Intermuscular coherence analysis can be used to assess the common drive to muscles. Coherence in the β-frequency band (15-35 Hz) is thought to arise from common cortical sources. Intermuscular coherence analysis is a potentially attractive tool for the investigation of motor cortical excitability changes because it is non-invasive and can be done relatively quickly. We carried out this study to test the hypothesis that intermuscular coherence analysis was able to detect cortical excitability changes in healthy subjects following transcranial direct current stimulation (tDCS). tDCS has been shown to increase (anodal stimulation) or decrease (cathodal stimulation) the size of the muscle potential evoked by TMS. We found that anodal tDCS caused an increase in motor evoked potential (MEP) size that was paralleled by an increase in β-band intermuscular coherence. Similarly, the reduction in MEP size produced by cathodal tDCS was paralleled by a reduction in β-band intermuscular coherence, while sham stimulation did not result in any change in either MEP amplitude or β-band intermuscular coherence. The similar pattern of change observed for MEP and intermuscular coherence may indicate similar mechanisms of action, although this cannot be assumed without further investigation. These changes do suggest that at least some of the action of tDCS is on cortical networks, and that combined tDCS and intermuscular coherence analysis may be useful in the diagnosis of pathologies affecting motor cortical excitability.

Objective-Multiple point stimulation (MPS) is a widely used technique to estimate the number of motor units in a muscle. Test-retest reliability must be high for the motor unit number estimates to be clinically useful. We hypothesized that the reliability of MPS can be improved by using a long stimulus pulse width (1 ms), in addition to the standard 0.05 ms pulse width.Methods-Median nerve innervated thenar muscles were examined in 11 young subjects, 27 ± 3 (mean ±SD) years old and in 5 elderly subjects, 71 ± 11 years old. The experimenter conducted the studies twice on each subject.Results-Test-retest reliability, using the modified MPS technique, was substantially higher (Cronbach's α of 0.88) than using the standard method (Cronbach's α of 0.80) in the young subjects. In contrast, for the elderly subjects the test-retest reliability of MUNE was high for both the modified technique (Cronbach's α of 0.96) and the standard technique (Cronbach's α of 0.99).Conclusions-Test-retest reliability of MPS can be significantly improved by using a long stimulus pulse width and the standard stimulus pulse width in younger subjects. However, this is not necessary in elderly subjects.Significance-The greater reliability of this modified MPS method should enhance its clinical utility.

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