Inverse correlation between resting motor threshold and corticomotor excitability after static magnetic stimulation of human motor cortex.

BACKGROUND: High-strength static magnetic field stimulation (SMS) results in a period of reduced corticomotor excitability that may be mediated through a decrease in membrane excitability. OBJECTIVE: As resting motor threshold (RMT) is thought to reflect membrane excitability, we hypothesized that SMS may increase RMT and that there would be an inverse relationship between RMT and motor-evoked potential (MEP) amplitude. METHODS: Ten healthy subjects (aged 20-29; 4 females) participated in a double-blinded crossover design comparing MEP amplitude and RMT before and after a 15-min period of SMS or sham stimulation over primary motor cortex (M1). RESULTS: MEP amplitude was initially significantly reduced post-SMS (∼20%), and returned to baseline by 6 min post-intervention. MEP amplitude and RMT were inversely correlated (r(2) = 0.924; P = 0.001). Sham stimulation had no effect on MEP amplitude (P = 0.969) or RMT (P = 0.549). CONCLUSION: After SMS, corticomotor excitability is transiently reduced in association with a correlated modulation of RMT. SMS after effects may be mediated in part by a reduction in membrane excitability, suggesting a possible role for non-synaptic (intrinsic) plasticity mechanisms.