Muscle fiber types and recruitment

Muscle Fibers - Slow vs. Fast Twitch

📌 Mnemonic: "1 slow red ox" → Type 1 fibers are slow-twitch, red (↑ myoglobin/mitochondria), and use oxidative phosphorylation.

⭐ Size Principle of Recruitment: Motor units are recruited from smallest to largest. Smaller, low-threshold motor units innervate slow-twitch (Type I) fibers, so they are recruited first for most movements. Larger, high-threshold units innervating fast-twitch (Type II) fibers are recruited later for more forceful contractions.

Recruitment Rules - The Size Principle

• Principle: Motor units are recruited in order of size, from smallest (Type I) to largest (Type IIx), as the required force of contraction increases.
• Recruitment Order & Rationale (Henneman's Size Principle):
  • Small Motor Neurons (Type I fibers):
    • Have ↑ input resistance & lower activation thresholds.
    • A given synaptic current generates a larger voltage potential ($V=IR$), reaching threshold first.
    • Recruited for light, sustained efforts (e.g., posture).
  • Large Motor Neurons (Type II fibers):
    • Have ↓ input resistance & higher activation thresholds.
    • Recruited progressively for powerful, forceful contractions.

⭐ Asynchronous recruitment of motor units during submaximal contractions helps delay fatigue by allowing some units to rest while others are active. This is critical for endurance.

Training Adaptations - Use It or Lose It

• Endurance (Aerobic) Training:

  • ↑ Mitochondrial density & oxidative enzymes.
  • ↑ Capillary supply & myoglobin content.
  • Enhances fatigue resistance.
  • Fiber type shift: Type IIx → Type IIa.

• Resistance (Anaerobic) Training:

  • ↑ Muscle fiber diameter (hypertrophy), especially Type II.
  • ↑ Glycolytic enzyme activity & glycogen stores.
  • ↑ Contractile proteins (actin, myosin) → ↑ strength.

• Detraining (Immobilization):

  • Muscle atrophy (↓ size); Type I fibers affected most in early stages.
  • Rapid loss of mitochondrial and strength gains.

⭐ Initial strength gains in resistance training (first 2-4 weeks) are primarily due to improved neuromuscular coordination, not hypertrophy.

High‑Yield Points - ⚡ Biggest Takeaways

• Type I fibers (slow-twitch, red) are for endurance (e.g., posture); they rely on oxidative phosphorylation.
• Type II fibers (fast-twitch, white) are for power and sprints; they rely on anaerobic glycolysis.
• Muscle fiber recruitment follows the size principle: Type I → Type IIa → Type IIx.
• Marathoners show a predominance of Type I fibers.
• Sprinters and weightlifters have more Type II fibers.
• Weight training can induce hypertrophy, primarily in Type II fibers.