Impaired regulation of force and firing pattern of single motor units in patients with spasticity.

Patients with spasticity were unable to maintain a constant force of the anterior tibial muscle. The force at maximal effort was reduced to less than 40% of normal, partly because motor units fired at a reduced rate even at high levels of contraction. Force and instantaneous frequency fluctuated slowly. The fast regulation of the firing rate, which characterises normal muscle, was absent. The variation between successive intervals was less than normal and the serial correlation coefficient between intervals increased.