Hypertonicity is an increased tension of the muscles, meaning the muscle tone is abnormally rigid, hampering proper movement. This condition is the opposite of hypotonicity. Hypotonicity is a decreased tension in muscle tone. A lack of muscle tone inhibits proper movement as the muscle is not developed or is too soft to support the body.

Neonatal or congenital hypotonia is fairly common and causal factors include infections, effects of drugs, brain hemorrhage, or chromosomal or cerebral pathologies. An infant diagnosed with generalized hypotonia has abnormally decreased muscle tone affecting posture for at least the first month of life.

Neonatal or congenital hypertonia, on the other hand, is usually a result of severe brain damage. Infants experiencing hypertonicity often have joint contractures and general discomfort as well as difficulty feeding. Neonatal hypertonicity is managed with neurodevelopmental techniques in the positioning and handling of the infant. The presence of neonatal hypertonicity with an exaggerated startle reflex is often associated with the neuropediatric disease Hyperekplexia. Hyperekplexia is treatable, but must be caught early or death from apnea may result.

Chronic back pain is often associated with hypertonicity. Physical causes of hypertonicity in back muscles may be due to either physical damage or emotional stress. Hypertonicity in back muscles may cause joint compression and an excess of lactic acid as well as a decrease in movement.

Hypertonicity is also linked to neurologic disorders of the basal ganglia such as Parkinson's and Huntington's diseases. The basal ganglia, at the brain's base, is made up of the caudate nucleus, putamen, and globus pallidus. These three neuron clusters work together to control voluntary movements.

Parkinson's disease is caused by degeneration of the basal ganglia as well as low amounts of the neurotransmitter dopamine. Parkinson's disease slows voluntary movements. Hypertonicity from Parkinson's often presents as short, stiff leg movements and overall muscle weakness. Postural abnormality is also a result of hypertonicity from Parkinson's disease. Surgery on the globus pallidus section of the basal ganglia may reduce some of the hypertonicity associated with Parkinson's disease in some cases.

Huntington's disease is mostly caused by a genetic inheritance to the disorder. The Huntington's gene protein is called huntingtin and only one gene copy has to be inherited to cause Huntington's disease. Damage from Huntington's disease often results in a smaller putamen area of the basal ganglia. Severe mood abnormalities such as mania or psychosis are often the first sign of Huntington's disease, but rigid or spastic hypertonicity of muscles is often quite pronounced in the late stages.