Influence of Voluntary Muscle Contraction on the Duration of the Silent Period Evoked by Transcranial Magnetic Stimulation

Introduction: It is known that the duration of the silent period (SP) following transcranial magnetic stimulation (TMS) can be used as a measure of intracortical inhibition. Thus, the SP is used in the clinical routine and in many studies, e.g., in order to evaluate the prognosis of motor recovery after stroke or for examining the cortical excitability in epilepsy patients. In most studies using TMS, recordings of the SP are performed by asking the subjects to contract their muscles (e.g., the first dorsal interosseous muscle, FDI) with maximal or sub-maximal strength. However, a possible modulation of the SP depending on the grade of voluntary muscle contraction strength has not been sufficiently examined. If present, such a modulation could influence the assessment of the results of TMS examinations. The purpose of our study was to investigate the duration of the SP in the contralateral FDI with different levels of contraction strength. Methods: 6 young healthy subjects (i.e., 12 hemispheres) participated in this study. We used a focal stimulating coil, which was placed over the hand area of the primary motor cortex. After the motor hot spot of the contralateral FDI was identified, the motor threshold (MT) and the maximal voluntary contraction strength (MVC) were determined. TMS was performed with 25%, 50%, 75% and 100% of the MVC, using 120% and 150% of the MT. Contraction strength was controlled and made visible to both the subject and the investigator using a hydraulic pinch dynamometer which allowed comfortable pressure application by the finger tips. Results: Using stimuli with an intensity of 120% of the MT, no changes in the duration of the SP became apparent when the subjects contracted their muscles at the different strength levels. The mean SP values for all subjects (in ms,±standard deviation) were 110.8 (±36) for 25% MVC, 112.3 (±35) for 50% MVC, 105.6 (±28) for 75% MVC and 109.4 (±32) for 100% MVC. With stimulation using 150% of the MT, the duration of the SP tended to be longer when the force applied from the subjects was higher, but it did not reach significant levels (137.5±39 ms for 25% MVC, 147.3±36 ms for 50% MVC, 146.6±28 ms for 75% MVC and 153.1±42 ms for 100% MVC). Conclusions: The contraction strength does not influence the duration of the SP significantly. This may be explained by modulations of the intracortical inhibition taking place in relation with the changing innervation level of the target muscle.