Ms. Kim et al., Synchronization of single motor units during voluntary contractions in theupper and lower extremities, CLIN NEU, 112(7), 2001, pp. 1243-1249
Objective: To investigate motor unit synchronization in the time and freque
ncy domains and compare the amount and nature of this synchronization betwe
en upper and lower extremity muscles in human subjects.
Methods: A total of 120 motor unit pairs from biceps brachii (BB), first do
rsal interosseous (1DI), vastus medialis (VM), and tibialis anterior (TA) o
n the dominant side were analyzed and compared. Pairs of motor unit spike t
rains were recorded from two concentric needle electrodes inserted within t
hese muscles in healthy volunteers. Subjects were instructed to maintain a
weak isometric contraction of these muscles so that an individual motor uni
t recorded from each concentric needle discharged at a steady rate of appro
ximately 10 impulses/s. Pairs of motor unit spike trains were cross-correla
ted in the time domain, and coherence analysis in the frequency domain was
performed on the same spike train data.
Results: Synchronization was seen in all the muscles studied. Strength of m
otor unit synchronization, expressed as synchronization index (SI), was gre
ater in 1DI muscles compared to other muscles (P < 0.01). Coherence analysi
s revealed significant association between motor unit firings in the 1-5 an
d 25-30 Nz frequency ranges in all the muscles studied. The incidence of 25
-30 Hz coherence peaks were found to be greater fur 1DI muscles compared to
other muscles.
Conclusion: The above results suggest a possible role for corticospinal pro
jections in producing pre-synaptic inputs responsible for synchronization o
f motor unit firings and 25-30 Hz coherence peaks. (C) 2001 Elsevier Scienc
e Ireland Ltd. All rights reserved.