Reporting for Duty: The duty cycle in Functional Electrical Stimulation research. Part I: Critical commentaries of the literature

Abstract

There are several parameters that can be modulated during electrical stimulation-induced muscle contraction to obtain external work, i.e., Functional Electrical Stimulation (FES). The literature has several reports of the relationships of parameters such as frequency, pulse width, amplitude and physiological or biomechanical outcomes (i.e., torque) when these parameters are changed. While these relationships are well-described, lesser known across the literature is how changing the duty cycle (time ON and time OFF) of stimulation affects the outcomes. This review provides an analysis of the literature pertaining to the duty cycle in electrical stimulation experiments. There are two distinct sections of this review – an introduction to the duty cycle and definitions from literature (part I); and contentions from the literature and proposed frameworks upon which duty cycle can be interpreted (part II). It is envisaged that the two reviews will highlight the importance of modulating the duty cycle in terms of muscle fatigue in mimicking physiological activities. The frameworks provided will ideally assist in unifying how researchers consider the duty cycle in electrical stimulation (ES) of muscles.

Home-Based Functional Electrical Stimulation of Human Permanent Denervated Muscles: A Narrative Review on Diagnostics, Managements, Results and Byproducts Revisited 2020

“…The outcomes included a significant increase in muscle strength and an increase in the number and size of fast myofibers, that are the first to respond to electrical stimulation and whose content and size are correlated to muscle strength. In muscle biopsies, Pax7- and NCAM-positive muscle satellite cells were also increased in the absence of muscle damage and cellular inflammation [ 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 ]. Finally, there are many applications of in-home or in-hospital functional electrical stimulation managements of disuse muscle atrophy related to organ diseases, from chronic cardiovascular failures to functional electrostimulation cycling in SCI [ 52 , 53 , 54 , 55 , 56 ].…”

The duty cycle in Functional Electrical Stimulation research. Part II: Duty cycle multiplicity and domain reporting

“…This concept may be further concurred by examination of other studies. By the findings of Pournezam et al 5 and others (table 2, in part I) 1 , it could be suggested that continuous electrical stimulation is much more fatiguing than intermittent, sequential stimulation. The authors do for example, argue that recovery is better for sequential as opposed to continuous stimulation.…”

“…The relative ability of intermittent and continuous protocols to bestow fatigue upon muscles is thus conflicting, as illustrated from literature (Part I) 1 Therefore, it follows that some theoretical guidance could facilitate the development of a more unified approach to discussing the relative degree of intermittency as compared with continuity of electrical stimuli. A fundamental approach is outlined in (Table 2), with relevant literature discussed.…”

“…There is no standard definitions of what constitutes continuity versus intermittency in the muscle international literature. This is an issue that renders comparison of those studies that aim to compare continuous versus intermittent stimulation minimally comparable (e.g., compare some of the studies in table 2 of this review Part I) 1 .…”

Functional electrical stimulation of permanently denervated muscles, updated 2020

“…In muscle biopsies, a significant increase in the number of Pax7-and NCAM-posi-tive muscle satellite cells was also observed, but there were no signs of muscle damage and/or cellular inflammation. [34][35][36][37][38][39][40][41][42][43][44] Furthermore, there are many more applications of home or in hospital FES to manage disuse muscle atrophy related to several organ diseases, from chronic cardiovascular failures to functional electrical stimulation cycling in spinal cord injury. [45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] conclusive remarks In recent years, we have demonstrated the long-term clinical value of co-activation of thigh muscle by home-based Functional Electrical Stimulation training using large surface electrodes.…”