It's my understanding that, to some extent, nerves and muscles operate with hysteresis based on charge thresholds, and rely on pulse-density encoding for variability rather than proportional voltage or current modulation. Presumably, that seems to be why your muscles shake when under heavy strain.
It sorta makes sense, considering your body is generally electrically conductive everywhere. Signals propagate as waves of charge sustained by cells opening and closing little pores that selectively release ions. "Conductors" have pores, and "insulators" don't. That's also why nerve signals are relatively slow to propagate vs. straight up electricity. That's also why you can measure muscle activity via an ECG; you're measuring the transient voltages being generated by the waves of charge moving around. The voltages themselves don't have much meaning to your body. Of course, if you apply enough voltage to build up a charge inside a muscle, you can cause it to actuate.
I recently had to get a pacemaker, and apparently the particular lower threshold to trigger my heart muscle to contract is around 0.7V for 350uS, at whatever impedance the lead happens to be. Below that, and my heart muscle does nothing. Anything above that, any my heart muscle does a full beat. The device applies a 2V pulse so that there's plenty of margin, and can go up to 5V if needed, in case the lead's impedance increases. The lower the voltage the better, in terms of battery life. The cool thing is that the device can safely coexist with any natural electrical activity my faulty nerves may have, since the muscle simply responds to whichever pulse happens to arrive first.