Society For Neuroscience Abstract | 2004

CORRELATED MOTOR UNIT ACTIVITY HAS ONLY A MINOR INFLUENCE ON THE FLUCTUATIONS IN ACCELERATION DURING ANISOMETRIC CONTRACTIONS. KW Kornatz1, JG Semmler3, MA Pascoe1, FG Meyer2, and RM Enoka1. 1

Dept. of Integrative Physiology and 2 Dept. of Electrical and Computer Engineering, University of Colorado at Boulder, U.S.A., and 3 School of Health Sciences, Deakin University, Australia. The purpose of the study was to determine the association between correlated motor unit activity and fluctuations in index finger acceleration as young and old adults performed shortening and lengthening contractions with the first dorsal interosseus (FDI) muscle. Subjects lifted and lowered a light load with abduction-adduction movements of the index finger over 10 deg by performing 6-s anisometric contractions with FDI. The task was repeated 10-20 times and the discharge of action potentials by pairs of motor units (24 young, 24 old) was recorded with intramuscular electrodes. Correlated motor unit activity was quantified with the cross correlation histogram and the coherence spectrum. Fluctuations in motor output were expressed as the standard deviation (SD) of index finger acceleration in the abduction-adduction plane. The SD of acceleration was similar for young and old adults, but greater for lengthening contractions (0.20 ± 0.07 m/s2) compared with shortening contractions (0.15 ± 0.06 m/s2, P < 0.05). Furthermore, the strength of synchronization, as indicated by the peak in the cross-correlogram, was similar in young and old adults, but greater during the lengthening contractions for both groups. There was a weak positive relation (P < 0.05), between synchronization and the acceleration fluctuations for both contraction types (shortening: r2 = 0.14, lengthening: r2 = 0.12). However, motor unit coherence was not associated with SD of acceleration for either contraction type, although coherence was less for young adults during shortening contractions. Thus, the fluctuations in index finger acceleration during anisometric contractions performed by a hand muscle must be caused by mechanisms other than the correlated discharge of motor units. Supported by NINDS award NS42734.