Taslim,
Typically signal to noise ratio is defined on simple signals such as sine waves. Even if you have a "clean" version of the ECG signal, you cannot simply subtract or divide it because it will have a a slight phase shift and probably a small difference in the waveform due to the separation of the ECG electrodes from the EMG electrodes.
The SINAD VI in the Signal Processing >> Waveform Measurements palette uses FFT methods to identify the fundamental frequency and to isolate that "signal" from other components in the total signal.
With a signal which looks much like noise (the EMG) you must define exactly what you mean by signal to noise ratio and how it could be measured.
You might be able to take the RMS of the composite signal and compare that to the RMS of a clean ECG, but you would need to normalize the amplitudes in some way to make the result reproducible and meaningful.
Lynn
