Lecture 5

Muscle Physiology Overview

  • Course Context: KIN2992 - Fitness Assessment and Strength Training, Western Health Sciences, School of Kinesiology.

Learning Objectives

  • Understand the structure-function relationship of muscle.
  • Consider muscle performance:
    • Strength
    • Endurance
  • Learn about muscle adaptation:
    • Stimulus (training)
    • Response (growth/adaptation)

Structural Hierarchy of Skeletal Muscle

  • Whole Muscle: The complete muscle
  • Fascicles: Bundles of muscle fibers
  • Fibers (cells): Individual muscle cells
  • Myofibrils: Subunits within the muscle fibers
  • Sarcomeres: Basic contractile units of myofibrils
  • Myofilaments: Actin and myosin proteins within sarcomeres.

Sliding Filament Theory

  • Mechanism of muscle contraction.
  • Each crossbridge consumes 1 ATP to generate:
    • 3.4 pN force
    • 10 nm length change

Muscle Function

  • Muscular Strength: Force exerted by a muscle or muscle group in one maximal effort.
  • Muscular Endurance: Ability to perform repeated or sustained contractions.

Factors Influencing Muscle Strength

  • Muscle Size (Cross-sectional area): Larger muscles can generate more force due to increased number of crossbridges.
  • Neural Drive: Activation of motor units.

Factors Influencing Muscle Endurance

  • Primarily determined by Metabolic Adaptations:
    • Increased capillarization: More blood vessels enhance oxygen delivery.
    • Increased mitochondrial density/function: Optimal energy production for sustained contractions.

Muscle Growth Mechanisms

  • Two primary methods of muscle tissue growth:
    • Hypertrophy: Increase in size of individual muscle fibers (common).
    • Hyperplasia: Increase in the number of muscle fibers (rare).
  • Growth achieved through:
    • Increase in the number of myofibrils per fiber.
    • Increased mitochondria and capillaries per fiber.

Myonuclear Domain Theory

  • Each nucleus of a muscle cell governs a finite volume of cell space.
  • For hypertrophy, the number of nuclei must increase which may involve:
    • Satellite Cells: Quiescent cells that donate nuclei during activation for repair and growth.

Exercise Stimuli and Response

  • Muscle reacts to overload with the following:
    • Disruption of ion balance (e.g., Ca²⁺ influx).
    • Depletion of substrates (e.g., phosphocreatine, glycogen).
    • Accumulation of metabolites (e.g., lactate, H⁺).
    • Triggering of inflammation.

Signaling Pathways in Muscle Adaptation

  • Mechanical Forces:
    • Triggers AMPK: Involved in energy balance regulation.
    • Increases mitochondrial biogenesis via PGC-1α.
  • Muscle Protein Synthesis:
    • Mediated by mTOR signaling, leading to muscle growth.
  • Satellite Cell Activation: Essential for supplying new nuclei necessary for growing fibers.

Summary of Muscle Adaptation Responses

  • Muscle growth is triggered by:
    • Accumulation of metabolic signals.
    • Detection of mechanical stress.
    • Inflammatory response from muscle damage.
  • Stimulus for adaptation relies heavily on the loading imposed on the muscle.

Final Thoughts on Muscle Function

  • Muscle performance is influenced by:
    • Structural properties of muscle.
    • Activation and efficiency based on neural recruitment.
    • Adaptations to exercise involve increases in cellular components, including new nuclei as a result of satellite cell differentiation.
  • Understanding these principles is critical for effective fitness assessment and strength training practices.