Article

Motor control of low-threshold motor units in the human trapezius muscle.

NeuroMuscular Research Center, Boston University, Boston, Massachusetts 02215, USA.
Journal of Neurophysiology (impact factor: 3.32). 04/2001; 85(4):1777-81. pp.1777-81
Source: PubMed

ABSTRACT The firing pattern of low-threshold motor units was examined in the human trapezius and first dorsal interosseous (FDI) muscles during slowly augmenting, low-amplitude contractions that were intended to mimic contractile activity in postural muscles. The motor unit activity was detected with a special needle electrode and was analyzed with the assistance of computer algorithms. The surface electromyographic (EMG) signal was recorded. Its root-mean-square (RMS) value was calculated and presented to the subject who used it to regulate the muscle force level. In the trapezius, there was minimal, if any, firing rate modulation of early recruited motor units during slow contractions (< or =1% EMG(max)/s), and later recruited motor units consistently presented higher peak firing rates. As the force rate of the contraction increased (3% EMG(max)/s), the firing rates of the motor units in the trapezius approached an orderly hierarchical pattern with the earliest recruited motor units having the greatest firing rate. In contrast, and as reported previously, the firing rates of all motor units in the FDI always presented the previously reported hierarchical "onion-skin" pattern. We conclude that the low-threshold motor units in the postural trapezius muscle, that is the motor units that are most often called on to activate the muscle in postural activities, have different control features in slow and fast contractions. More detailed analysis revealed that, in the low force-rate contractions of the trapezius, recruitment of new motor units inhibited the firing rate of active motor units, providing an explanation for the depressed firing rate of the low-threshold motor units. We speculate that Renshaw cell inhibition contributes to the observed deviation of the low-threshold motor units from the hierarchical onion-skin pattern.

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Keywords

active motor units
 
computer algorithms
 
firing pattern
 
firing rate
 
force rate
 
hierarchical onion-skin pattern
 
higher peak
 
human trapezius
 
low force-rate contractions
 
low-threshold motor units
 
mimic contractile activity
 
motor unit activity
 
new motor units inhibited
 
orderly hierarchical pattern
 
postural activities
 
postural trapezius muscle
 
recruited motor units
 
reported hierarchical
 
special needle electrode
 
surface electromyographic
 

R H Westgaard