Muscle Fiber Types

How are skeletal muscle fibers classified?

By ATP production method, motor neuron type, and myosin heavy chain (MHC) type.

What are key characteristics of fast-twitch fibers?

• High myosin ATPase activity

• Rapid Ca²⁺ release/uptake

• Fast contraction speed

• High glycolytic (anaerobic) capacity

What activities primarily use fast-twitch fibers?

Sprinting and high-power, anaerobic movements.

What are key characteristics of slow-twitch fibers?

• Low ATPase activity

• Slow contraction speed

• High mitochondrial density

• High fatigue resistance

What activities rely on slow-twitch fibers?

Endurance and prolonged aerobic exercise.

Are fiber types strictly separate categories?

No—many properties exist on a continuum (not purely distinct types).

What determines a muscle fiber’s characteristics?

The motor neuron that innervates it.

What happens when nerve supply to fibers is switched?

Fibers adopt characteristics of the new nerve (slow ↔ fast).

What are the three main motor unit types?

• Type I: Slow, low force, fatigue-resistant

• Type IIa: Fast, moderate force, fatigue-resistant

• Type IIx: Fast, high force, fatigable

How does Ca²⁺ handling differ between fibers?

Faster in Type II, slower in Type I.

Key difference between Type I and Type II fibers?

• Type I: Endurance, fatigue-resistant

• Type II: Power, fast fatigue

What energy systems do fiber types rely on?

• Type I: Aerobic

• Type II: Anaerobic (glycolytic)