Muscle fatigue

Question 1

What is fatigue?

Answer 1

Failure to maintain the required or expected outcome/task

Question 2

Why are muscle fibres transient?

Answer 2

• Recovery is quick after force produced from contraction

- Doesn’t last

Question 3

What does muscle fatigue depend on?

Answer 3

1) Training status
2) Task (duration and load)
3) Fibre type composition
4) Type of contraction

Question 4

What does everything below the muscle constitute as?

Answer 4

= central

Question 5

What does everything after the nerve constitute?

Answer 5

= peripheral

Question 6

What is the spectrum of human performance?

Answer 6

Patients > normal > athletes (sprinters, marathon runners)

= different types of muscle fatigue experienced

Question 7

What can muscle fatigue be?

Answer 7

1) Central
2) Peripheral
3) Both

Question 8

What is the command chain?

Answer 8

Intention to move > activating muscle from brain > goes through spinal cord to muscle fibres > contraction

Question 9

What is central fatigue?

Answer 9

• occurs proximal to NMJ
• may occur ± peripheral fatigue
• failure of CNS to activate muscle
  1) increase inhibition
  2) decrease excitation - amount of AP or e- signal you
  are producing

Question 10

What is peripheral fatigue?

Answer 10

• occurs distal to NMJ
• may occur ± central fatigue
• decreases intrinsic ability of muscle to produce force
  (muscle can’t be activated then contract properly)

Question 11

What are some non-invasive techniques to assess neuromuscular function?

Answer 11

1) Transcutaneous stimulation
2) Surface electromyography (EMG) - directly induce contraction by using nerve stimulation –> look at e- signal (how it goes through the muscle) and force produced

Question 12

Why is an EMG useful?

Answer 12

• Gives an idea of the e- signal that is going through the membrane to the TT
• Is an indication of the AP that’s propagated inside the muscle
  = can detect if something is wrong

Question 13

What is an electric shock?

Answer 13

• only one pulse
• equivalent of a twitch
  = can measure amplitude of e- signal

Question 14

What happens in prolonged isometric contraction?

Answer 14

• Muscle length doesn’t change, sustained contraction
• Decrease in force but no change in excitation = something goes wrong after the AP goes to the TT –> so linked to calcium release or inactivation of CBs
  = not related to AP propagation

Question 15

What happens in prolonged exercises (running or cycling)

Answer 15

• Same measurements taken before + after different exercises
• A problem with the contraction e.g. calcium release and inefficient CB cycling (like isometric) –> not the excitation or APs

Question 16

Why is calcium release + uptake important during muscle fatigue?

Answer 16

• Beginning = complete mechanism

- Release more calcium, after a while doesn’t work + lose a lot of force

Question 17

What is the problem in calcium transient?

Answer 17

???

Question 18

What is calcium transient?

Answer 18

The movement of calcium going in and out of the SR

Question 19

What is phosphagen depletion?

Answer 19

Pattern of CP/ATP depletion

• Associated during intensive exercise
• CP and ATP deplete rapidly
• CP supplementation delays onset of task failure

Question 20

What is glycogen depletion?

Answer 20

• Glycogen depletion associated with prolonged submaximal exercise
• Glycogen depletion is fibre type specific depending on intensity of exercise
• Glycogen depletion impairs ability to generate ATP

Question 21

What happens when Pi accumulates?

Answer 21

• Pi enters SR
• Calcium - Pi precipitates by binding to SR
• Decrease free calcium* concentration

*to be released = why you don’t get a “nice” calcium transient + why the force is very low

(ATP hydrolysed = ADP + Pi)
Use electrons to produce metabolites

Question 22

Where does H+ come from?

Answer 22

Metabolic acidosis unrelated to lactate - lowers PH

Question 23

What is lactate?

Answer 23

• A by-product of muscle fatigue

- Doesn’t cause fatigue (only an indicator)

Question 24

What happens in McArdle’s disease?

Answer 24

MD = genetic disease w/ muscle weakness

• Muscle phosphorylase deficiency
• Rapid muscle fatigue
• But no increase in plasma H+
• Affects an enzyme producing H+
• Fatigue quickly w/o H+ = over-contributors

Question 25

What is the fibre composition of power and endurance athletes?

Answer 25

Power = fast-twitch fibres
Endurance = slow-twitch fibres

• Respond differently to muscle fibres

Question 26

What is the role of H+ in muscle fatigue?

Answer 26

Contributor of muscle fatigue but not the sole contributor.

Lower PH =/= muscle fatigue

Question 27

What are Dorsi flexors and Plantar flexors?

Answer 27

Dorsi = mainly type I fibres
Plantar = mainly type II fibres

Force decreases for both.
E.g. mitochondria content greater in type I fibres = more ATP –> more calcium released = why type I less easily fatigued

Question 28

What are phosphagens?

Answer 28

Energy storage compounds

Question 29

What are the 2 main causes of fatigue?

Answer 29

• energy depletion (ATP and glycogen)

- accumulation of metabolites (H+)

Question 30

What is the command chain for the central elements?

Answer 30

1) activity in cerebral cortex, basal ganglia and cerebellum
2) motor cortex during
3) impulses in descending motor pathways
4) motor neurone excitation

Question 31

What are the peripheral elements of the command chain?

Answer 31

1) impulse propagation to axon terminals
2) neuromuscular transmission
3) impulse propagation along muscle fibre plasmalemma
4) impulses into TT
5) calcium release into SR

Excitation coupling

6) engagement of CB
7) MOVEMENT (OR FORCE)