This question is two parts.
First, can tightening your muscles repeatedly increase their strength similarly to lifting weights? Second, if this is true, can electrical stimulation of the muscles make this happen automatically?
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To answer your first question: no. If done correctly, isometric contractions of any muscle WILL cause an increase in strength, but it will not be comparable to the increase in strength from lifting weights. Whether your goal is pure strength or hypertrophy, you need to break down the muscle tissue by taking it to its absolute limits, and with the absence of a load you won't be doing that. Again, it's not to say that isometric contractions are completely useless, but you will probably not see any significant changes in muscle size from it.
And to answer your second question, EMS by itself probably wouldn't do much, but combined with a proper training regime it could potentially be a useful tool.