Muscle Fiber Types

Three types:

  1. Type 1 (slow twitch)

  2. Type IIa (fast twitch)

  3. Type IIx (super fast twitch)

Type 1: Slow Twitch

  • slow contraction time

  • endurance fiber

  • low peak forces

  • high resistance to fatigue

  • energy delivered from aerobic metabolism

Type IIa: Fast Twitch

  • resemble slow twitch fibers

  • can get energy from anaerobic and aerobic metabolism

  • moderate forces and contraction times

  • can be trained

Types IIx: Super Fast Twitch

  • specialized for high speed contraction

  • fatigue easily

  • high peak force

  • energy delivered from anaerobic metabolism

Control of Muscle Fibers:

Motor Unit — one motor neuron and all the skeletal muscle fibers innervated by that neuron

  • smallest functional unit

  • single muscle fiber can only be innervated by one motor neuron

  • single motor neuron can innervate multiple muscle fibers

Size Principle:

— Motor units recruited from smallest to largest depending on the force needed

  • important for strength training, hypertrophy, and rate of force development

  • recruit Type 1 first for low force tasks

Processes to Replenish ATP:

  1. Phosphocreatine

  2. Glycolysis

  3. Mitocondrial / aerobic reparation