Passive resistance to movement of the extremities is called tone. Normally infants give only a moderate amount of resistance to you when you move their extremities. The amount of tone present is one way of assessing the condition of the nervous and muscular system in an infant.
Infants with too much tone, too much resistance to passive movement, are called hypertonic and an extreme example of this is spasticity. Infants with too little tone (too little resistance to passive movement) are called hypotonic. In many cases, hypotonia can mean simply low muscle tone and increased flexibility or laxity of ligaments; in one who is severely ill it can mean an inability to sit up, crawl, walk, or eat correctly.
