L6.1 Skeletal muscle fatigue

Question 1

What is skeletal muscle fatigue and how does it differ from weakness?

Answer 1

• Fatigue definition: Loss of max or potential performance
• Weakness definition: Inability to develop an initial force

Question 2

What is central fatigue and what are the 2 school of thoughts associated with it?

Answer 2

• Due to emotional & physiological factors
  
  • ↓ motivation
  • ↓drive of motor neurons
  • Impaired spinal transmission
  • Neuromuscular transmission failure
  • ↓recruitment of motor units & firing frequency
  • Inhibitory afferent muscle inputs
• 2 school of thoughts:
  
  • Sense muscular effort due to pathway reaching conscious centres
    
    • “Tells” muscles what to do
  • Sense of effort from afferent info from active muscles

Question 3

What are the arguments FOR and AGAINST central fatigue?

Answer 3

• FOR:
  Auditory/visual stimuli → ↑ force
  Electrical stimulation of fatigued muscles → ↑ force
• AGAINST:
• During voluntary muscle fatigue → electrical stimuli was unable to restore tension

Question 4

What is peripheral fatigue

Answer 4

• Neural, mechanical or biochemical
• ↓force & slower force developed
• Relaxation prolonged
  
  • ↓cross-bridge cycling
  • ↓SR pump functions (↓reuptake)

Question 5

Peripheral fatigue in - NMJ?

Answer 5

• Depletion of neurotransmitters (x Ach) - unlikely
  
  • Perhaps only during prolonged isometric contraction

Question 6

Peripheral fatigue in - T-tubules?

Answer 6

• K+ accumulation in t-tubules
• K+/Na+ exchange failure

Question 7

What are the 2 types of peripheral muscle fatigue?

Answer 7

• High frequency fatigue:
  
  • Fast onset, fast recovery
  • Likely to be caused by t-tubules conduction failure
• Metabolic fatigue:
  
  • Slow onset, slow recovery
  • Characterised by metabolic stress

Question 8

ATP

Answer 8

• FOR
  
  • Mismatch btw supply & demand
  • ATP also required for cellular homeostasis
  • Fatigue → protective mechanism → ↓rate of ATP hydrolysis
• AGAINST
  
  • Intramuscular ATP falls only to 60% at fatigue
  • At ↓[ATP], resting fatigue muscles would be rigor

Question 9

ATP & ADP balance

Answer 9

• ADP competes with ATP for sites on RyR → ↓ RyR opening and thus Ca2+ release

Question 10

Lactic acid & lactate

Answer 10

• ↑ lactate → ↑ H+ → ↓ pH
  
  • Especially during high intensity → fast twitch = fewer mito → less mito take up H+ → acidosis

Question 11

pH

Answer 11

• FOR
  
  • H+ → ↓ force:
  • ↓sensitivity of trroponin for Ca2+
  • Slows glycolysis
  • Interferes with actin-myosin cross bridge cycling
• AGAINST
  
  • Mcardle’s disease: ↓ muscle glycogen phosphorylase → no glycogen breakdown, no glycolysis & thus no lactate/H+ accumulation
  • Yet still fatigues more rapidly

Question 12

Other biochemicals

Answer 12

• Ca2+
  
  • Caffeine → ↑ Ca2+ release after fatigue → ↑ force
• Pi
  
  • ↑ ex → accumulate Pi → ↓ msucle fibre force by 30%
  • ↓ free [Ca2+] in SR
• Mg2+
  
  • Harder for DHPR to open RyR
• ROS
  
  • ↑ ex → ↑ ROS → damage SR → alters Ca2+ release