SYNCHRONIZATION OF MOTOR-UNIT FIRINGS IN SEVERAL HUMAN MUSCLES

1. Synchronization of concurrently active motor-unit firings was studi ed in six human muscles performing isometric constant-force contractio ns at 30% of the maximal level. The myoelectric signal was detected wi th a quadrifilar needle electrode and was decomposed into its constitu ent motor-unit action-potential trains with the Precision Decompositio n technique, whose accuracy has been proven previously. 2. Synchroniza tion was considered as the tendency of two motor units to fire at fixe d time intervals with respect to each other more often than would be e xpected if the motor units fired independently. A rigorous statistical technique was used to measure the presence of peaks in the cross-inte rval histogram of pairs of motor-unit action-potential trains. The loc ation of the center of peak as well as their width and amplitude were measured. A synch index was developed to measure the percentage of fir ings that were synchronized. The percentage of concurrently active mot or-unit pairs that contained synchronized firings was measured. 3. Syn chronization of motor-unit firings was observed to occur in two modali ties. The short-term modality was seen as a peak in the cross-interval histogram centered about zero-time delay (0.5 +/- 2.9 ms, mean +/- SD ) and with an average width of 4.5 +/- 2.5 ms. The long-term modality was seen as a peak centered at latencies ranging from 8 to 76 ms. On t he average, the peaks of the long-term synchronization were 36% lower but had approximately the same width as the peaks for the short-term s ynchronization. Short-term synchronization was seen in 60% of the moto r-unit pairs, whereas long-term synchronization was seen in 10% of the pairs. 4. Short-term synchronization occurred in bursts of consecutiv e firings, ranging in number from 1 to 10, with 91% of all synchronize d firings occurring in groups of 1 or 2; and the bursts of discharges appeared at sporadic times during the contraction. 5. The amount of sy nchronization in motor-unit pairs was found to be low. In the six musc les that were tested, an average of 8.0% of all the firings were short -term synchronized, and an average of 1.0% were long-term synchronized . The synch index was statistically indistinguishable (P = 0.07-0.89) among the different muscles and among 9 of the 11 subjects tested. 6. Sixty percent of concurrently active motor-unit pairs displayed short- term synchronization, 10% of the pairs displayed long-term synchroniza tion, and 8% displayed both modalities. Motor-unit synchronization lev el was significantly lower in the larger muscles [tibialis anterior (5 4% short-term, 13% long-term). and deltoid (45% short-term, 12% long-t erm)] than in the smaller muscles [first dorsal interosseous (71% shor t-term, 15% long-term), extensor carpi ulnaris (74% short term, 11% lo ng-term), and the extensor carpi radialis longus (69% short-term, 9% l ong-term)], and in 2 of the 11 subjects. 7. The amount of synchronizat ion was not found to be dependent on the recruitment threshold of the motor units or on the difference between the recruitment thresholds of any two motor units that displayed synchronization. 8. Our results ar gue against the common presynaptic fiber-branches hypothesis and the m uscle afferent feedback hypothesis that have previously been used to e xplain the occurrence of synchronization. 9. We suggest that synchroni zation among motor-unit firings may not have a significant physiologic al purpose; instead, it may be a by-product of the activity of other p hysiological mechanisms. 10. We introduce a new hypothesis for explain ing synchronization. Oscillators within the CNS drive motoneurons to f ire in synchrony occasionally. This hypothesis requires no direct comm on physical connection to drive the motoneurons and is capable of expl aining all our observations.