Hi @Grant_Powicki! What processing board do you have the Motor Shield attached to? No voltages are “generated” (per-se) on the Motor Shield itself. If your board brings 3.3V to a header pin, as the Arduino Uno and others like it do, that 3.3V is “available” on the corresponding Motor Shield header pin. The pin that carries 3.3V is a no-connect on the Motor Shield, and you’d have to solder a wire to bring it (and presumably ground as well) out to your muscle sensor.
If you were using, for example, a WildFire (wildfire.wickeddevice.com), all the header pins are also have convenient holes adjacent to the headers so you can solder directly to them on the processing board.