Capacitive electrodes are a promising alternative to the conventional adhesive ECG electrodes. They provide more comfort to the patient when integrated in everyday objects (e.g. beds or seats) for long-term monitoring. However, the application of such electrodes is limited by their high sensitivity to motion artifacts. Artifacts caused by variation of the coupling capacitance are studied here. An injection signal is proposed to track these variations in real-time. An adaptive filter then estimates the motion artifact and cancels it from the recorded ECG. The amplitude of the motion artifact is reduced in average by 29 dB in simulation and by 20 dB in a lab environment. Our method has the advantages that it is able to reduce motion artifacts occurring in the frequency band of the ECG and that it does not require knowledge about the measurement system.