Synchronization of motor unit discharges
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Abstract
Synchronous behavior of motor units is considered as the tendency for pairs of motor units to contract with similar latencies, relative to each other, more often than would be expected if the motor units were functioning independently. Therefore, synchronization is a form of interdependence among simultaneously active motor units. This study was undertaken to identify and quantify synchronization using statistically based techniques.
Myoelectric signals were detected using a selective needle electrode from healthy subjects during voluntary isometric contractions. The signals were decomposed, by a proven accurate technique, into their constituent motor unit action potential trains (MUAPTs). Pairs of MUAPTs were then analyzed to determine if synchronization existed. Histograms of the time intervals between the firing of a triggering or conditioning motor unit to the subsequent forward and backward firings of the alternate or conditioned motor unit of the pair were created. Assuming motor units operate as independent, Gaussian point processes the histograms should be flat with an expected value dependent upon the histogram bin width and the mean firing rate of the conditioned motor unit. Bins containing 1.96 standard deviations more than the expected number of occurrences indicated, at a 95% confidence level, statistically significant interdependence or synchronization.
The latency at which synchronization was maximum and the range of bins over which it occurred were computed. The amount of synchronization was then calculated as the ratio of the average number of firings in each bin in the above described range, minus the expected value to the expected standard deviation. The frequency of occurrence was also measured.
Synchronization measurements were performed on motor units from several muscles engaged in a variety of contraction paradigms. The results indicate that more than 60% of the motor unit pairs examined exhibited synchronization. Of this percentage, synchronization occurred most often with near zero latency (0+-5 ms). However, significant amounts of synchronization were also evident at the approximate mono- or disynaptic reflex arc latencies for the muscles examined.
The results support the hypothesis that synchronization is indeed a phenomenon of most contracting muscles. Indications for a relationship between afferent feedback and the existence of synchronization were also observed. Confidence in these results is warranted because of the proven accuracy of the decomposition technique used to obtain the MUAPTs and the statistical nature of the investigation.
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Thesis (M.S.)--Boston University
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