What causes the muscles to contract?

Spasticity is one of the features of upper motor neuron (UMN) syndrome. UMN syndrome is caused by damage to one (or more) areas of the central nervous system (CNS) involved in controlling voluntary movement. UMN syndrome can be divided into two broad groups – negative phenomena and positive phenomena.

Negative phenomena refer to what is lost, such as fine motor skills, strength, and motor control.

Positive phenomena are characterized by an abundance of muscle hyperactivity, such as spasticity, hypereflexia (increased stretch reflexes), clonus (uncontrollable movement of a body limb, especially the ankle), co-contractions (involuntary contractions of a limb or a part of the limb while the patient is actively moving another part (e.g., lower limb) (e.g., adduction of the shoulder while walking), or muscles spasms.