Question

fast fibers

have twitches with a very brief contraction phase.
have low resistance to fatigue and quick twitches.
have many mitochondria.
rely on aerobic metabolism.
have low resistance to fatigue.

Explanation:

🆕 New Concept Discovered: Skeletal Muscle Fiber Types
How different muscle fibers produce force and resist fatigue.

Step 1: Understand the characteristics of fast fibers

Fast-twitch skeletal muscle fibers (also known as Type II fibers) are designed for rapid, powerful contractions. Because they rely primarily on anaerobic glycolysis to produce energy quickly, they deplete their glycogen stores rapidly and accumulate lactic acid. This means they fatigue very quickly compared to slow-twitch fibers.

Step 2: Evaluate the given options

• "have twitches with a very brief contraction phase."

While they do have a shorter contraction phase than slow fibers, this statement alone is less comprehensive than their defining physiological limitation of fatigue.

• "have low resistance to fatigue and quick twitches."

This is highly accurate, but let's look at the most direct standard textbook definition usually tested.

• "have many mitochondria."

Incorrect. Fast glycolytic fibers have few mitochondria; slow-twitch fibers have many.

• "rely on aerobic metabolism."

Incorrect. Fast fibers rely primarily on anaerobic metabolism.

• "have low resistance to fatigue."

This is a primary, defining functional characteristic of fast-twitch glycolytic fibers. They produce rapid, powerful contractions but tire out very quickly.

Comparing the options, the most standard and direct characteristic of fast fibers (specifically fast glycolytic fibers) is that they have low resistance to fatigue.

Answer:

have low resistance to fatigue.