M04.05.06 Mechanical Response to a Single Action Potential

Hypertrophy

• Definition: Increase in muscle size due to an increase in the size of individual muscle fibers.
• Mechanism: Involves the addition of myofibrillar proteins and an increase in the number of sarcomeres.
• Stimuli: Primarily induced by resistance training, mechanical tension, and metabolic stress.
• Key Factors:
  • Protein Synthesis: Enhanced protein synthesis rates.
  • Hormonal Influence: Growth hormone, testosterone, and insulin-like growth factor 1 (IGF-1).
  • Nutritional Requirements: Adequate protein and caloric intake.

Atrophy

• Definition: Decrease in muscle size due to a reduction in the size of individual muscle fibers.
• Mechanism: Involves the breakdown of muscle proteins and loss of sarcomeres.
• Stimuli: Caused by disuse, immobilization, or denervation.
• Key Factors:
  • Protein Degradation: Increased activity of ubiquitin-proteasome and autophagy-lysosome pathways.
  • Reduced Load: Decreased mechanical load and neural activation.
  • Catabolic Hormones: Increased levels of cortisol and myostatin.

Endurance vs. Strength Training Adaptations

Endurance Training

• Primary Focus: Enhances aerobic capacity and mitochondrial density.
• Adaptations:
  • Increased Mitochondria: More mitochondria and improved oxidative enzymes.
  • Capillary Density: Enhanced capillary network for better oxygen delivery.
  • Fiber Type Shifts: Increased proportion of Type I (slow-twitch) fibers.
  • Metabolic Efficiency: Improved utilization of fat as an energy source.

Strength Training

• Primary Focus: Increases maximal force production and muscle hypertrophy.
• Adaptations:
  • Neural Adaptations: Improved motor unit recruitment and firing rate.
  • Fiber Hypertrophy: Increased size of Type II (fast-twitch) fibers.
  • Connective Tissue: Enhanced strength of tendons and ligaments.
  • Metabolic Changes: Increased glycolytic enzyme activity.

Muscle Fiber Types and Their Plasticity

Muscle Fiber Types

• Type I (Slow-Twitch) Fibers:
  • Characteristics: High oxidative capacity, fatigue-resistant, slow contraction speed.
  • Primary Use: Endurance activities.
• Type II (Fast-Twitch) Fibers:
  • Characteristics: High glycolytic capacity, quick to fatigue, fast contraction speed.
  • Subtypes:
    • Type IIa: Intermediate, both oxidative and glycolytic.
    • Type IIb/x: Primarily glycolytic, highest contraction speed and force.

Plasticity

• Adaptability: Muscle fibers can undergo transitions between types based on training and environmental factors.
• Influences:
  • Training: Endurance training can increase Type I fibers, while resistance training can increase Type II fibers.
  • Inactivity: This leads to a shift towards more glycolytic fibers.
  • Age and Disease: Aging and certain diseases can affect fiber type distribution and functionality.

Satellite Cells and Muscle Regeneration

Satellite Cells

• Definition: Muscle stem cells located between the basal lamina and sarcolemma of muscle fibers.
• Role in Muscle Regeneration:
  • Activation: Activated in response to muscle injury or stress.
  • Proliferation: Proliferate and differentiate into myoblasts.
  • Fusion: Myoblasts fuse to form new muscle fibers or repair damaged ones.

Regeneration Process

• Injury Response: Satellite cells are activated and proliferate.
• Myoblast Formation: Activated satellite cells differentiate into myoblasts.
• Repair and Growth: Myoblasts fuse to existing fibers or form new fibers, leading to muscle repair and growth.
• Regulatory Factors: Growth factors such as IGF-1 and FGF, along with inflammatory cytokines, play key roles.

Points to Remember

• Hypertrophy results from increased protein synthesis and muscle fiber size, while atrophy results from increased protein degradation and muscle fiber size reduction.
• Endurance training enhances aerobic capacity and mitochondrial density, whereas strength training increases maximal force production and muscle hypertrophy.
• Muscle fibers exhibit plasticity, adapting to different types of physical training and environmental factors.
• Satellite cells play a critical role in muscle repair and regeneration by proliferating and differentiating into myoblasts that fuse to repair or form new muscle fibers.

Bibliography

• Lieber, R. L. (2010). Skeletal muscle structure, function, and plasticity: The physiological basis of rehabilitation. Lippincott Williams & Wilkins.
• Bonaldo, P., & Sandri, M. (2013). Cellular and molecular mechanisms of muscle atrophy. Disease Models & Mechanisms, 6(1), 25-39.
• Schiaffino, S., & Reggiani, C. (2011). Fiber types in mammalian skeletal muscles. Physiological Reviews, 91(4), 1447-1531.
• Charge, S. B., & Rudnicki, M. A. (2004). Cellular and molecular regulation of muscle regeneration. Physiological Reviews, 84(1), 209-238.