It is possible to observe recently that application of the simple, known tools for biomedical signals processing is no longer justified. The problems under analysis frequently require application of more and more sophisticated tools. This is the case in attempt of estimation of rehabilitation progress referring to patients after ischemic brain stroke. Classic approach apply acquisition of EEG signal and following that application of different signal processing tools for analysis and classification of patients after ischemic brain stroke using e.g. Scandinavian scale. In this paper the totally different, indirect approach is discussed. Instead of EEG the application of relatively easier for acquisition signal such as HRV is applied. However, again classic analysis methods well established in Task Force report such as power spectral estimation present several disadvantages like e.g. loss of phase information. The trends turn the attention into more sophisticated methods and one among them concerns application of the second order statistics and following that calculation of both bispectrum and bicoherence. The presented considerations are in fact first serious study after years of existing of mentioned tool. However the difficulties in calculations put that tool into the background of signal processing method. The general trend in current analysis of such signal is focused on application of sort of hybrid method taking into account methods from different areas and creating in that way a novel approach by attempt of application of indirect method allowing effective conclusions.