Common Drive Among Concurrently Active Motor Units in Cerebral Palsy and Post-stroke Hemiparesis K.C. McGill1,2, C. Patten*1,2, Z. Lateva1, and J. Rose2. VA Rehab R&D Center1, Palo Alto, CA & Stanford University School of Medicine2, Stanford, CA
'Common drive', or modulation of the discharge activity of simultaneously active motor units (MUs), is thought to reflect net excitatory activity of the motoneuron pool. We evaluated common drive in 3 persons with cerebral palsy (CP, tib ant and gastroc.), 7 persons with post-stroke hemiparesis (CVA, vas med), and normal controls. MU activity was recorded during isometric ramp contractions using intramuscular electrodes. MU trains were identified using computer-aided decomposition yielding 6-25 continuous trains per contraction. For each contraction, the instantaneous firing rate (IFR) of each MU was calculated over a 3-4 second interval. The common modulation pattern (CMP) was estimated as the ensemble average of the IFR patterns of all the active MUs. The amplitudes of the CMPs ranged from 0.2 to 2.1 Hz rms, and were greater in persons post-CVA (1.0 Hz rms) compared to controls (0.6 Hz rms, p < 0.05). Each IFR was then characterized by its mean firing rate, its gain in tracking the CMP, and its rms tracking error. The mean firing rates ranged from 6 to 18 Hz and the gains from 0.5 to 1.7. All observed MUs followed the CMP closely, most with tracking errors less than 0.5 Hz rms. These results suggest that a common excitatory signal is widely distributed throughout the motoneuron pool of these lower extremity muscles. Moreover, this distribution is not markedly disrupted in neuropathology. Widespread observation of common drive across muscles and subject populations suggests it is an emergent property of the force regulation system. Supported by: VA Medical† & Rehab†‡ Research Svcs & NIH R03 HD 36705†§ (KCM†, CP‡, JR§) |