After immobilization for a leg injury, a patient shows decreased calf circumference due to which mechanism?

After immobilization for a leg injury, a patient shows decreased calf circumference due to which mechanism?

• A. Necrosis of muscle fibers
• B. Decreased glycogen synthesis
• C. Decreased myosin light chain phosphatase activity
• D. Increased protein degradation

Answer: (D)

Disuse atrophy from immobilization lowers muscle mass because the muscle’s protein balance shifts toward breakdown. When a muscle isn’t regularly stimulated, signals for growth weaken while proteolytic pathways become more active—most notably the ubiquitin–proteasome system—which degrades contractile proteins like actin and myosin. This increased protein degradation, along with reduced protein synthesis, leads to smaller muscle fibers and a decrease in overall muscle girth, such as the calf circumference. Necrosis would imply cell death, which is not the mechanism of gradual, reversible loss seen with disuse. Changes in glycogen synthesis affect energy stores rather than long-term muscle size, and altered myosin light chain phosphatase activity pertains to acute regulation of contraction rather than sustained atrophy. Thus, the rise in protein degradation best explains the observed thinning.

Disuse atrophy from immobilization lowers muscle mass because the muscle’s protein balance shifts toward breakdown. When a muscle isn’t regularly stimulated, signals for growth weaken while proteolytic pathways become more active—most notably the ubiquitin–proteasome system—which degrades contractile proteins like actin and myosin. This increased protein degradation, along with reduced protein synthesis, leads to smaller muscle fibers and a decrease in overall muscle girth, such as the calf circumference. Necrosis would imply cell death, which is not the mechanism of gradual, reversible loss seen with disuse. Changes in glycogen synthesis affect energy stores rather than long-term muscle size, and altered myosin light chain phosphatase activity pertains to acute regulation of contraction rather than sustained atrophy. Thus, the rise in protein degradation best explains the observed thinning.