© 2019 IEEE.Capacitive ECG (cECG), a technique older than 50, is able to replace the gold standard ECG only in certain applications where unobtrusiveness and conformity are aimed at the expense of reduced signal quality. Triboelectric surface charges, motion artifacts, and resulting time-variant coupling capacitances are among the reasons for the signal deformations in cECG. In this paper, the mechanical vibrations of the human body are proposed as the source of time-variant coupling capacitances causing motion artifacts, which questions the applicability of adaptive filtering approaches to the problem of time-variant coupling capacitance. Ballistocardiogram (BCG) measurements on cECG electrodes are recorded and analyzed to investigate how these mechanical vibrations reflect on a differential biopotential measurement. Furthermore, using measured signals in a human experiment, numerical and test bench simulations were conducted to replicate how triboelectric surface charges might cause deformation on cECG signals.