His further studies of the late 19th century noted symptoms including tremor, hypotonia, diminished or lost tendon jerks, and slurred speech.

The commonly quoted definition by Lance (1980) describes "a motor disorder, characterised by a velocity-dependent increase in tonic stretch reflexes with exaggerated tendon jerks, resulting from hyper-excitability of the stretch reflex as one component of the upper motor neurone (UMN) syndrome".

Dr. Wolpaw and his colleagues, for example, have shown that training monkeys and rats to alter the size of a simple reflex - a tendon jerk - produces lasting changes, both anatomical and physiological, in the spinal cord itself, and the changes persist when connections to the brain are severed.