Lacking in 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, in polio where the death of motor neurons causes the denervation of muscle fibers.