A 17-year-old soccer player suffered a fracture to the left tibia. After his lower leg has been in a cast for 8 weeks, he found that the left gastrocnemius muscle is significantly smaller in circumference than it was before the fracture. What is the most likely explanation?

Skeletal muscle continuously remodels in response to its level of use. When a muscle is inactive for an extended period, the rate of synthesis of the contractile proteins in individual muscle fibers decreases, resulting in an overall reduction in muscle mass. This reversible reduction in muscle mass is called atrophy.