Hypotonia is an abnormally low muscle tone (the amount of tension or resistance to movement in a muscle). Even when relaxed, muscles have a continuous and passive partial contraction which provides some resistance to passive stretching. Hypotonia thus manifests as diminished resistance to passive stretching. Hypotonia is not the same as muscle weakness, although the two conditions can co-exist. The phenotype is Hypotonia (HP_0001252, a Human Phenotype Ontology identifier). Also known as: Low muscle tone, Low or weak muscle tone, Muscle hypotonia, Muscular hypotonia.