The nonlinear nature of MEP amplitudes during cold-water immersion

OBJECTIVE: The objective of this study is to investigate whether motor-evoked potentials (MEPs) amplitudes are associated nonlinearly with cold-water immersion-induced experimental pain in muscle. METHODS: The cortical representation area of the right abductor pollicis brevis muscle of 10 subjects was stimulated using a multi-locus transcranial magnetic stimulation device, and the resulting MEPs were recorded with electromyography. The stimulation was delivered with and without the cold-water immersion. A generalized additive model (GAM) was applied to study the association of different parameters with the MEP amplitudes. RESULTS: The GAM results indicated a nonlinear association between MEP amplitude and cold-water immersion-induced experimental pain, with a curved component at lower pain levels [up to approximately 5 on the numerical rating scale (NRS)], followed by a rising trend that peaked at the upper end of the NRS. CONCLUSION: The formed GAM suggests a nonlinear association between the experienced pain level and the amplitudes of MEPs. These findings indicate that higher pain levels or prolonged cold-water exposure may be associated with changes in MEP amplitudes, though causality cannot be inferred.