Frequency-dependent corticospinal facilitation following tibialis anterior neuromuscular electrical stimulation.

The optimal stimulation frequency for inducing neuromodulatory effects remains unclear. The purpose of our study was to investigate the effect of neuromuscular electrical stimulation (NMES) with different frequencies on cortical and spinal excitability. Thirteen able-bodied individuals participated in the experiment involving NMES: (i) low-frequency at 25 Hz, (ii) high-frequency at 100 Hz, and (iii) mixed-frequency at 25 and 100 Hz switched every one second. All interventions were applied on the tibialis anterior muscle using a 10 sec ON / 10 sec OFF duty cycle for 10 min, using motor-level NMES at 120 % of the individual motor threshold for each stimulating frequency. Assessments were performed at baseline, immediately after, and 30 min after the interventions. Corticospinal excitability and intracortical inhibition were examined using transcranial magnetic stimulation by assessing the motor evoked potentials and cortical silent period, respectively. Spinal motoneuron excitability and neuromuscular propagation were assessed using peripheral nerve stimulation by evaluating F-wave and maximum motor (M_max) responses, respectively. Maximal voluntary contraction (MVC) was evaluated during isometric dorsiflexion force exertion. Motor performance was also evaluated during the ankle dorsiflexion force-matching task. Our results showed that mixed frequency was most effective in modulating corticospinal excitability, although motor performance was not affected by any intervention. The cortical silent period was prolonged and M_max was inhibited by all frequencies, while the F-wave and MVC were unaffected. Mixed-frequency stimulation could recruit a more diverse range of motor units, which are recruited in a stimulus frequency-specific manner, than single-frequency stimulation, and thus may have affected corticospinal facilitation.