Motor unit synchronization and coherence after motor learning training in the hands of older adults
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Motor unit coherence has been suggested to be a cause of force un-steadiness. It is possible that motor learning training will improve hand steadiness and cause a change in motor unit synchronization and coherence within and across synergistic muscles. The purpose of this study was to compare force steadiness and motor unit synchronization and coherence in the first dorsal interosseous (FDI) and adductor pollicis (AdP) muscles of older adults during a pinching task before and after a motor learning training program. Pre and post tests consisted of 2-minute sustained contractions at 2, 4, 8, and 12 % maximal pinching force (MPF) with the non-dominant hand. Single motor units were recorded from the AdP and FDI muscles with intramuscular fine-wire electrodes. The training program consisted of 2 sessions per week for 4 weeks. 183 motor unit pairs before training and 158 motor unit pairs after training were analyzed to determine changes within and across muscles. Following motor learning training, the coefficient of variation in the force decreased significantly in the 2 and 4 % MPF levels in the finger and in the thumb. Synchronization and coherence indices pre- and post-training did not change at any level . Motor unit firing rate variability was not correlated to force variability. Motor learning improved hand steadiness; however, it did not impact the incidence of coherence or synchronization within or across the AdP and FDI muscles. Thus, motor unit synchronization and coherence were not related to changes in hand steadiness with training.