The lack of muscle size and number of muscle fibers that takes place with aging, reduction in blood supply, or following physical inactivity, and malnutrition: The amyotrophia that anyone has is often a progressive deficiency of muscle tissue, especially because that person doesn't move around and use his or her muscles.

There are numerous causes for the amyotrophias of muscles, including chronic malnutrition, immobilizations, and denervation which is a loss of nerve supplies that may be caused by a disease. For example, the death of motor neurons in polio causes the denervation of muscle fibers.