Measurement of Muscle Activity

Question 1

What are some of the non-sporting uses of EMG?

Answer 1

Ergonomics
Clinical settings
Surgery prep
Products

Question 2

What do we use EMG for in sport?

Answer 2

Is a muscle active and when is it active?
Does it fatigue?
Is there coactivation present?

Question 3

What conclusions cannot be drawn from sEMG?

Answer 3

Increased responses doesn’t imply an increase in motor unit recruitment

Question 4

What conclusions can be drawn from sEMG?

Answer 4

If a muscle is on/off
Timings of activation
Muscle excitement

Question 5

What are some of the neural factors affecting force generation?

Answer 5

Number of neurons
Size of the neurons
Rates of action potential discharge

Question 6

How is a signal generated?

Answer 6

Nerve impulses cause muscle fibre contraction

Motor end point stimulation reduces the electrical potential of the cell (depolarisation)

Question 7

What is termed the waveform, Spatio temporal summation and repetitive sequence?

Answer 7

Muscle Action Potential (MAP)
Motor Unit Action Potential (MUAP)
Motor Unit Action Potential Train (MUATP)

Question 8

What is the electromechanical delay?

Answer 8

Delay in the sending of the impulse and then seeing the movement occur

Question 9

What is the process of EMD?

Answer 9

Detection of fluctuations (electrodes)
Signal Transmission
Signal Modification
Storage of Waveform

Question 10

What are the intrinsic factors affecting the EMG signal?

Answer 10

1) Physical –> no. of active MU’s, fibre type, blood flow

2) Anatomical –> fibre diameter, depth of fibres

Question 11

What are the extrinsic factors affecting the EMG signal?

Answer 11

1) Indwelling –> deep muscles and isolated MU’s
2) Surface –> focus on large muscle groups (lowers sin prep but increases crosstalk risk)
3) Passive –> prone to placement errors and crosstalk

Question 12

What are the 3 causative extrinsic factors of EMG signal/measurements?

Answer 12

1) Electrode Location –> must be on muscle belly
2) Impedance –> correct skin preparation is vital
3) Orientation –> parallel with the muscle fibres

Question 13

What must we do to standardise EMG recordings?

Answer 13

Isometric tests (single joint exercises)
Sample at 1000Hz (due to Nyquist theory)

Question 14

What is crosstalk?

How can it be reduced?

Answer 14

Detected signal contains noise from another muscle

Reduce electrode size and place on muscle belly

Question 15

What is noise?

What are the frequencies at ambient, motion and physiological times?

Answer 15

Any signals not part of the physiological signal

50/60Hz, 0-20Hz, 80Hz

Question 16

How can you reduce noise of an EMG signal?

Answer 16

1) Tape the electrodes down
2) Differential Amplification –> subtract 2 sites and then amplify that answer to amplify it
3) FIlters –> Tissues (low pass), Electrode (high pass)

Question 17

How do you process the signal related to the time component?

Answer 17

Where you estimate the amplitude of the signal

1) Raw EMG –> basic identification
2) Rectification –> AREMG (average rectified value), RMS (root mean square), Linear Envelope

Question 18

How do you process the signal related to the frequency component?

Answer 18

Indicator of fatigue

Median, Mean, Zero Crossings, Time to Peak

Question 19

What is the role of the frequency spectrum?

Answer 19

Low frequencies contribute a low amplitude to the trace
Likewise with high frequencies
However when all things happen at once it fills the spectrum completely with ranges of amplitudes

Question 20

How does fatigue affect the frequency spectrum?

Answer 20

Compression of the spectrum towards the lower frequencies due to:

• fatigue of high threshold MU’s
• reduced conduction velocity

Question 21

What are the advantages and disadvantages of EMG normalisation testing?

Answer 21

ADV –> compare muscles/subjects

DISADV –> eliciting a MVC differs, poor repeatability of EMG within MVC’s

Question 22

What does threshold analysis accurately assess?

Answer 22

When the muscle is active/inactive

Define a threshold and assess when a signal is above that

Question 23

What is the EMG-tension relationship?

Answer 23

Muscle tension and EMG signal are both affected by the same factors:
No. and firing rate of muscle fibres/MU’s