Paired Associative Stimulation Targeting the Tibialis Anterior Muscle using either Mono or Biphasic Transcranial Magnetic Stimulation

Paired associative stimulation (PAS) protocols induce plastic changes within the motor cortex. The objectives of this study were to investigate PAS effects targeting the tibialis anterior (TA) muscle using a biphasic transcranial magnetic stimulation (TMS) pulse form and, to determine whether a reduced intensity of this pulse would lead to significant changes as has been reported for hand muscles using a monophasic TMS pulse. Three interventions were investigated: (1) suprathreshold PA(bi)-PAS (n = 11); (2) suprathreshold PA(mono)-PAS (n = 11) where PAS was applied using a biphasic or monophasic pulse form at 120% resting motor threshold (RMT); (3) subthreshold PA(bi)-PAS (n = 10) where PAS was applied as for (1) at 95% active motor threshold (AMT). The peak-to-peak motor evoked potentials (MEPs) were quantified prior to, immediately following, and 30 min after the cessation of the intervention. TA MEP size increased significantly for all interventions immediately post (61% for suprathreshold PA(bi)-PAS, 83% for suprathreshold PA(mono)-PAS, 55% for subthreshold PA(bi)-PAS) and 30 min after the cessation of the intervention (123% for suprathreshold PA(bi)-PAS, 105% for suprathreshold PA(mono)-PAS, 80% for subthreshold PA(bi)-PAS. PAS using a biphasic pulse form at subthreshold intensities induces similar effects to conventional PAS.