Bipolar configuration: bandpass filter
• Amplifier: bandpass filter

Question 19

What do AREMG and RMS stand for?

Answer 19

• 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)

Question 20

What three reasons cause fatigue results in compression of the frequency spectrum towards lower frequencies?

Answer 20

• Fatigue of higher threshold MU’s
• Decreased conduction velocity – metabolite accumulation
• MU synchronisation

Question 21

What does Normalisation mean?

Answer 21

Expression of muscle activity in relative terms

Question 22

MVC normalisation advantages and disadvantages?

Answer 22

• 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

Question 23

Threshold Analysis of EMG

Answer 23

▪ 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