Measuring ipsilateral silent period: Effects of muscle contraction levels and quantification methods

Ipsilateral silent period (iSP) is a frequently measured index of interhemispheric inhibition. However, the methodology used across studies has been inconsistent and variable. We investigated the optimal contraction level and quantification methods for achieving iSP measurement consistency. Twenty-five healthy adults performed right isometric thumb abduction under three conditions (30%, 50%, and 100% of maximal voluntary contraction) while transcranial magnetic stimulation was applied over the primary motor cortex representational area of the abductor pollicis brevis. iSP was quantified by: iSP duration, iSP area and normalized iSP. Measurement consistency was determined by the homogeneity of variance test and by the coefficient of variation. iSP was consistent across all contraction levels when measured by iSP duration and normalized iSP. Normalized iSP showed the least measurement variability. We propose that future investigations examining interhemispheric inhibition use normalized iSP for measurement consistency and the ability to compare results across studies.