ECGs are subject to many different kinds of noise internally and externally. Figure 6 shows the different kinds of artifacts that ECGs typically experience.
These artifacts can be filtered digitally as well as through analog practices. For example, the analog band stop filter shown in figure 7 can be used to modulate 60 Hz noise from power lines based on what resistor, capacitor, and inductor values are chosen.
Digitally, Matlab allows one to implement filters with the help of FFTs. For example, an FFT can extract the ECG and muscle shaking frequency components. From there, one can design a filter to obtain only the ECG components of the signal. For ECG circuits however, typically low pass filters will suffice to eliminate outside noise due to the frequency of a heartbeat.