Excitability changes of motor evoked potentials dependent on muscle properties and contraction modes

Using transcranial magnetic stimulation (TMS), differences in the excitability changes of motor evoked potentials (MEPs) between isometric (force task) and isotonic (movement task) muscle contractions in a distal (first dorsal interosseous; FDI) and a proximal (middle deltoid; MD) muscle were studied. In the FDI muscle, the active threshold of MEP recruitment was significantly lower in the isotonic than that in the isometric muscle contraction in spite of identical background EMG activity levels. Additionally, the dependence of the MEP amplitude on background EMG activity was significantly greater in the isotonic than in the isometric muscle contraction at low EMG activity levels, but the difference disappeared beyond middle EMG activity levels. In the MD muscle, the dependence of the MEP amplitude on background EMG activity was significantly greater in the isotonic than in the isometric muscle contraction, and further this dependence was kept at all muscle contraction levels. These results indicate that the dependence of the MEP amplitude on background EMG activity is modulated not only by the different muscle contraction modes (isotonic and isometric), but also by muscle properties (distal and proximal). Thus, the present findings suggest that the task-specific extra excitation in the proximal muscle is definitely produced corresponding to task differences (task-dependent subliminal fringe), which might be explained by the predominant frequency principle if applied to the proximal muscle. On the other hand, the lack of task-dependent extra excitation in the distal muscle is explained by the predominant recruitment principle for force grading in small hand muscles.