Objective: The clinical use of Transcranial Magnetic Stimulation (TMS) as a technique to assess corticospinal excitability is limited by the time for data acquisition and the measurement variability. This study aimed at evaluating the reliability of Stimulus-Response (SR) curves acquired with a recently proposed rapid protocol on tibialis anterior muscle of healthy older adults. Methods: Twenty-four neurologically-intact adults (age:55–75 years) were recruited for this test-retest study. During each session, six SR curves, 3 at rest and 3 during isometric muscle contractions at 5% of maximum voluntary contraction (MVC), were acquired. Motor Evoked Potentials (MEPs) were normalized to the maximum peripherally evoked response; the coil position and orientation were monitored with an optical tracking system. Intra- and inter-session reliability of motor threshold (MT), area under the curve (AURC), MEP_max, stimulation intensity at which the MEP is mid-way between MEP_max and MEP_min (I50), slope in I50, MEP latency, and silent period (SP) were assessed in terms of Standard Error of Measurement (SEM), relative SEM, Minimum Detectable Change (MDC), and Intraclass Correlation Coefficient (ICC). Results: The relative SEM was ≤10% for MT, I50, latency and SP both at rest and 5%MVC, while it ranged between 11% and 37% for AURC, MEP_max, and slope. MDC values were overall quite large; e.g., MT required a change of 12%MSO at rest and 10%MSO at 5%MVC to be considered a real change. Inter-sessions ICC were >0.6 for all measures but slope at rest and MEP_max and latency at 5%MVC. Conclusions: Measures derived from SR curves acquired in <4 minutes are affected by similar measurement errors to those found with long-lasting protocols, suggesting that the rapid method is at least as reliable as the traditional methods. As specifically designed to include older adults, this study provides normative data for future studies involving older neurological patients (e.g. stroke survivors).

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