Lecture Notes on Muscle Fiber Types

Introduction

• Focus on Type 2 muscle fibers, specifically Type 2A and Type 2X
• Type 2A: Fast oxidative muscle fibers
• Type 2X: Fast glycolytic muscle fibers

Muscle Fiber Types Overview

• Slow Twitch (Type 1): Slow oxidative
• Fast Twitch (Type 2):
  • Type 2A: Fast oxidative (Type 2A)
  • Type 2X: Fast glycolytic (Type 2X)

Type 2A Muscle Fibers

• Characteristics:
  • Largest diameter of all skeletal muscle fibers
  • High capillary density
  • Moderate glycogen content
  • Lots of mitochondria
  • Fast contractility due to high myosin ATPase activity
  • Moderate fatigue resistance (operates less than 30 minutes)
  • Moderate power generation
• Metabolism:
  • Primarily aerobic but can switch to anaerobic
  • Uses glucose and oxygen in aerobic respiration
  • Can utilize glycogen and creatine phosphate pathways for ATP
• Activities:
  • Used in activities like walking and sprinting
  • Recruited second after Type 1 for generating moderate power

Type 2X Muscle Fibers

• Characteristics:
  • Intermediate fiber diameter
  • Low capillary density
  • Little mitochondria and myoglobin
  • Fastest contractility and highest myosin ATPase activity
  • Very low fatigue resistance (less than 1 minute)
  • Largest power generation
• Metabolism:
  • Primarily anaerobic
  • Relies on glycolysis and creatine phosphate pathways
• Activities:
  • Used for activities requiring quick bursts of power such as lifting heavy weights or hitting a baseball
  • Recruited last in muscle fiber order

Muscle Fiber Adaptation

• Muscle fiber composition can change due to genetics and environmental factors
• Endurance Training:
  • Increases capillary density, mitochondria size/number, and myoglobin
  • Switch from Type 2A to Type 1 fibers
• Resistance Training:
  • Muscle fibers can split and increase in size (hypertrophy)
  • Increase in myofibrils, mitochondria, glycogen
  • Switch from Type 2A to Type 2X fibers

Disuse Atrophy

• Occurs when muscles are not used, leading to a decrease in muscle size
• Muscle fibers can revert from Type 2X to Type 2A

Conclusion

• Regular use of muscles maintains their strength and composition
• Different exercises target different muscle fibers and adaptations

Note: The lecture emphasizes understanding the structural and metabolic aspects of muscle fibers and their adaptability based on activity levels.