The effect of electrical stimulation on excitability of the corticomotor pathway

Purpose: Peripheral electrical stimulation (ES) has been used extensively to treat acute and chronic pain states whereby it is believed to activate descending pain inhibitory systems and spinal gating mechanisms. It is also used to facilitate voluntary muscle contractions with the intent to change muscle structure/function, provide a functional contraction or experience of a contraction that can be trained. Recent research also demonstrates that ES is capable of moderating cortical excitability. However, the parameters tested vary considerably and are often not consistent with those used in the clinic. Therefore, this study aimed to investigate the effect of five clinical electrical stimulation paradigms on the corticomotor excitability. Relevance: This study has direct relevance for physiotherapists using electrical stimulation and will impact upon how and why physiotherapists use this electrophysical agent. Participants: Ten healthy subjects (5 males, mean (SD) age 26 (3.6) years). Methods: Using a same subject pre-post test design, transcranial magnetic stimulation (TMS) was used to measure the responsiveness of corticomotor pathway to biceps brachii and triceps brachii muscles before and after 30 minutes of electrical stimulation over the biceps brachii. Five ES paradigms (sensory 10 Hz, motor 10Hz, noxious 10 Hz, motor 30 Hz ramped and sensory 100 Hz) were applied in random order, at least three days apart. Analysis: To account for activity-dependant changes in muscle fibre action potentials resulting from the electrical stimulation, amplitudes of TMS-induced motor evoked potentials were normalised to maximal M-wave amplitudes. For each ES paradigm, a two way repeated measures analysis of variance with factors TIME (pre/post stimulation) and MUSCLE (biceps/triceps) was used. Results: Electrical stimulation delivered at sensory (10 Hz or 100 Hz) or noxious intensity (10 Hz) decreased corticomotor responsiveness (p < 0.03). Stimulation sufficient to induce a motor contraction (30 Hz) and applied in a 'functional' ramped pattern increased corticomotor responsiveness (p = 0.04) whereas motor stimulation at 10 Hz did not. Corticomotor excitability changes were consistent for agonist and antagonist muscles. Conclusions: Stimulus intensity and pattern has a significant impact on the nature of cortical excitability induced by ES. Electrical stimulation at sensory intensities reduces corticomotor responsiveness. Motor intensities increase corticomotor responsiveness but only when a functional motor contraction is created. Implications: There are two key implications. First, the differential effects of ES paradigms could be used clinically to drive adaptive plasticity or alter maladaptive plasticity. Maladaptive plasticity has been demonstrated in a number of chronic pain states such as low back pain and fibromyalgia. Second, current ES paradigms for pain relief may be producing an undesired effect in excitability of the cortiomotor pathway to surrounding musculature