Muscles And Motor Units - Lecture 6

Question 1

The EMG signal is concerned with what event in muscle contraction?

Answer 1

Action potential spreads across the sarcolemma / muscle membrane and into the fibre along the t-tubules

Question 2

What is a motor unit?

Answer 2

One alpha motor neuron and all the muscle fibres it innervates

Question 3

What are the two ways that muscles can generate more force?

Answer 3

• recruiting more motor units
• increasing the frequency of firing

Both these factors increase the EMG response

Question 4

The force of one single motor unit is tiny, how therefore can the response be seen?

Answer 4

By the process of averaging. With more than 10 spikes, the signal to noise ratio increases with increased numbers of traces forming an average.

Question 5

What three things can twitch properties be affected by?

Answer 5

Fatigue, temperature and fibre type (twitch duration far longer in calf than head for example)

Question 6

What is the relationship between muscle activity (EMG) and force?

Answer 6

The amplitude of the surface EMG signal is proportional to the force produced by the muscle. (See diagram for reference)

An EMG signal is rectified and then integrated and this final product parallels force from a isometric contraction

Question 7

Muscles act as low-pass filters, what does this mean?

Answer 7

Neural input signal is high frequency, force output is low frequency

Question 8

Some muscles show a non-linear relationship between EMG and Force, give an example and explain why?

Answer 8

The biceps brachii displays a less linear relationship, this is due to the combination of slow and fast fibres present in the bicep. The fast twitch fibres become more dominate at the higher forces, thus producing higher EMG.

Question 9

Compared to slow, what properties do fast muscle fibres have?

Answer 9

• Higher resting membrane potentials
• greater density of sodium channels
• faster action potentials
  Therefore fast fibres generate larger electrical responses

Question 10

Explain the graph concerning force/torque+ velocity on concentric and eccentric muscle contraction

Answer 10

Force/torque is higher for eccentric contraction, alongside the negative velocity

Positive velocity is accompanied with a banana 🍌 shaped concentric muscle contraction much lower than eccentric

Question 11

How does EMG/force relationship depend on muscle lengthening/shortening?

Answer 11

At constant velocity, for a given tension (KG) concentric contraction yield higher integrated electrical activity.
Both are positively linear however.

(see graph for reference)

Question 12

How does the EMG/force relationship depend on muscle contraction?

Answer 12

For a given knee angle (rad), eccentric muscle contraction was able to produce more torque than concentric. Supramaximal eccentric electric shocks yielded even higher torques.

Question 13

Analysis of graphs show us that more force can be seen with eccentric contraction but less EMG Is seen. Why is this?

Answer 13

Possibly due to prevent muscle / joint / tendon damage.

Question 14

Explain concept that EMG / force relationship depends on joint angle?

Answer 14

For a given torque, a higher degree of angle produces a higher EMG.
Interpreted that muscle is stronger at lower angles due to higher force output with less EMG scores.

Can be explained by mechanics and length tension relationship

Question 15

Explain EMG and force during fatiguing contraction (prolonged contractions at 50% and 100% MVC)

Answer 15

With 50% MVC, force is maintained by recruiting more motor units to compensate to failing contraction in other fibres

With 100% MVC, no scope for further recruitment therefore force drops off, EMG also drops off due to ‘muscle wisdom’

Question 16

What are the changes in fusion frequency after fatigue??

Answer 16

After fatigue, an increase in relaxation time can be seen, and also an increase in the degree of fusion of an unfused tetanus.

Question 17

Why does muscle fatigue prolong twitch duration?

Answer 17

Due to biochemical changes in the muscle

Question 18

Explain the phenomenon of muscle wisdom?

Answer 18

The frequency of motor unit firing falls over time to compensate for prolonged twitch time.

This causes a drop in EMG amplitude (both single units and also the surface EMG)

Question 19

How does the nervous system ‘know’ when to reduce the motor neuron firing rate??

Answer 19

3 possible mechanisms.

• Peripheral detection of the build up of metabolites and pain via group III and group IV receptors
• spinal suppression of motor neuron firing rates
• changes in voluntary activation

Question 20

Why does the nervous system reduce the firing rate?

Answer 20

• possibly to reduce the likelihood of neuronal fatigue (neuromuscular transmission failure)
• more likely to maintain fine motor control (if firing rate is way above fusion frequency, force modulation is limited)

Question 21

High frequency fatigue can only occur by artificial means and hence is not a common feature of most movement. How does it occur and what helps prevent it?

Answer 21

It is caused by continuous high frequency stimulation (80Hz for example) leading to a failure of transmission along the muscle membrane, but recovers immediately following the removal of stimulation. Prevented by muscle wisdom.

Question 22

When does low frequency occur and what is the most likely cause?

Answer 22

Occurs after mild muscle damage, could be intense unaccustomed exercise. Most likely caused by reduced calcium release from the sarcoplasmic reticulum, due to mechanical damage

There is a need to activate muscle at a higher frequency to get the same force (see diagram)

Question 23

What happens to the M waves in high and low frequency fatigue?

Answer 23

In high frequency - the M wave is abolished/reduced (EMG reduced)

In low frequency - the M wave is unaffected

Question 24

What do central mechanisms of fatigue refer to (neural contributions) ??

Answer 24

Planning of willed movement -> motor cortical and other corticospinal outputs + other supraspinal and propriospinal outputs -> alpha and gamma motor neurons

Question 25

What is an interpolated twitch torque (ITT)?

Answer 25

During contraction, an electrical stimulus is superimposed onto a muscle or its nerve, and the evoke ITT is measured.

The ITT is a measure of the number of motor units that are not maximally recruited during a voluntary contraction.

Question 26

How do the effects of stimulation (before fatigue) differ depending on varying excitation levels?

Answer 26

At low levels of contraction, the twitch is much larger, indicating extra capacity in the muscle.

Question 27

Discuss muscle stimulation during fatigue (what happens and why)

Answer 27

As fatigue develops, stimulation produces a greater twitch force

Evidence that the neural drive to the muscle is reduced (indicating central fatigue) - lies upstream of motor cortex

Question 28

When looking at voluntary vs electrical muscle training, what was the key finding?

Answer 28

Voluntary contraction training produced an increase in strength over 8 weeks

Question 29

What were the changes seen after voluntary strength training?

Answer 29

With voluntary contractions - 33% force increase

With electrical stimulations - 11% force increase

Question 30

What are the changes seen after electrical strength training?

Answer 30

No force increase for either experimental group.

Yielding conclusion that the majority of the strength improvements early on can be explained predominantly by increased neural drive

Question 31

What is acoustomyography?

Answer 31

It is a method for measuring muscle activity, main frequency of muscles is around 25Hz

Question 32

Explain the changes in AMG and EMG with force output?

Answer 32

AMG amplitude decreases during involuntary muscle fatigue whilst surface EMG does not, therefore AMG represents muscle force.

With voluntary changes in volition, force AMG and EMG all decrease simultaneously

Question 33

What is the EMG/AMG ratio?

Answer 33

It can be used to monitor changes in muscle control due to changes in fatigue

Question 34

What can an ultrasound tell us?

Answer 34

Monitors changes in muscle length, tells us if muscle is lengthening or shortening

Question 35

Why does the calf muscle appear to get shorter duringforward sway (concentric)?

Answer 35

Due to elastic Achilles’ tendon. During forward sway EMG increases but the muscle length decreases