Neither of these is truly sinusoidal. At best, you might say the first has an oscillatory behavior. Note that the first one has a strong semi-linear trend in it too. But sinusoidal? Not so. And if it is not actually sinusoidal, your question becomes a bit moot, as now one can only ask about the amplitude of semi-periodic behavior.
So if you wanted that, then you might consider an FFT. See what it can tell you about those patterns. For example, if you saw a strong amplitude at some frequency, what might it tell you?
Or perhaps the wavelet toolbox might be even more appropriate for your purposes.
