Muscles And Motor Units - Lecture 6 Flashcards
The EMG signal is concerned with what event in muscle contraction?
Action potential spreads across the sarcolemma / muscle membrane and into the fibre along the t-tubules
What is a motor unit?
One alpha motor neuron and all the muscle fibres it innervates
What are the two ways that muscles can generate more force?
- recruiting more motor units
- increasing the frequency of firing
Both these factors increase the EMG response
The force of one single motor unit is tiny, how therefore can the response be seen?
By the process of averaging. With more than 10 spikes, the signal to noise ratio increases with increased numbers of traces forming an average.
What three things can twitch properties be affected by?
Fatigue, temperature and fibre type (twitch duration far longer in calf than head for example)
What is the relationship between muscle activity (EMG) and force?
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
Muscles act as low-pass filters, what does this mean?
Neural input signal is high frequency, force output is low frequency
Some muscles show a non-linear relationship between EMG and Force, give an example and explain why?
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.
Compared to slow, what properties do fast muscle fibres have?
- Higher resting membrane potentials
- greater density of sodium channels
- faster action potentials
Therefore fast fibres generate larger electrical responses
Explain the graph concerning force/torque+ velocity on concentric and eccentric muscle contraction
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
How does EMG/force relationship depend on muscle lengthening/shortening?
At constant velocity, for a given tension (KG) concentric contraction yield higher integrated electrical activity.
Both are positively linear however.
(see graph for reference)
How does the EMG/force relationship depend on muscle contraction?
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.
Analysis of graphs show us that more force can be seen with eccentric contraction but less EMG Is seen. Why is this?
Possibly due to prevent muscle / joint / tendon damage.
Explain concept that EMG / force relationship depends on joint angle?
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
Explain EMG and force during fatiguing contraction (prolonged contractions at 50% and 100% MVC)
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’
What are the changes in fusion frequency after fatigue??
After fatigue, an increase in relaxation time can be seen, and also an increase in the degree of fusion of an unfused tetanus.
Why does muscle fatigue prolong twitch duration?
Due to biochemical changes in the muscle
Explain the phenomenon of muscle wisdom?
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)
How does the nervous system ‘know’ when to reduce the motor neuron firing rate??
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
Why does the nervous system reduce the firing rate?
- 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)
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?
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.
When does low frequency occur and what is the most likely cause?
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)
What happens to the M waves in high and low frequency fatigue?
In high frequency - the M wave is abolished/reduced (EMG reduced)
In low frequency - the M wave is unaffected
What do central mechanisms of fatigue refer to (neural contributions) ??
Planning of willed movement -> motor cortical and other corticospinal outputs + other supraspinal and propriospinal outputs -> alpha and gamma motor neurons
What is an interpolated twitch torque (ITT)?
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.
How do the effects of stimulation (before fatigue) differ depending on varying excitation levels?
At low levels of contraction, the twitch is much larger, indicating extra capacity in the muscle.
Discuss muscle stimulation during fatigue (what happens and why)
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
When looking at voluntary vs electrical muscle training, what was the key finding?
Voluntary contraction training produced an increase in strength over 8 weeks
What were the changes seen after voluntary strength training?
With voluntary contractions - 33% force increase
With electrical stimulations - 11% force increase
What are the changes seen after electrical strength training?
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
What is acoustomyography?
It is a method for measuring muscle activity, main frequency of muscles is around 25Hz
Explain the changes in AMG and EMG with force output?
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
What is the EMG/AMG ratio?
It can be used to monitor changes in muscle control due to changes in fatigue
What can an ultrasound tell us?
Monitors changes in muscle length, tells us if muscle is lengthening or shortening
Why does the calf muscle appear to get shorter duringforward sway (concentric)?
Due to elastic Achilles’ tendon. During forward sway EMG increases but the muscle length decreases
