What term describes a decrease in muscle mass due to inactivity?

The term that accurately describes a decrease in muscle mass due to inactivity is "atrophy." This condition occurs when muscles are not being used, leading to a reduction in their size and strength over time. Factors contributing to atrophy may include prolonged bed rest, a sedentary lifestyle, or immobilization from injury. When muscles are regularly engaged in physical activity, they experience hypertrophy, which is the opposite process involving muscle growth and an increase in size.

Stagnation refers more broadly to a lack of growth or development, without specifically indicating a decrease in muscle mass, while weakness pertains to a general lack of strength that can occur for various reasons, not solely inactivity. Thus, atrophy is the most precise term to denote the specific loss of muscle mass associated with inactivity.