Neural Activation and EMG Lectures

Question 1

What are the two activation parameters behind muscular force?

Answer 1

Recruitment and Firing Frequency

Question 2

What is the Hennemans Size Priniciple?

Answer 2

Describes a linear relationship between motor unit recruitment and muscular force output.

At low levels of force, the smaller motor units (known as slow twitch MU), then increasing to medium forces (T resistant MU) and again to large forces (Type II fibres) are recruited.

Basically, with increasing for, the bigger MU are recruited, however, these fibres are more fatiguable, hence slow twitch fibres are recruited first.

Question 3

What is firing frequency?

What is the relationship between firing frequency and force production?

Answer 3

Definition: the rate at which electrical impulses (action potentials) are delivered to the muscle fibres.

Relationship: as firing frequency increases, the muscle has less time to relax between APs, which results in a build up of force until a linear force trace, or tetanus, is reached.

Question 4

What is the onion skin model of MU firing rate?

Answer 4

At any given force, the MU that were recruited first have the highest firing rate.

Question 5

What is the minimal physiological firing rate?

What is the minimum firing rate to achieve MVC?

Answer 5

5-8 Hz

30-50Hz

Question 6

What does interpolated mean?

Answer 6

Adding an external stimuli to deliver twitch contractions on top of voluntary contractions.

Question 7

Describe the linear ITT relationship?

Answer 7

ITT applies a superimposed twitch on top of a voluntary contraction. The method suggests that as voluntary contraction force goes up, the size of the superimposed twitch will decrease.

To generate more voluntary force, humans activate (more recruitment and higher rate coding) more MU. This means that there are less available MU for the superimposed twitch to activate.

The method suggests that during full activation, there will be no twitch response.

Question 8

How does the linear ITT calculate %VA?

Answer 8

The size of the superimposed twitch on top of voluntary contraction is compared to the size of a superimposed twitch upon a muscle at rest.

This equation is then used

Voluntary activation= [1 - (superimposed twitch/control twitch)) x 100]

Question 9

What are the problems associated with the Linear ITT equation?

Answer 9

The equation assumes a linear relationship between interpolated twitch force and voluntary force. Several studies (tillen et al., 2011 for example) have found that this relationship is in fact curvilinear.

thus, a linear relationship produces a low estimate of voluntary force required to achieve full activation. In addition to an overestimated value of voluntary activation.

Question 10

What is the definition of EMG and what are some of EMGs uses?

Answer 10

A technique for evaluating and recording the electrical activity within skeletal muscle during contraction

Uses:

• assessing neural activation
• can be used to infer force
• Assess the timing and coordination
• index of fatigue

Question 11

Describe antagonist activation using the Alkner et al. (2000) paper?

Answer 11

During leg extension, BF (a hamstring muscle) activation increases creating opposing torque to the quadriceps.

This is highly unwanted in sports performance.

Question 12

What did Behm et al., 2002 report, using the ITT to determine inactivation properties of several muscles?

Answer 12

the ability to activate different muscles is varied.

The graph showed the dorsi flexors can almost achieve full activation whereas the quadriceps cannot by nearly 20%

Question 13

What findings did Lanza et al. (2012) report when comparing neural activation during a variety of joint angles?

Answer 13

Quad activation was higher at 80 and 110 degrees. More activation was achieved in more flexed positions.

With increasing flexion, however, antagonist muscle activation also increased. Creating opposing torque to the quads.

Question 14

What did Babault et al. (2004) report when comparing neural activation during different contraction types in sedentary individuals?

Answer 14

It is known that an isolated muscle can produce more force eccentrically than a human. This discrepancy suggests that humans cannot achieve full activation eccentrically.

The paper reported that untrained humans produce the smallest activation during eccentric contractions.

Question 15

Describe the findings of Amiridis et al., 1996; comparing torque-angular velocity traces between elite high jumpers and sedentary individuals?

Answer 15

throughout all angular velocities, Elite high jumpers can produce more torque.

Elite high jumpers were also able to produce more force eccentrically than isometrically; something that was not present in sedentary individuals.

With practice, the high jumpers are able to activate quads to a much greater extent during eccentric contractions.

Question 16

Explain what Dudley et al. (1990) reported when applying a superimposed twitch upon sedentary eccentric contractions?

What is the voluntary strength deficit?

Answer 16

With stimulation, eccentric contractions were able to produce much greater torque than voluntarily.

With voluntary contraction. some neural inhibition of failure of activation prevents full actiation of the full torque that could be achieved in the situation

The VSD is the space between the voluntary and the stimulated force traces.

Question 17

What happens to the rate of force development when the muscle is stimulated electrically?

Answer 17

Participants can achieve much quicker and greater forces

There is a normal 50ms time delay in attaining full muscle activation. This time delay is not present when muscle is stimulated.

Question 18

What relationship did de Rutler (2007) report between explosive strength and neural activation?

Answer 18

Those with fast neural activation were able to elicit greater explosive strength

R2= 0.74

Question 19

What were the main findings in the Vecchio et al. (2019) paper that used high density grid electrodes to understand recruitment and firing frequency properties within MUs during explosive contractions?

Answer 19

Both firing rate and speed of recruitment, especially firing rate, influence explosive strength during the early phase of the contraction

Question 20

What were the theoretical implications of the 50ms time delay in muscle activation in regard to cycling?

Answer 20

As the RPM of the cycle crank goes up, because the time required to activate and deactivate a muscle is relatively fixed, the actual activation profile in comparison to the optimal activation profile becomes more and more sub optimal

Question 21

What where the findings of the Van Soest and Casius (2000) paper when comparing theoretical power output (no activation/deactivation time delay) with normal power output?

Answer 21

Theoretical power output was 60% higher

Question 22

What are the fundamental problems with EMG signal?

Answer 22

Surface EMG signal is the net electromagnetic field at the point of measurement- it does not measure any muscular level variables and any electrical signal will be picked up, not just the muscle

EMG is the superposition of many EMF from different sources

Question 23

What might occur to mean that two consecutive EMG signals are not comparable?

Answer 23

If the sensor was moved between contractions, the participant was to sweat or if any technical problems occured.

Question 24

What is signal cancellation with EMG data?

Answer 24

Positive and negative EMG signals cancel each other out, providing a raw EMG output of 0.

Question 25

What are the practical limitations of EMG data

specifically signal-to-noise ratio

Answer 25

Connective tissues
Fat
Skin (sweat)
Other muscle 
Heart beat
Movement artefact
Ambient noise

factors changing with time- anthropometrics
age (VA)

Question 26

How do differential bipolar electrodes measure more accurate EMG data?

Answer 26

They measure the electromagnetic field between two sensors, that removes much of the background noise.

Question 27

What considerations should be taken when positioning electrodes?

Answer 27

To ensure there is no movement of sensors during contraction. This changes their placement relative to IZ, changing the signal recording.

Do not put sensors over the innervation zone. Signal is much smaller due to distance and speed to AP conduction velocity appearing greater.

Put sensors parallel to muscle fibres. Signal is measured between two sensors. Across fibres, no signal is the same, thus activation and deactivation will not represent the activity of a muscle fibre correctly.

The farther the electrodes are apart the more noise they are subject to. AMP is proportionate to the spacing distance.

Question 28

Why is normalising EMG data to %MVC useful?

Answer 28

Raw EMG signals cannot be compared reliably in most cases. Thus, normalising allows for a valid comparison.

Question 29

What is frequency content?

How might this be useful when analysing muscle fibre types?

Answer 29

The average frequency of EMG signal.

when fatiguing, medium to high frequency decreases due to fast twitch muscle fibres dropping out. A lot of signal content is due lots of AP running down the muscle fibre. Fast twitch fibres
have fast conduction velocity, so finish and drop out sooner. However, frequency content is affected by many things so its usefulness is debatable.

Question 30

What considerations are required when sorting out sampling frequency?

Answer 30

2000 Hz is the minimum frequency to get clean data.

If you don’t sample high enough high frequency background noise gets wrapped around as low frequency noise and disrupts the signal output. This is known as aliasing.

Question 31

When analysing EMG data, how do you identify the start of activity?

Answer 31

Change occurs when signal surpasses 3 STDEV from the baseline noise.

Question 32

What does it mean to rectify, low pass filter and RMS EMG data?

Answer 32

Rectify- turn all values positive

Low pass filter- smooths the curves out

RMS- dont have to rectify, EMG is presented in an Si unit and is more useful