Relating to, caused by, or exhibiting a lack of normal muscle tone or strength: "A person'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; for example, the arm muscles may have atonicity resulting from damage to the brachial plexus (nerve roots in the neck that go into the arms)."