I’d like to underline a very important and misunderstood feature of EMS, recruitment order. In a nutshell: muscle fibers are not recruited in reversed order and there is no apparent sequencing related to muscle fiber type; recruitment reversal reflects beliefs based on past findings that are being questioned and proven incorrect. The next quote is an example of how rooted this belief is.
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:
The orderly recruitment of MUs (Motor Units) is controlled by the size of motorneurons (Hennemann’s size principle): Small motorneurons are recruited first, and requirements for higher forces are met by the activation of the large motoneurones that innerfate fast MUs.
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):
For isolated motor nerve situation, the pattern of recruitment tends to be in order from largest-diameter to smallest-diameter fiber…
Stimulated contraction occurs by an activation of type FF (fast-twitch, fatigable) motor units first, followed by type FR (fast-twitch, fatigue-resistant) and ending with type S (slow-twitch, fatigue-resistant) units. This reversed order of motor unit recruitment in electrically induced contractions is not stable as that for voluntary contraction. For example, if the axons of fatigue-resistant units are located significantly closer to the stimulating electrodes than axons for type FF units, these units may be recruited before the fatigable type.
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:
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Previous studies as well as some commonly used textbooks,18,19 presume the reversal of recruitment pattern based on studies of lower mammals. However, factors that affect current flow, and therefore muscle activation in vivo (ie, skin impedance, subcutaneous fat, peripheral nerve orientation, and so on), result in a different physiological environment relative to the animal studies. Thus, although the neurophysiological principles commonly used to support a reversal of recruitment order are based on well designed studies, these principles do not strictly apply during typical EMS applications to humans.
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:
The present findings confirm the suggestions made by Gregory and Bickel, that MU recruitment pattern during NMES is random and nonselective. Over-the-muscle electrostimulation would neither result in motor unit recruitment according to Henneman’s size principle nor would it result in a reversal in voluntary recruitment order. During electrostimulation, muscle fibres are activated without obvious sequencing related to fiber type.
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.
The importance of this very recent finding, for those who train for force development, will be explained in a future more-in-depth posting on stimulation frequency.