Type I muscle fibers vs Type II muscle fibers
How to tell Type I muscle fibers and Type II muscle fibers apart when a board question makes them look alike.
Type I fibers: slow-twitch, oxidative, fatigue-resistant (red); type II fibers: fast-twitch, glycolytic, powerful but fatigue-prone (white).
Side by side
Type I fibers are slow-twitch oxidative, fatigue-resistant, rich in mitochondria/myoglobin, and used for endurance posture.
Type II fibers are fast-twitch, more glycolytic depending on subtype, larger force generation, and fatigue more quickly.
Type I fibers are slow-twitch oxidative, fatigue-resistant, rich in mitochondria/myoglobin, and used for endurance posture.
Type II fibers are fast-twitch, more glycolytic depending on subtype, larger force generation, and fatigue more quickly.
The trap
Type I is slow oxidative endurance; Type II is fast powerful glycolytic-biased movement.
The repair move
Before naming the answer, force the stem through the separator: Type I is slow oxidative endurance; Type II is fast powerful glycolytic-biased movement.
Try a free question
CoreA histology slide of skeletal muscle from the soleus muscle shows fibers with high myoglobin content (red color), abundant mitochondria, high oxidative enzyme activity, and small fiber diameter. These fibers are predominantly used for sustained postural activity and endurance. Which fiber type is this?
Show answer and explanation
Answer: A. Type I. Type I fibers are slow-twitch oxidative: high myoglobin (red), high mitochondrial density, high oxidative enzymes, small fiber diameter, fatigue-resistant. Predominate in postural muscles (soleus, paraspinals) and endurance athletes (marathon runners). Mnemonic: 'One Slow Red Ox' (Type I, Slow twitch, Red, Oxidative).
Why the other choices are tempting
- B. Type IIa fibers are intermediate (fast oxidative-glycolytic) with moderate endurance and power.
- C. Type IIx fibers are fast glycolytic, white, fatigue-prone, large fiber diameter.
- D. Cardiac muscle has a different fiber type system.
- E. Smooth muscle is nonstriated.
Flashcards
Tap a card to flip it.
What clue supports Type I muscle fibers?
Choose Type I muscle fibers when the stem gives this pattern: Type I fibers are slow-twitch oxidative, fatigue-resistant, rich in mitochondria/myoglobin, and used for endurance posture.
What clue supports Type II muscle fibers?
Choose Type II muscle fibers when the stem gives this contrasting pattern: Type II fibers are fast-twitch, more glycolytic depending on subtype, larger force generation, and fatigue more quickly.
Quick answers
What is the fastest way to tell Type I muscle fibers from Type II muscle fibers?
Type I fibers: slow-twitch, oxidative, fatigue-resistant (red); type II fibers: fast-twitch, glycolytic, powerful but fatigue-prone (white).
What is the key clue for Type I muscle fibers?
Type I fibers are slow-twitch oxidative, fatigue-resistant, rich in mitochondria/myoglobin, and used for endurance posture.
What is the key clue for Type II muscle fibers?
Type II fibers are fast-twitch, more glycolytic depending on subtype, larger force generation, and fatigue more quickly.
What is the common trap with Type I muscle fibers vs Type II muscle fibers?
Type I is slow oxidative endurance; Type II is fast powerful glycolytic-biased movement.