Reliability of motor-evoked potentials during resting and active contraction conditions

Purpose: To determine the reliability of motor-evoked potentials (MEP) obtained using transcranial magnetic stimulation (TMS) in the first dorsal interosseous (FDI) and biceps brachii muscles. Methods: Fourteen college subjects attended the laboratory on three separate days. TMS was used to obtain MEP with the subject relaxed (resting condition) at stimulation intensities of 70%, 85%, and 100% of maximal stimulator output. MEP were also obtained during four active contraction conditions involving contractions of 25%, 50%, 75%, and 100% of maximal effort (MVC). Reliability was measured using an intraclass correlation analysis of variance (ANOVA) design. Results: In the resting condition, substantial increases in MEP amplitude were observed for both muscles from day 1 to day 2. Intraclass reliability estimates were higher for the biceps muscle (ICC = 0.95-0.99) than for the FDI muscle (ICC = 0.60-0.81). During the active conditions, the greatest MEP were observed at 25% and 50% MVC, with smaller MEP at 75% and 100% MVC. Intraclass correlations in the active condition were approximately 0.63-0.73. Conclusions: Moderate to good reliability of MEP amplitude in the biceps and FDI muscles can be obtained using TMS in both resting and active contraction conditions.