Diminished task-related adjustments of common inputs to hand muscle motor neurons in older adults.

The purpose of this study was to quantify correlated motor unit activity during isometric, shortening and lengthening contractions of a hand muscle in older adults. Thirteen old subjects (69.6+/-5.9 years, six women) lifted and lowered a light load with abduction-adduction movements of the index finger over 10 degrees using 6-s shortening and lengthening contractions of the first dorsal interosseus muscle. The task was repeated 10-20 times while activity in 23 pairs of motor units was recorded with intramuscular electrodes. The data were compared with 23 motor-unit pairs in 15 young (25.9+/-4.6 years, five women) subjects obtained using a similar protocol in a previous study. 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 all contractions, there was no difference with age for the strength of motor-unit synchronization, although age-related differences were observed for synchronous peak widths (young, 17.6+/-7.4 ms; old, 13.7+/-4.9 ms) and motor-unit coherence at 6-9 Hz (z score for young, 3.0+/-1.8; old, 2.2+/-1.5). Despite increased synchrony during lengthening contractions and narrower peak widths for shortening contractions in young subjects, there was no difference in the strength of motor unit synchronization (CIS approximately 0.8 imp/s), or the width of the synchronous peak (approximately 14 ms) during the three tasks in old subjects. Furthermore, no significant differences in motor-unit coherence were observed between tasks at any frequency for old adults. These data suggest that the strategy used by the central nervous system to control isometric, shortening, and lengthening contractions varies in young adults, but not old adults. The diminished task-related adjustments of common inputs to motor neurons are a likely consequence of the neural adaptations that occur with advancing age.