Final answer:
Atrophy is the medical term for the reduction in muscle size due to non-use following injury or surgery, and can lead to various health problems, though disuse atrophy can sometimes be reversed with exercise.
Step-by-step explanation:
The medical term for the reduction in size of tissues, such as muscle, when they are not used following an injury or surgery is atrophy. When muscles undergo atrophy, they decrease in mass because structural proteins are lost and muscle cells may die, especially if this atrophy is prolonged. This can have varied implications for health, as decreased muscle mass can lead to impairments in locomotion, balance, and posture, potentially causing joint problems and other medical issues. While disuse atrophy can often be reversed with exercise and physical therapy, age-related muscle atrophy, known as sarcopenia, is irreversible and results in a decline of muscle strength and function.