Electromyography 2.2 - Recording, Analysing, and Processing EMG

Question 1

what kind of filters are recommended for surface electrodes?

Answer 1

a high-pass filter set above 10 Hz and a low-pass filter set at 1 kHz

however, options vary at the exact bands-pass characteristics to be used

software filters can be used after recording to apply further digital signal processing

Question 2

what is the usual sample frequency for surface electrodes?

Answer 2

approx. 1000 samples per second

a low-pass filter set to the Nyquist limit should be implemented to prevent high signals distorting the signal

Question 3

give 3 methods of analysing and interpreting EMG signals

Answer 3

1) amplitude
2) peak-to-peak amplitude
3) root mean square amplitude
4) linear envelope
5) onset-offset analysis
6) normalisation of the EMG signal

Question 4

explain the following technique of analysis and interpreting the EMG signal:

EMG amplitude

Answer 4

major variables used to define amplitude: peak-to-peak amplitude, arEMG, RMS EMG, linear envelope, iEMG

Question 5

explain the following technique of analysis and interpreting the EMG signal:

peak-to-peak amplitude

Answer 5

• useful when signal is highly synchronous (e.g. the m-wave)
• when intensity of stimulation is inc^ sufficiently, all motoneurons are activated, resulting in max EMG activity the muscle is capable of producing

Question 6

how can the m-wave be calculated?

Answer 6

the m-wave can be calculated by working out the value of the negative to the positive peak amplitude

Question 7

explain the following technique of analysis and interpreting the EMG signal:

Onset-Offset Analysis (3 points)

Answer 7

• one criterion for ensuring onset & offset is that the high-frequency components of the signal have not yet been filtered or otherwise attenuated to the appropriate extent
• filtering can delay detection of onset & offset times
• many algorithms designed to detect these produce subjective results & require experimenter interpretation

Question 8

explain the following technique of analysis and interpreting the EMG signal:

normalisation of the EMG signal

Answer 8

• MVC commonly used as a reference
• max m-wave amplitude can be used (electronically stimulate muscle to produce max m-wave)
• errors in interpretation can occur with normalisation techniques when working with dynamic movements
• one solution is to use the MVC during a dynamic contraction for a reference

Question 9

state what it is meant by the key term - signal cancellation

Answer 9

signal cancellation refers to the cancellation of positive and negative phases of MUAPS

Question 10

explain ‘signal cancellation’ (2 points)

Answer 10

• an estimation of EMG amplitude from a raw EMG signal is less than that obtained by summing the amplitudes of the individual MUAPs
• signal cancellation can lead to an underestimation of the neural drive by up to 62% during a MVC (Keenan et al., 2005)

Question 11

further explain ‘signal cancellation’

Answer 11

• further inc^ with fatigue
• signal conduction velocity dec^ with fatigue, inc^ AP duration
• causes greater overlap of signal recording, causing underestimation
• signal cancellation may explain why EMG signals are considerably lower at the end of time to exhaustion tests

Question 12

alongside MVC, what else can be used to normalise the EMG signal, and why?

Answer 12

sub-max contractions

assumption - relationship between muscle activation and contraction intensity is linear –> 20% EMG should equate to 20% force

• shown to be true in some muscles, but not others (Lawrence., 1983) –> synergists or single muscle?

Question 13

state 2 drawbacks of using sub-max contractions for the normalisation of EMG signals

Answer 13

1) leads to misinterpretation of the role of some muscles in a given task
2) this method is further compromised in clinical populations as motor coordination strategies are often altered in individuals with a MSK disease

Question 14

what is the common method of determining muscle onset?

Answer 14

when the EMG amplitude passes a pre-determined threshold

this is typically 15-20% of a peak EMG (0r 1-3 SD’s above baseline)

also associated with a min duration period that the signal must be above the threshold

Question 15

what is the issue with the common method to determine onset of muscle activity

Answer 15

this method produces results highly dependent on the signal to noise0ratio and the choice of amplitude/duration ratios

Question 16

what is a secondary method of determining muscle onset? (3 points)

Answer 16

• Wavelength Transform (Marlo et al., 2013)
• takes advantage of the Teager-Kaiser energy operator (TKEO) to dec^ background noise of the signal
• shows higher accuracy than classical methods when the signal to noise ratio is low, it does induce a time delay for the onset of activation of approx. 20 ms

Question 17

what is still important to do when determining onset and offset times of a muscle’s activation?

Answer 17

visual inspection to reduce the potential for unusual occurrence of artefact/noise in the EMG signal to influence the reported data

Question 18

explain why EMG amplitude does NOT equal muscle force (3 points)

Answer 18

• strong linear relationship for some, but not all, muscles
• EMG amplitude normalised for a MVC is considered an index of torque
• however, torque depends on several biomechanical factors, such as PCSA

Question 19

explain why EMG onset is NOT Torque onset (5 points)

Answer 19

• onset of EMG depends on onset of activation & recording delay
• EMG onset depends on electrode placement
• assuming average conduction velocity of 5 m/s, every 1 cm away from MU causes a 2ms delay
• electromechanical delays occur between neural drive & torque
• as depends on electrochemical & mechanical processes, that can vary between muscles & individuals

Question 20

the concept that neuromuscular fatigue causes an inc^ in EMG amplitude does not hold up during dynamic tasks. Explain why

Answer 20

this is due to it being difficult to dissociate the effects of neuromuscular fatigue and the possible changes of muscle coordination strategies

• changes in predominant muscle
• inc^ in number of MU’s recruited
• both