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Abstract
The purpose of this study was to investigate how the recruitment properties of the
corticospinal pathway are modulated in the soleus muscle of the lower limb during
lengthening (LEN) and shortening (SHO) contractions by comparing the shape of the
input-output (I/O) relation of the corticospinal pathway. To this end, we investigated
the relationship between various stimulus intensities applied via transcranial magnetic
stimulation and the size of motor-evoked potentials in 14 healthy subjects during
voluntary plantarflexion and dorsiflexion (active lengthening) with a similar background
activity (BGA) level. The shape of this relationship was sigmoidal and was characterized
by a plateau value, maximum slope and threshold. The plateau value was clearly lower
during LEN contractions than during SHO contractions. Likewise, the maximum slope
was lower during LEN contractions. However, the threshold did not differ significantly
between the two tasks. Since the plateau value and the maximum slope clearly differed
between LEN and SHO contractions despite the similarity of their BGA levels, the central
nervous system appears to have a different activation strategy for each of these tasks.
Namely, the relative balance between excitatory and inhibitory components of the corticospinal
volleys, as well as the subliminal fringe of the corticospinal pathway, were reduced
during LEN contractions compared with SHO contractions. These strategies may help
to avoid reflexive contractions brought about by higher discharge of muscle spindles
and enable fine motor actions in voluntary lengthening contractions.