The upper motor neuron syndrome is composed of negative, positive, and rheologic features. The positive features have to do with muscle overactivity, which results from abnormal signal processing in the spinal cord, from altered supraspinal inputs and/or dysfunctional segmental spinal modulatory mechanisms. The negative features are characterized by a reduction of muscle activity and loss of selective muscle control. Third are rheologic features characterized by changes in the physical properties of muscle. These 3 types of clinical features share perpetuating cause-and-effect relationships. This discussion highlights pathophysiology involved in muscle overactivity in the context of the upper motor neuron syndrome.