Muscle Physiology Notes

Muscle Contraction

• Muscle contraction involves several physiological processes.
• Key concepts: sliding filament theory, excitation contraction coupling, cross-bridge cycling, all or none principle.
• Sliding Filament Theory: Actin filaments slide over myosin filaments.
• Excitation Contraction Coupling: Action potential triggers calcium release from the sarcoplasmic reticulum.
• Crossbridge Cycling: Repeated attachment and detachment of myosin head to actin filament.
• All or None Principle: Motor unit fibers contract fully or not at all.
• Neural impulse triggers action potential, releasing acetylcholine at the neuromuscular junction.
• ATP energizes myosin head to attach and pull actin.
• The mousetrap analogy requires energy to release energy.

Types of Muscle Contraction

• Isotonic contractions: Constant force production with shortening or lengthening.
• Concentric: Muscle shortens (positive work).
• Eccentric: Muscle lengthens (negative work).
• Isokinetic contractions: Constant velocity.
• Isometric contractions: Muscle length remains constant, zero work is done.

Muscle Fibre Types

• Slow (Type I): Fatigue resistant, aerobic, lower force production.
• Fast Glycolytic (Type 2X): High power output, fatigue easily.
• Fast Oxidative (Type 2A): Mix of Type I and Type 2X characteristics.
• Genetics primarily determine muscle fibre distribution.
• Training can shift fibre types, but rarely from slow to fast glycolytic.

Force Production

• Neural Factors: Motor units activate all fibers fully or not at all.
• Recruiting additional motor units increases contractile force.
• Mechanical Factors:
  • Length-Tension Relationship: The amount of actin-myosin overlap affects force production.
    F = k * x
• Force-Velocity Relationship: As velocity decreases, force increases and vice versa.
• Power-Velocity Relationship: Power is positively related to velocity to a certain point. P = F * v

Fibre Type and Athletic Performance

• Muscle fibre typing helps understand performance capabilities.
• Aerobic capacity correlates with slow-twitch fibres.
• Power athletes possess more fast-twitch fibres.
• Athletic performance involves psychological, neurological,biochemical, cardiovascular, respiratory, muscular, and biomechanical collaboration.

Neuromuscular Aspects of Movement

• Central Nervous System (CNS) integrates information; Somatic Nervous System controls muscle fiber contraction.
• Neurons: Dendrites receive signals, the cell body integrates, the axon transmits action potential.
• Resting Neuron: Polarized with potassium leaking out and sodium-potassium pump maintaining balance.
• Depolarization: Sodium channels open, reversing polarity and triggering action potential.
• Repolarization: Sodium channels close, potassium gates open, restoring resting potential.
• Neuromuscular Junction: Acetylcholine releases to trigger muscle contraction.
  *Myotatic/Stretch Reflex: Prevents overstretching.
• Golgi Tendon Organs (GTOs): Activated by muscle contraction or stretch, limits muscle damage.
  Volitional Control of Movement:
• Primary motor cortex initiates movement.
• Neural impulse transmits from brain to descending tract.

Muscular Adaptation

• Specificity: Adaptations are specific to the muscle group, contraction type, and movement speed.
• Individualization: Genetics, particularly muscle fiber type, play a significant role in adaptation.
• Overload: Stressing the body enough to disrupt homeostasis is necessary for adaptation.
• Rest and Recovery: Adaptation occurs during recovery, not during training.
  Progression: Too slow or too quick progression limits adaptation.
• Diminishing Returns: Adaptation slows with training age and genetic potential.
• Detraining: Strength is retained longer than hypertrophy due to neural pathways.
  Upper Body Muscles:
• Lower stimulus threshold than the lower body in the early adaptation phase.
  Neural Adaptations:
  Increased neural drive.
  *Improved motor unit synchronisation.
  Lower GTO activity.
• Reduced antagonist co-activation to improve force production.
• Intracellular signalling interference diminish strength gains
  Concurrent training is completing resistance training in close proximity to cardio training. This can diminish strength gains if heavy focus is on aerobic training.