Something that is caused by, or exhibits a lack of normal muscle tone or strength: Wilber's atonicity can result in a loss of tension or strength in a muscle, so that it is completely flaccid or floppy.

Some atonicities take place because of a nervous system disorder or after an injury to the nerves: The arm muscles may have atonicity resulting from damage to the "brachial plexus" or nerve roots in the neck that go into the arms.