Changes in motor cortex excitability during ipsilateral hand muscle activation in humans

Objectives: To test whether unilateral hand muscle activation involves changes in ipsilateral primary motor cortex (M1) excitability. Methods: Single- and paired-pulse transcranial magnetic stimulation (TMS) of the right hemisphere was used to evoke motor evoked potentials (MEPs) from the resting left abductor pollicis brevis (APB) in 9 normal volunteers. We monitored changes in motor threshold (MT), MEP recruitment, intracortical inhibition (ICI) and intracortical facilitation (ICF) while the ipsilateral right APB was either at rest or voluntarily activated. Spinal motoneuron excitability was assessed using F-wave recording procedures. Results: Voluntary muscle activation of the ipsilateral APB significantly facilitated the MEPs and F-waves recorded from the contralateral APB. Facilitation was observed with muscle activation >50% of the maximum voluntary force and with stimulus intensities >20% above the individual resting motor threshold. Intracortical inhibition significantly decreased in the ipsilateral M1, while there was no significant change in intracortical facilitation during this maneuver. Conclusions: Unilateral hand muscle activation changes the excitability of homotopic hand muscle representations in both the ipsilateral M1 and the contralateral spinal cord. While the large proportion of MEP facilitation most likely occurred at a spinal level, involvement of the ipsilateral hemisphere may have contributed to the enlargement of magnetic responses.