Evoked responses to transcranial and electrical stimulation during isometric and lengthening contractions of the soleus muscle
The aim of this study was to assess differences in motor control between isometric and
lengthening contractions of the soleus muscle. Evoked responses to TMS (MEPs) and
electrical stimulation of the peripheral nerve (H-reflexes) where recorded at rest and during
isometric and lengthening contractions of the soleus muscle at 20%, 40%, 60% and 80%
MVC. Torque and background EMG were averaged over a time window of 100ms prior to
stimulation. MEPs were found to be significantly lower (P < 0.05) during lengthening
contractions compared to isometric contractions at 40%, 60% and 80% MVC (10.95 ± 0.05
mV vs 9.93 ±0.06 mV ; 1.47 ± 0.07 mV vs 9.7 ± 0.08 mV; 11.48 ± 0.08 mV ± vs 10.14 ±
0.07 mV). The H-reflex-to-Mmax ratio was significantly lower (P < 0.05) during passive
lengthening compared to the passive isometric condition (2.5 ± 1.11 mV vs 1.4 ±0.88 mV).
In an active muscle, the H-reflex-to-Mmax ratio was similar between isometric and
lengthening modes of contraction. Torque production during passive lengthening (1.8 ± 2.8
Nm vs 4.6 ± 3.2 Nm) and at 20% MVC lengthening (215.25 ± 68.3 Nm vs 300.8 ± 155
Nm) was higher (P < 0.05) compared to the corresponding torque produced in isometric
conditions.
MEPs represent the excitability of both supraspinal and spinal neurons; H-reflex, on the
other hand, reflects only the excitability of the spinal MN pool. Therefore, it was concluded
that the motor cortex generates a descending command of lower amplitude in the case of
lengthening contractions. This, however, could not be considered as neural inhibition, since
recorded torque during muscle lengthening was similar or higher compared to the torque
recorded during isometric muscle actions.
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