Removing noise by superimposing quasi periodic bio-medical signals

Hi everyone,

I'm fairly new to signal processing. I'm doing a project which involves using matlab to superimpose quasi periodic bio-medical signals (emg or egg signals) to remove the noise (the noise approaches zero because its mean is zero) so I can calculate the signal to noise ratio (SNR). The problem is that I think the frequency changes over time (quasi periodic) so I don't know how to superimpose them. Is there a way to superimpose signals with different frequencies to cancel noise or is there an algorithm to change frequencies. How should I do it? I'm very confused. Thanks in advance.

John