What happens to muscles that are not used?

• A. They grow larger due to increased protein synthesis.
• B. Connective tissue replaces them, and then adipose tissue.
• C. They are replaced by bone tissue.
• D. They spontaneously regenerate into new muscle fibers.

Answer: (B)

When a muscle isn’t used, it undergoes disuse atrophy: the contractile fibers shrink because protein synthesis slows and protein breakdown increases. If inactivity lasts a long time, the muscle tissue can be progressively replaced by connective tissue (fibrosis) and, later, by fat (adipose tissue), leading to fibrofatty degeneration. This pattern fits the idea that under prolonged disuse, the muscle is not simply smaller but is structurally replaced by other tissues. The other possibilities don’t align with how muscles respond to inactivity: they don’t enlarge from lack of use, they aren’t replaced by bone, and new muscle fibers don’t spontaneously form without injury or regenerative signaling.