Reliability of rapid TMS stimulus-response curves during tibialis anterior contractions on healthy elderly

Transcranial Magnetic Stimulation (TMS) is a widespread technique to study corticospinal excitability. Its use in longitudinal studies is crucial to analyze the plasticity induced by rehabilitation treatments on neuromotor-impaired subjects. As TMS-related measures fluctuate with time and attention, it is crucial to collect them rapidly. Recently, it has been shown that stimulus-response (SR) curves can be acquired in less than two minutes (Mathias et al.). This technique favors the clinical use of TMS-related measures, but its intersession reliability has not been investigated yet. In this study we propose a test-retest reliability analysis of five parameters extracted from SR curves acquired on the preactivated Tibialis Anterior (TA) using the fast acquisition method. A secondary aim was to compare the reliability of these parameters for two different calculation methods of the Motor Evoked Potential (MEP) amplitude: the peak-to-peak value, referred as MEPpp, and the root mean square value, referred as MEParea. The intraclass correlation coefficients (ICC), the standard error of measurement (SEM) and the minimum detectable changes (MDC) were evaluated. Thirteen neurologically-intact elders (age 62.2±5.1) were recruited. The acquisition protocol was set up to minimize the variability of the measurement between the two sessions, carried out on separate days: an optical tracking system was used to maintain the coil placement, the EMG data were normalized over the maximum peripherally evoked action potential and a visual feedback was displayed to the testing subject to maintain the TA activation level. All the parameters, regardless the method used to compute the MEP amplitude , showed good to excellent reliability (0.75<0.95). Our results suggest that TMS-related measurements extracted from SR curves rapidly acquired are reliable in healthy elderly. Future studies should increase the sample size to confirm the analysis.