Lower excitability of the corticospinal tract to transcranial magnetic stimulation during lengthening contractions in human elbow flexors

The purpose of this study was to characterize the neuromuscular control dur ing shortening (SHO) and lengthening (LEN) contractions by investigating th e input-output (I/O) property in the corticospinal tract. To this end, the relation between various stimulus intensities applied via transcranial magn etic stimulation and the size of motor evoked potentials was investigated i n six healthy subjects during elbow flexion and extension. The measured I/O property demonstrates a sigmoidal shape, and is characterized by a plateau value, maximum slope and threshold. The results demonstrated that both the plateau value and maximum slope were significantly lower during LEN contra ction than during SHO contraction (P < 0.05), whereas the threshold was fou nd not to be significantly different. These results suggest that both the m aximum excitation level and the gain of the corticospinal tract are reduced during LEN contractions. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.