Transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) elicits muscular response called motor evoked potential (MEP). MEP is produced by temporal and spatial summation of EPSPs elicited by multiple descending volleys, which is composed by several indirect waves or I-waves. The contribution of each I-wave is not the same, and it is proposed that later I-waves exert large influence on MEP amplitude. The exact amount of I-waves, however, has to be measured by invasive recording such as epidural spinal recording. This report describes a method to infer the degree of later I-wave contribution to MEP waveforms. Independent component analysis (ICA) was applied to a set of MEP waveforms which contained 10 stimulus conditions, each of which had 10 MEPs. In two settings, the results were compatible with the previous ones obtained by invasive recordings: (1) when stimulus intensity was increased, MEP from a single TMS had larger later I-wave component, and (2) in short-interval intracortical inhibition (SICI) and facilitation (SICF), decrease and increase of later I-wave contribution was found. Analyses of surface EMG waveforms using ICA might be a useful way to estimate later I-wave component of the descending volleys non-invasively.