I’d like to underline a very important and misunderstood feature of EMS: recruitment order.
Voluntary muscle activation activates muscle fibers in a very specific order, from smallest Slow-Twitch (ST) fibers, to largest Fast-Twitch (FT) fibers. Early research seemed to point out that this was reversed in EMS exercise: FT fibers were activated more easily than ST fibers. However, very recent research has demonstrates that EMS does not reverse the natural recruitment order, and it rather activates fibers based on position relative to the electrode pads.
Voluntary recruitment order, as quoted from Zatsiorsky’s & Kraemer’s - Science and Practice of Strength Training, Human Kinetics, Champaign, IL:
Early studies that experimented on single fibers in a lab setting, found that FT larger fibers were easier to excite: see Gorman and Mortimer, The effect of stimulus parameters on the recruitment characteristics of direct nerve stimulation; IEEE Trans. Biomed.Eng. 1983
However, those studies were performed on anesthetized animals, attaching electrodes directly to muscle fibers by cutting through the skin. Since this were the only available findings at the time, the literature on EMS started using this as a self fulfilling benchmark.
Clinical Electrophysiology, a sacred reference textbook on EMS says (Robinson, Snyder-Mackler; 1995, Williams and Wilkins):
So you can start seeing above some exception to the, until then, complete belief in the reversed-recruitment-order principle. More recent research has actually confirmed that there isn’t actually an order, but rather an indiscriminate recruitment: the peer-reviewed article, Recruitment Patterns in Human Skeletal Muscle During Electrical Stimulation, Gregory and Bickel, Phys Ther, Apr.2005, says:
.
Jubeau et al. verified this hypothesis in the peer-reviewed published research, Random Motor Unit Activation by Electrostimulation, Int J Sports Med Nov.2007, concluding:
Understanding recruitment correctly is very important. The consequence is enormous: recruitment percentage is dictated by the depth of the electric field (which increases with increasing current intensity). ST and FT fibers are normally mixed in a muscle bundle, i.e. equally distant from the pads. Therefore stimulation frequency becomes the most important factor in deciding what type of work we are going to do with EMS training.
Stimulation frequency will be the subject of another more-in-depth posting.