Muscles and Motor Units

Question 1

Historical study of muscle

Answer 1

• Francesco Redi 1666
  
  • first to recognise the connection between muscles and generation of electricity
• Luigi Galuani
  
  • credited as the father of neurophysiology for his work with frogs’ legs - 1791
  • showed that “eletrical stimulation of muscular tissue produces contraction and force”

Question 2

First demo of electromyography (EMG)

Answer 2

• Emil Du Bois - Reymond 1850
  
  • demonstrates that the human body can generate electrical current
  • Du Bois - Reymond washed his hands, immersed them in the saline, and grasped the dowels. With both arms relaxed, he waited for the galvanometer needle to rest at zero. Suddenly and powerfully, he contracted all the muscles in one arm. The needle, as predicted, deflected instantly in response

Question 3

History of EMG

Answer 3

• Erlanger, Gasser and Newcomer 1920’s make the first recordings of EMG, using the recently invented cathode ray oscilliscope. Win nobel prize in 1944
• In the 1920s, Gasser, Newcomer and Erlanger developed a triode vaccum tube amplifier for use with the newly invented cathode ray oscilloscope. The amplified signals from a pair of electrodes connected to muscle, EMG signals could now be displayed. To store these readings, photographic film was held up against the cathode tube instead of anything resembling today’s storage media

Question 4

Chain of events in muscle contraction

Answer 4

1) action potential (AP) stimulates the release of a neurotransmitter across the neuromuscular junction
2) AP spreads across sarcolemma/muscle membrane and into fiber along the T-tubules (affected by high frequency fatigue)
3) causes release of calcium from the sarcoplasmic reticulum (affected by low frequency fatigue)
4) calcium binds to muscle and causes cross-bridge cycling

Question 5

the motor unit

Answer 5

• one alpha motor neuron and all of the muscle fibres it activates
• one action potential generates a single twitch
• multiple action potentials generate continuous force
• force produced by a single motor unit is tiny but can be seen with averaging

Question 6

twitch fusion and motor unit recruitment

Answer 6

• continuous smooth muscle contraction is generated by the fusion of multiple twitches
• muscles generate extra force by:
  
  • recruiting more motor units
  • increasing the frequency of firing
• both of these factors also increase the electrical activity generate i.e. increase EMG activity

Question 7

twitch properties depend upon fibre types

Answer 7

• twitch properties are also affected by fatigue and temperature
• this requires changes in motor unit firing rates to compensate (‘muscle wisdom’)

Question 8

movement precision determined by number of motor units

Answer 8

• muscles differ greatly in both numbers of fibres and numbers of motor neurons
• more motor neurons = finer force modulation = better control
• innervation number = fibres/motor neurons
• lower innervation number = more control

Question 9

measuring muscle activity

Answer 9

• action potentials propagate along the sarcolemma, starting at the neuromuscular junction to the ends of the muscle fibre
• this electrical signal can be recorded, either by inserting a needle electrode into the muscle, or a surface electrode at the level of the skin
• this technique is termed electromyography (EMG)
  
  • action potentials from numerous motor units summate to produce the surface EMG signal

Question 10

relationship between muscle activity and force

Answer 10

• the amplitude of the sEMG signal is proportional to the force produced by the muscle
• muscles act a low pass filters: neural input signal is high frequency, force output is low frequency

Question 11

non-linear EMG / force relationship

Answer 11

• filtered and integrated EMG is generally proportional to force
• however, some muscles show a non-linear relationship

Question 12

muscle fibre types and EMG

Answer 12

• compared with slow muscle fibres, fast fibres have
  
  • higher resting membrane potentials
  • greater density of sodium channels
  • faster action potentials
• therefore, fast fibres generate larger electrical responses
• biceps made up of a mixture of fast and slow twitch fibres
• fast twitch may become more dominant at higher forces. thus producing more EMG

Question 13

types of muscle contraction

Answer 13

muscle torque / load torque
= isometric
>1 concentric
<1 eccentric

Question 14

EMG force relationship

Answer 14

• EMG/force relationship depends on muscle lengthening / shortening
• EMG-force relationship is also affected by joint angle
  
  • this can be explained by mechanics and the length-tension relationship

Question 15

EMG and force during fatiguing contraction

Answer 15

• during 50% MUC, more motor units are recruited to compensate for failing contraction in other fibres. force is maintained
• during 100% MUC there is no scope for further recruitment. therefore force drops off. EMG also drops off

Question 16

changes in fusion frequency after fatigue

Answer 16

muscle fatigue causes prolonged twitch duration, due to biochemical changes in the muscle

Question 17

muscle wisdom

Answer 17

• frequency of motor units firing falls over time to compensate for prolonged twitch time. this causes a drop in the EMG amplitude (both single units and also surface EMG)
• how does the nervous system ‘know’ when to reduce motor neuron firing rate?
  
  • peripheral detection the build of metabolites (e.g. lactate) and pain via group III and 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 e.g. failure of neuromuscular transmission
  • more likely to maintain fine motor control (if firing rate is way above fusion frequency, force modulation is limited)

Question 18

high frequency fatigue - only occurs artificially

Answer 18

• caused by continuous high frequency stimulation
• recovers immediately following cessation of stimulation
• probably not a common feature of most movement: ‘muscle wisdom’ normally helps to prevent it
• caused by failure of transmission along the muscle membrane
• ‘M’ wave reduced / abolished (i.e. reduces EMG signal)

Question 18

low frequency fatigue - occurs after mild muscle damage

Answer 18

• caused by intense exercise, particularly when unaccustomed to it - wobbly legs the following day
• need to actuate muscle at higher frequency to get some force
• takes hours/days to recover
• caused (probably) by reduced calcium release from the sarcoplasmic reticulum due to mechanical damage
• ‘M’ wave unaffected (i.e. no change to EMG)

Question 18

what causes loss of force during a fatiguing contraction?

Answer 18

• mainly caused by biochemical changes in the muscle itself
• possible neural mechanisms
  
  • central
  • peripheral

Question 18

twitch interpolation

Answer 18

• the interpolated twitch technique was first described in 1928 by Denny-Brown, and it is frequently used to study the degree of motor unit activation during voluntary effort. During contraction, an electrical stimulus (typically a single or double twitch) is superimpased onto a muscle or its nerve, and the evoked interpolated twitch torque (ITT) or superimposed twitch force is measured. The ITT is a measure of the number of motor units that are not maximally recruited during voluntary contraction. Therefore, the ITT is an index of the level of completeness of muscle activation
• voluntary force is compared with electrically evoked force

Question 19

effect of stimulation (before fatigue)

Answer 19

• at low levels of contraction, stimulation produces a large twitch - indicates extra capacity in the muscle
• at high contraction levels, this capacity is reduced

Question 20

muscle stimulation during fatigue

Answer 20

• as fatigue develops, stimulation produces greater twitch force
• evidence that neural drive to the muscle is reduced - central fatigue

Question 21

motor cortex stimulation during fatigue

Answer 21

• the same effect is observed with TMS stimulation
• this suggests that the brain is not driving the motor cortex as much as it could
• therefore central fatigue lies upstream of the motor cortex

Question 22

voluntary vs electrical muscle training

Answer 22

• eighty maximal 10s voluntary contractions per day for 8 weeks OR 60Hz electrical stimulation for the same period
• voluntary muscle contraction training produced an increase in strength over 8 weeks
• the majority of strength improvements early on in training can be explained predominantly by increased neural drive to the muscle

Question 23

acoustomyography (AMG)

Answer 23

the muscles of the body are continually contracting and relaxing in the wake human being. During contraction, they generate sounds. Under ordinary conditions, these sounds are not heard, but if you place both thumbs in your ears and make a fist, you will hear a low rumble. The tighter the fist, the louder the sound. You are hearing the sounds of the forearm muscles as they contract. The main frequency of the muscle sound is 25Hertz, which is at the lower limit of human hearing

Question 24

changes in AMG and EMG with force output

Answer 24

• AMG is intrinsically tied to contraction, and its amplitude declines during muscle fatigue while the surface EMG signals do not
• i.e. AMG represents muscle force
• therefore EMG/AMG ratio can be used to monitor changes in muscle control due to fatigue

Question 25

alternative methods for measuring muscle activity

Answer 25

• ultrasound tells us if the muscle is lengthening (eccentric) or shortening (concentric) contraction
• EMG tells is how ‘activated’ the muscle is
• force tells us the overall output of the muscle

Question 26

ultrasound reveals ‘paradoxial’ muscle movement during human standing

Answer 26

• calf muscle appears to get shorter during forward sway (i.e. concentric) due to elastic achillies tendon
• during forward sway EMG increases but muscle length decreases