Modulation of corticospinal excitability during lengthening and shortening contractions in the first dorsal interosseus muscle of humans.

Lengthening and shortening contractions are the fundamental patterns of muscle activation underlying various movements. It is still unknown whether or not there is a muscle-specific difference in such a fundamental pattern of muscle activation. The purpose of this study was, therefore, to investigate whether or not the relationship between lengthening and shortening contractions in the modulation of corticospinal excitability in the first dorsal interosseus (FDI) muscle is the same as that of previously tested muscles because the hand muscles are anatomically and functionally different from the other muscles. To this end, we investigated the relationship between the input-output curves of the corticospinal pathway (i.e., the relationship between the stimulus intensities vs. the area of motor-evoked potentials) during lengthening and shortening contractions in 17 healthy subjects. The shape of this relationship was sigmoidal and characterized by a plateau value, maximum slope, and threshold. The plateau value was at the same level between lengthening and shortening contractions. However, the maximum slope (P < 0.01) and threshold (P < 0.01) were significantly higher during lengthening contractions than during shortening contractions. These findings were different from the results of other muscles tested in previous studies (i.e., the soleus muscle and the elbow flexors). That is to say, the plateau value and the maximum slope during lengthening contractions were significantly lower than those during shortening contractions in previous studies. This study provides tentative evidence that the relationship between lengthening and shortening contractions in the modulation of corticospinal excitability differs between muscles, indicating that the underlying neural control is not necessarily the same even though the fundamental patterns of muscle activation are carried out.