Muscle overactivity and movement dysfunction in the upper motoneuron syndrome.

The upper motoneuron syndrome (UMN) is a collective term that refers to different types of motor behaviors produced by patients who have lesions of the descending corticospinal system. Lesions involving the upper motoneuron, its pathways, and its connections can occur at the level of the cortex, internal capsule, brain stem, or spinal cord. The clinical features of UMN have traditionally been classified as "positive" phenomena, referring to overt behaviors generated by various forms of muscle overactivity, and "negative" phenomena, referring to loss of overt behaviors secondary to impaired muscle activation, impaired control of motor behavior, and impaired motor performance. The clinical impact of UMN on patients is broad and tends to limit functional capacity. The negative and positive signs lead to reduced mobility and limb usage.