Analyzing sEMG frequency using FFT

I am trying to find the wave frequency of an imported raw sEMG signal data. I have previously performed a digital Butterworth Filter and FFT iteratively to obtain the most constructive wave during a certain time period. Are there any suggestions for correctly analyzing this data to produce afrequency vs. time graph that shows where the bursts in activity are? Below is the current resultant of our filtering and analysis process. THANKS!

[Star Strider]

If you want to analyse frequency as a function of time, consider using the spectrogram (link) function. You can do a STFT of your own design if you want to, however spectrogram makes this much easier.

Wavelet decomposition is another option.