Duxx,
It’s a good question! I think Charlie may help in here. (Glad you are enjoying the EMS Theory thread)
My educated guess is that since EMS recruits preferentially fast fibers and then slow fibers (this is not an absolute since fibers are mixed), and voluntary contraction goes the other way around (first slow then fast), the two could complement each other.
However, if you are able to increase current intensity to the max, and recruit all fibers possible, you may not be able to augment at all, because all fibers are already contracted. If one is not able to increase current intensity to the max, or if one doesn’t have a good professional stimulator, then simultaneous voluntary contraction may help. Again this is my educated guess.
At the National Strength and Conditioning Association, 2007 convention in Atlanta, I remember a Sport Medicine doctor presented a study in which he showed the increased benefits from doing EMS dynamically (as opposed to isometrically). This is another subject altogether, but it shows that it’s possible to do things differently and obtain results.
Giovanni