Neuromuscular electrical stimulation (NMES), delivered transcutaneously via electrodes placed on the skin over the target muscles, can be used to evoke muscle contractions. Devices have been constructed so that NMES can be used as a form of exercise. One common approach is NMES leg cycling, often known as functional electrical stimulation (FES) cycling. For practical purposes, we will use the common nomenclature “FES” when describing certain NMES approaches (such as “FES leg cycling”, “FES rowing”, etc) and the term “NMES” for others (such as “NMES resistance training”) despite the fact that all approaches use NMES technology. In this approach, a computer controls the phasic cycling activation of different leg muscle groups so that contractions are coordinated to power a leg cycle. Other approaches can be used, such as pairing NMES with resistance exercise movements (such as knee extension). It should be noted that some investigators have used low-intensity/high-volume NMES approach where the muscle is tonically activated with a low level of stimulation that is sustained for a prolonged period of time. This approach will not be covered in this chapter.
Fourteen studies have used FES leg cycling exercise (FES-LCE) as a training intervention with cardiorespiratory outcomes, ranging in duration from 6 weeks to 52 weeks. A single RCT study by Gorgey et al. (Gorgey, Graham et al. 2017) compared the efficacy of 16 weeks of FES-LCE versus arm-cycle ergometry (ACE) training. These authors reported that peak oxygen uptake was not significantly changed in either of the intervention groups nor was resting heart rate or blood pressure. By contrast, of the single-cohort longitudinal studies, most reported concurrent improvements to both VO2peak and POpeak (or endurance time) during or following FES-LEC training. Of those that examined ventilatory parameters, only two out of six noted increased peak ventilation during graded stress tests (Janssen & Pringle 2008; Pollack et al. 1989), and Hjeltnes et al (Hjeltnes et al. 1997) did not observe any changes to resting lung volumes after training. Seven of the FES-LEC studies assessed blood pressure or arterial parameters. While resting blood pressure seems largely unchanged with training (four studies including an RCT), both Pollack (Pollack et al. 1989) and Faghri (Faghri et al. 1992) observed lower blood pressure responses to exercise, indicating positive cardiovascular adaptations with FES-LEC training. This is further supported by findings from Hopman (Hopman et al. 2002) and Zbogar (Zbogar et al. 2008) indicating favourable vascular adaptations. Furthermore, most studies reporting on heart rate (HR) data found increased peak or submaximal exercising HR, though the RCT from Gorgey (Gorgey, Graham et al. 2017) did not see significant changes to HRpeak.
Of twelve studies involving hybrid FES training, five used hybrid FES that combines FES-LEC with voluntary arm cycling exercise (FES-LEC+ACE), and seven involved FES-rowing training. There was a single RCT by Bakkum (Bakkum et al. 2015) which compared FES-LCE+ACE versus ACE, though this study did not include cardiorespiratory data related to oxygen uptake or exercise performance, and otherwise only saw some reductions in resting diastolic blood pressure following the 16-week intervention. All studies but one reported improvements to VO2peak in the ranges of ~10-100%, not only in FES-hybrid modalities but in some cases translating to other modalities like ACE alone and/or FES-LCE alone (Brurok et al. 2011; Gibbons et al. 2016; Krauss et al. 1993). Most often the improvements in oxygen uptake were accompanied by greater POpeak during graded stress tests. The only study that didn’t see improved VO2peak was Kim (Kim et al. 2014) whose training duration was a relatively short 6 weeks and did otherwise report improved muscular strength. In contrast to findings from the FES-LCE studies, all hybrid FES studies reporting on ventilation found increases in peak ventilation responses during graded stress tests. Two studies assessing cardiac structure and function also noted increased cardiac output. Of note, Gibbons et al. (Gibbons et al. 2016) performed detailed echocardiographic assessments in participants following FES-ROW training, and observed augmented heart mass, dimensions, and improved ejection and filling function. Finally, two of the studies noted increased peak HR during FES stress tests, while HRpeak was unchanged in three others.
Two studies using FES ambulation training, both with the commercial Parastep® system, found that training promoted favourable adaptations to the vascular system (i.e. larger arteries and greater flow responses) or improvements to VO2peak following the training program. A single prospective cohort study by Carty et al. (Carty et al. 2012) assessed the efficacy of 8 weeks of NMES resistance training and found that participants improved VO2peak in an incremental wheelchair test following their training. They also found increased peak HR across their full cohort. Only two other studies using NMES have reported cardiorespiratory data: Stoner (Stoner et al. 2007) and Sabatier (Sabatier et al. 2006) have assessed arterial structure and function but did not observe any changes to femoral artery diameter or resting function. Stoner (2007) did note improved blood flow responses post-occlusion, but these were not significant in Sabatier’s (Sabatier et al. 2006) participants.
There is Level 2 evidence (Carty et al. 2012) that 8 weeks of 1-hour NMES training, 5 times per week can improve aerobic capacity (ie. peak oxygen uptake), and allow individuals to achieve higher peak exercising HR.
There is level 4 evidence (Berry et al. 2008; Faghri et al. 1992; Gerrits et al. 2001; Hjeltnes et al. 1997; Hooker et al. 1992; Hooker et al. 1995; Hopman et al. 2002; Janssen & Pringle 2008; Mutton et al. 1997; Pollack et al. 1989) that 6-52 weeks training with 2-3 sessios per week of FES training promotes improvements to aerobic capacity (ie. peak oxygen uptake) and exercise performance
There is Level 4 evidence (Janssen & Pringle 2008; Pollack et al. 1989) that 6-28 weeks, 2-3 sessions per week FES training can result in increased ventilatory capacity without no changes in lung volumes, per se.
There is Level 4 evidence (Hopman et al. 2002; Zbogar et al. 2008) that 6-12 weeks, 3 sessions per week of FES training can lead to positive cardiovascular adaptations, including favourable alterations to arterial structure and function.
There is Level 4 evidence (Brurok et al. 2011; Gibbons et al. 2016; Gurney et al. 1998; Jeon et al. 2010; Krauss et al. 1993; Mutton et al. 1997; Qiu et al. 2016; Solinsky et al. 2020; Taylor et al. 2014; Wheeler et al. 2002) that 6-36 weeks FES-hybrid training (FES-LES+ACE and FES-rowing), results in improved aerobic capacity (ie. peak oxygen uptake) and exercise performance.
There is Level 4 evidence (Brurok et al. 2011; Mutton et al. 1997; Pollack et al. 1989; Qiu et al. 2016; Taylor et al. 2014) that 8-36 weeks of FES-hybrid training can improve peak ventilation during exercise.
There is Level 4 evidence (Brurok et al. 2011; Gibbons et al. 2016) that FES-hybrid training (FES-LES+ACE and FES-rowing) can promote positive cardiovascular adaptations, including increased heart size and improved pumping and filling function. This training may also increase peak exercising HR.
There is Level 4 evidence (Stoner et al. 2007)that 18 weeks of NMES training, 2 times per week, may promote cardiovascular adaptations, including positive alterations to arterial structure and function.