A.3.3.1 Fatigue and recovery (HL)

Fatigue is:

• a reversible, exercise-induced decline in performance.

OR

• a disabling symptom in which interactions between performance fatigueability and perceived fatiguability limit physical and cognitive function

There are two types of fatigue:

• Peripheral fatigue:a term where fatigue is caused by factors that reside outside the CNS, distal to the neuromuscular joint

• During prolonged or intense exercise, chemical changes in the brain can impair its ability to effectively recruit and activate muscle fibres. This results in reduced muscle performance, characterized by decreased force and slower contraction speeds.

• Central fatigue: is there a term for fatigue caused by factors that reside within the CNS

• Even when the brain sends signals, factors such as depleted energy stores, accumulation of metabolites, or mechanical stress can hinder muscle contractions. This leads to diminished muscle performance despite optimal neural input.

Causes of peripheral fatigue in high-intensity activity include:

• Depletion of energy systems (creatine/ATP)

  • CP is used for anaerobic conditions, and maintains ATP levels in the muscle; depletion of both will cause fatigue in this type of exercise

• Accumulation of metabolic by-products (lactate/H ions)

  • Lactate is a byproduct of anaerobic glycolysis

  • These activities rely heavily on anaerobic glycolysis and produce large amounts of lactate and H+ ions

  • An increase in these ions decreases the pH of the cell

  • Decrease pH = decreased energy production = decreased contraction force in the muscle

  • Interferes with the role of calcium in cross-bridge formation

• Accumulation of inorganic compounds

  • When muscles work hard, inorganic phosphate (Pi) builds up from ATP breakdown:

    • Interfering with cross-bridge cycling: Pi binds to myosin heads, weakening their interaction with actin and reducing force production.

    • Reducing calcium release: Pi forms calcium-phosphate (CaPi), lowering free Ca²⁺ availability and impairing contraction strength.

    • Slowing ATP regeneration: Pi disrupts ATP resynthesis by affecting creatine kinase and oxidative phosphorylation, reducing energy for contraction.

Causes of peripheral fatigue in low-intensity/endurance activity include:

• Depletion of muscle and liver glycogen stores

  • Aerobic glycolysis relies on stored glycogen to produce ATP

  • It stores glycogen in the muscles and the liver

  • Prolonged exercise glycogen concentration decreases and depletes after 2 hours of activity

• Reduction in calcium ion release

  • Plays a role in muscle contractions of muscle fibres (sliding filament theory)

  • Reduced levels of Ca will inhibit muscle contractions as the sarcoplasmic reticulum is repeatedly stimulated

• Electrolyte loss (Na and K)

  • Disruption in the availability of sodium and potassium can lead to hyponatremia, which can be intensified in endurance events when athletes consume too many fluids.

  • Electrolytes like sodium (Na⁺) and potassium (K⁺) are crucial for nerve signals and muscle contractions. During an action potential, sodium enters the neuron (depolarisation), making it more positive, and potassium exits (repolarisation), restoring the resting state.

  • This process enables the signal to travel along the neuron, triggering muscle contraction.

• Depletion of acetylcholine

  • Depleted levels of ACh inhibit the ability of the muscle to contract.

• Dehydration

  • Dehydration leads to a decrease in blood plasma volume

  • This causes an increase in HR to maintain CO (cardiovascular drift). This means the heart needs to work harder, which can cause fatigue.

• Overheating

  • Increased core body temperatures during prolonged exercise result in blood being vasodilated to the skin away from where it’s needed in the working muscles for contraction, which can cause fatigue.

Exam Question:

Discuss how the causes of peripheral fatigue differ for a weightlifter compared to a marathon runner. What would be the main cause of peripheral fatigue for each athlete? (6 marks)

Marathon runners experience peripheral fatigue primarily due to the depletion due to the depletion of muscle and liver glycogen stores, reducing the availability of ATP and its production, as well as dehydration, impairing the removal of waste products and over-muscle function. Electrolyte loss is prominent as well as it disrupts nerve function, leading to cramping and reduced performance. For a weightlifter, peripheral fatigue occurs due to the rapid depletion of ATP and CP due to high-intensity efforts, along with the accumulation of by-products such as lactate and hydrogen ions. These reduced the force of the muscle contractions. A reduction in Ca ions too further limits the ability of muscles to sustain their contractions continuously.