Patterns of pathological firing in human motor units.

Modulation of motor unit firing rates can change muscle force production. Motor unit firing rates are often reduced during voluntary contractions of muscles influenced by disorders such as stroke or multiple sclerosis, while higher firing rates are typical of muscles innervated by a reduced number of motoneurones. An expanded range of motor unit firing rates is characteristic of disorders in which damage to various systems and neurons occur. Most neuromuscular disorders result in an increase in motor unit discharge variability, in part due to a higher incidence of doublets. In spinal cord injured subjects, long lasting involuntary contractions are common. This activity may reflect persistent inward currents that are revealed due to a lack of (voluntary) inhibition. Some of these changes in motor unit behaviour may actually work to enhance muscle force rather than to reduce it.