Which statement about muscle fiber number in adults is true?

Which statement about muscle fiber number in adults is true?

• A. It can increase with training
• B. It cannot increase in number
• C. It always decreases with aging
• D. It increases only in the neonatal period

Answer: (B)

The main idea is that the number of skeletal muscle fibers in adults is essentially fixed. After birth, muscles grow primarily by hypertrophy—existing fibers get larger as they accumulate more contractile proteins and other cellular components—rather than by forming new fibers. Satellite cells help with repair and can contribute nuclei to existing fibers to support growth, but they don’t reliably create new muscle fibers in healthy adults. That’s why the statement about fiber number not increasing in adults best matches how adult muscle adapts to training: gains come from enlarging the current fibers, not from adding more fibers. The other ideas—training increasing fiber number, aging always reducing fiber count, or increases occurring only in the neonatal period—don’t align with typical adult muscle physiology.

The main idea is that the number of skeletal muscle fibers in adults is essentially fixed. After birth, muscles grow primarily by hypertrophy—existing fibers get larger as they accumulate more contractile proteins and other cellular components—rather than by forming new fibers. Satellite cells help with repair and can contribute nuclei to existing fibers to support growth, but they don’t reliably create new muscle fibers in healthy adults. That’s why the statement about fiber number not increasing in adults best matches how adult muscle adapts to training: gains come from enlarging the current fibers, not from adding more fibers. The other ideas—training increasing fiber number, aging always reducing fiber count, or increases occurring only in the neonatal period—don’t align with typical adult muscle physiology.