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Muscle atrophy refers specifically to a decrease in muscle size and strength, which can occur for several reasons such as disuse, aging, or illness. When muscles are not regularly engaged in physical activity, they can gradually shrink and weaken, a phenomenon often observed in individuals who are bedridden or leading a sedentary lifestyle. Aging also contributes to muscle atrophy, as the body's ability to maintain and regenerate muscle tissue diminishes over time. Similarly, certain illnesses can lead to a decline in muscle mass due to a lack of physical activity or as a consequence of the body's metabolic changes.

This understanding distinguishes muscle atrophy from other options, such as increases in muscle size due to weight training or muscle soreness following intense exercise, both of which involve the enhancement rather than the reduction of muscle mass. Additionally, while aging does affect muscle fibers, it does not specifically encapsulate the concept of atrophy, which emphasizes the loss of muscle size and function.