You can get a very forceful contraction even if you are an endurance athlete, with a high proportion of red, slow-twitch fibers (type 1). Look up this posting, in a forum devoted to endurance/triathlon athletes using Globus and Compex EMS.
standard units are made to travel through fat
What do you mean? Fat cells and muscle mass each resist/oppose the passage of current in different ways (resistance or impedance). Current from any unit will travel in both. However, what sets a unit apart from another are the parameters that cause training contractions, the know-how distilled in the electrostimulation parameters that the device is capable to achieve, and in the electrostimulation programs that come with it. Most units I’ve seen discussed by users in this forum are TENS units made for therapy, which have inadequate pulse width for sport training.