During exercise, analysis of the spectral components of the heart rate variability (i.e. R-R interval series variability; HRV) points out a new high frequency component related to the pedalling frequency. This component is independent of the classical low and high frequency bands related to the autonomic nervous system modulation of the heart period. It is not related to ECG distortion or artifacts, and could be linked to the mechanical influence of the muscle pump on venous return. To assess this new component, we develop and propose a method based on a time-varying filter, designed in the time-frequency plane, as well as a new model to link the potential input of the modulation (the muscle pump) to the pedalling frequency component of HRV.