W7 - Peripheral Fatigue

Question 1

What is fatigue?

Answer 1

Calcium, Hydrogen, Phosphates and Carbohydrates can all be involved in fatigue
* Physiological definition:
– Failure to maintain the required or expected force or power
* Not to be confused with weakness:
– Failure to generate the required or expected force or power

Question 2

What are the 2 cites of fatigue?

Answer 2

Generation of the signal then down to the CNS (causing fatigue)
Fatigue can also happen in the muscle
- Could have an electrical stimulation before and after task, if results in same power output = no muscle fatigue
If you ask the person to do the same movement without peripheral electrical stimulation = slower/ cannot move muscle –> then CNS fatigue

Question 3

When is energy required during muscle work?

Answer 3

• Excitation-contraction coupling: ATP-dependent processes
  – Myosin head force development
  – Na+ - K+ -ATPase (3 Na+ out, 2 K+ in) (→ generating a resting potential)
  – Calcium pump - important for muscle contraction
  In an energy crisis, enzyme activity could drop - impairing performance

Question 4

How does the type of fibre affect how quickly they fatigue?

Answer 4

Type II fatigue quickly
- Note changes in intracellular Ca2+ concentration has come down dramatically, as well as force generation

Type I are “essentially unfatigable” (have mitochondria) - fatigue resistant
- Intracellular Ca2+ concentration stays the same

Question 5

What are some factors that can cause inhibitory effects to performance?

Answer 5

Inhibitory effects: Ca2+ release highly effected by these products
* High AMP/IMP
* High ADP
* VERY low ATP (little effect in range of 2-8 mmol/L)
* Mg2+ (concentration doubles at fatigue as ADP, AMP and IMP have lower affinity for Mg2+ than ATP) - low levels are associated with fatigue
- Increasing Mg2+ concentration calcium release decreases

Question 6

How does fatigue affect action potentials?

Answer 6

• Fatigue: Accumulation of K+ in extracellular space
  – T tubular membrane: large surface - goes directly to the middle of the cell (carrying an action potential)
  – T tubular network: small volume
  → Rapid K+ accumulation
• More difficult to induce action potentials
  Action potential wave forms are depressed when there is no rested state(high potassium accumulation)

Question 7

How does pH play a role in fatigue?

Answer 7

• pH: importance of physiological range!
  
  • Decrease in pH cannot explain fatigue alone
    – muscle fatigue is shown at moderate drops of pH (from 7.05 to 6.8)
    – Long-lasting activity causes little/no acidosis (marathon)
    – pH takes longer than force to recover
    – Activation of Ca2+ release not inhibited even at pH 6.2(low pH)
    – Patients with McArdle’s disease don’t accumulate H+ but fatigue extremely - physiological disease
    
    • pH can induce fatigue, but physiological state is more important to determine what causes fatigue

Question 8

Explain compartmentalisation

Answer 8

Compartmentalisation: when metabolite concentrations vary within different compartments of the cell
* ADP tightly regulated and does not accumulate - on its own can induce fatigue(accumulation by ATPases can impact how they work)
* ATP average levels don’t fall far enough to affect cross-bridge function
* H+ does seem to play minor role in physiological conditions (pH, temperature)

Question 9

How does maintaining CHO levels increase performance levels for a longer period of time?

Answer 9

• maintains CHO oxidation (little glycogen used from muscle between 3h and 4h…)
  → still fatigue when CHO is being fed continuously - plasma glucose continuously falls so becomes a point where the person must stop exercising
  
  • Sodium potassium
  • pH localised/not
  • Inorganic phosphate through calcium metabolism
  • Low/high ATP levels
  • In both situations people fatigue at some point