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* To whom correspondence should be addressed. E-mail: semmler{at}deakin.edu.au.
The purpose of the study was to quantify the strength of motor-unit coherence from the left and right first dorsal interosseous muscles in untrained, skill-trained (musicians), and strength-trained (weightlifters) individuals who had long-term specialized use of their hand muscles. The strength of motor unit coherence was quantified from a total of 394 motor unit pairs in 13 subjects using data from a previous study in which differences were found in the strength of motor unit synchronization depending on training status. In the present study, we found that the strength of motor unit coherence was significantly greater in the left, compared with the right hand of untrained right-handed subjects, with the largest differences observed between 21-24 Hz. The strength of motor unit coherence was lower in both hands of skill-trained subjects (21-27 Hz) and the right (skilled) hand of untrained subjects (21-24 Hz), whereas the largest motor unit coherence was observed in both hands of strength-trained subjects (3-9 Hz and 21-27 Hz). A strong curvilinear association was observed between motor unit synchronization and the integral of coherence at 10-30 Hz in all motor unit pairs (r2 = 0.77), and was most pronounced in strength-trained subjects (r2 = 0.90). Furthermore, this association was accentuated when using synchronization data with broad peaks (> 11 ms), suggesting that the 10-30 Hz coherence is due to oscillatory activity in indirect branched common inputs. The altered coherence with training may be due to an interaction between cortical inhibition and the number of direct common inputs to motor neurons in skill- or strength-trained hands.
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