Society For Neuroscience Abstract | 2003

CORRELATED MOTOR UNIT DISCHARGE IS SIMILAR IN YOUNG AND OLD ADULTS FOR SLOW CONCENTRIC, BUT NOT ECCENTRIC, CONTRACTIONS OF A HAND MUSCLE. KW Kornatz1, JG Semmler3, FG Meyer2, B Poston1, MA Pascoe1, 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 quantify correlated motor unit activity in the first dorsal interosseous muscle (FDI) of young and old adults during anisometric contractions. Subjects lifted and lowered a light load with abduction-adduction movements of the index finger over 10 deg using 6-s concentric and eccentric contractions of the FDI. The task was repeated 10-20 times while activity in pairs of motor units (15 young, 16 old) was recorded with intramuscular electrodes. Correlated motor unit activity was quantified using time-domain (synchronization index; Common Input Strength) and frequency-domain (coherence) analyses for the same motor unit pairs. For the concentric contractions, no differences were observed between young and old adults in the strength of motor unit synchronization (young, 0.8 ± 0.46 imp/s; old, 0.69 ± 0.41), width of the central synchronous peak (young, 10.8 ± 4.2 ms; old, 10 ± 2.6), or coherence at any frequency. In contrast, the strength of motor unit synchronization (young, 1.30 ± 0.65 imp/s; old, 0.77 ± 0.65; P < 0.05), and the width of the central synchronous peak (young, 17.4 ± 7.7 ms; old, 10.3 ± 3.1; P < 0.05) were greater during eccentric contractions in young adults. Furthermore, the strength of low-frequency coherence (1 - 4 Hz, all P values < 0.05) for motor unit discharge was greater during eccentric contractions in older adults. These data suggest that the proportion of common input, including inhibitory inputs, shared by FDI motor neurons are similar for concentric, but not eccentric contractions in young and old adults. These findings may reflect diminished task-related plasticity of common inputs to motor neurons in older adults. Supported by NIH AG09000 and NS42734.