Muscle Fatigue

Central and Peripheral Fatigue

Muscle fatigue refers to a reversible decrease in muscle tension as a result of previous contractile activity. It can be classified into central and peripheral fatigue, each with distinct mechanisms and effects.

• Central Fatigue: Originates in the central nervous system (CNS). It is characterized by a feeling of tiredness and a desire to cease activity, often before physiological cell fatigue occurs. Central fatigue may be influenced by psychological factors and protective mechanisms.

• Peripheral Fatigue: Occurs within the muscle fibre itself. It is associated with physiological changes such as accumulation of inorganic phosphate, ADP, and H+ ions, which can impair muscle contraction.

Key Points:

• Fatigue may result from acid production during ATP hydrolysis, which can reduce the sensation of fatigue perceived by the brain.

• Peripheral fatigue is often due to limitations in ATP or glycogen stores, impaired excitation-contraction coupling, and altered ion concentrations (e.g., increased extracellular K+).

• Repeated action potentials can lead to accumulation of ions in the T-tubule, raising the threshold for action potential generation in muscle fibres.

• Fatigue can involve reduced Ca2+ release and reuptake by the sarcoplasmic reticulum, decreased activation of thin filament proteins, and direct inhibition of myosin cross-bridge cycling.

Example: During prolonged high-intensity exercise, both central and peripheral fatigue contribute to the decline in muscle performance.

Sources of ATP in Skeletal Muscle

ATP Production Pathways

Muscle contraction requires ATP, which can be supplied by several metabolic pathways. The duration and intensity of activity determine which pathway predominates.

• Intracellular ATP: Immediate source, but limited in quantity; supports only a few seconds of contraction.

• Creatine Phosphate: Rapidly regenerates ATP from ADP; supports short bursts of activity (seconds).

• Anaerobic Metabolism: Produces ATP without oxygen, primarily through glycolysis; supports moderate-duration activities (minutes).

• Aerobic Metabolism: Produces ATP using oxygen, primarily in mitochondria; supports prolonged activities (hours).

Example: Sprinting relies on creatine phosphate and anaerobic metabolism, while marathon running depends on aerobic metabolism.

Types of Skeletal Muscle Fibres

Classification by Contraction Velocity and Metabolic Pathway

Skeletal muscle fibres are classified based on their maximal velocity of shortening (fast or slow) and their primary ATP production pathway (oxidative or glycolytic).

• Type I (Slow Oxidative): Contain myosin with slower ATPase activity, high mitochondrial content, and rely on oxidative metabolism. These fibres are fatigue-resistant and suited for endurance activities.

• Type IIA (Fast Oxidative-Glycolytic): Intermediate properties; fast contraction velocity, moderate mitochondrial content, and can use both oxidative and glycolytic pathways. These fibres are moderately fatigue-resistant.

• Type IIX (Fast Glycolytic): Contain myosin with rapid ATPase activity, low mitochondrial content, and rely on glycolytic metabolism. These fibres fatigue quickly and are suited for short, intense bursts of activity.

Key Points:

• Oxidative fibres are surrounded by blood vessels and contain myoglobin to aid in oxygen delivery.

• Glycolytic fibres have fewer mitochondria but abundant glycolytic enzymes and glycogen stores.

Example: Postural muscles contain more Type I fibres, while muscles used for sprinting contain more Type IIX fibres.

Determinants of Muscle Force/Tension Development

Factors Affecting Individual Muscle Fibre and Whole Muscle Force

The force or tension developed by skeletal muscle depends on several factors at both the cellular and whole muscle levels.

• Individual Fibre:

  • Fibre Diameter: Larger diameter fibres can generate more force due to more contractile proteins.

  • Fatigability: Fatigue-resistant fibres maintain force longer.

  • Initial Resting Length: Optimal overlap between actin and myosin filaments maximizes cross-bridge formation and force.

  • Frequency of Activation: Increased frequency of action potentials leads to summation and greater force.

• Whole Muscle:

  • Number of Muscle Cells Activated: More active fibres produce greater force.

  • Number of Muscle Cells per Motor Unit: Larger motor units generate more force.

  • Number of Motor Units Activated: Recruitment of additional motor units increases total muscle force.

Key Concepts:

• Muscle length influences tension by determining the degree of overlap between actin and myosin filaments.

• The amount of tension developed is directly proportional to the number of cross-bridges formed.

• Summation occurs when multiple action potentials lead to increased Ca2+ release, activating more troponin complexes and allowing more cross-bridges to form.

• Tetanus is a sustained contractile response to repeated stimuli. In unfused tetanus, the muscle fibre partially relaxes between stimuli; in fused tetanus, maximal tension is maintained without relaxation.

Example: Lifting a heavy object requires recruitment of multiple motor units and high-frequency stimulation to achieve fused tetanus and maximal force.

Equations

• Force Proportionality:

• ATP Hydrolysis:

Additional info: The notes have been expanded to include definitions, examples, and academic context for clarity and completeness.