Functional Electrical Stimulation (FES)

Computer-assisted FES during leg cycling has been shown to be an important and practical means of exercising a relatively large muscle mass in persons with SCI (Hooker et al. 1992). These devices also permit the activation of the skeletal muscle pump during leg cycling. For these reasons, FES training has been advocated widely as an effective treatment strategy for SCI. It is important to note, that the physiological responses to FES training appear to be distinct from arm ergometry training. For instance, arm exercise has been shown to lead to faster VO₂ kinetics (oxygen metabolism/uptake) (at a constant workload), greater changes in HR, and lower post-exercise blood lactates than FES leg cycling (Barstow et al. 2000).

We identified one longitudinal (Berry et al. 2012), one prospective experimental (Fornusek et al. 2014), one post (Hakansson et al. 2012), and 13 pre-post (Ragnarsson et al. 1988; Faghri et al. 1992; Hooker et al. 1992; Barstow et al. 1996; Hjeltnes et al. 1997; Mohr et al. 1997; Gerrits et al. 2001; Hopman et al. 2002; Crameri et al. 2004; Janssen and Pringle 2008; Zbogar et al. 2008; Griffin et al. 2009; Kahn et al. 2010) studies that examined the effectiveness of FES leg cycle ergometry on indices of cardiovascular fitness and/or health in SCI. We also identified 7 pre-post (Pollack et al. 1989; Krauss et al. 1993; Mutton et al. 1997; Gurney et al. 1998; Thijssen et al. 2005; Thijssen et al. 2006; Brurok et al. 2011), 1 RCT (Bakkum et al. 2015) and 1 cross-sectional (Bakuum et al. 2014) investigations that examined hybrid FES (combined leg and arm) on cardiovascular fitness in SCI.

There was one further prospective cohort study (Carty et al. 2012), and 11 pre-post (Jacobs et al. 1997; Nash et al. 1997; Solomonow et al. 1997; Wheeler et al. 2002; de Groot et al. 2005; Sabatier et al. 2006; Stoner et al. 2007; Berry et al. 2008; Jeon et al. 2010; Taylor et al. 2011; Ryan et al. 2013) investigations that examined the effects of other electrically assisted training programs on cardiovascular fitness and/or health.

Discussion

There is a growing body of literature indicating that FES exercise training is an effective way of improving cardiovascular health, peak power output, and exercise tolerance/capacity in persons with SCI (Table 6). These studies generally employ a cycling motion, although rowing and bipedal ambulation have also been evaluated. It appears that moderate-to-vigorous intensity FES training (relative to baseline capacity) may be effective in enhancing cardiovascular fitness in persons with SCI. The majority of the investigations are pre-post designs (level 4) with investigators reporting marked changes in VO₂max or VO₂peak after FES training. Similar to aerobic training, 20–40% changes in aerobic capacity are often observed after FES training. However, improvements in excess of 70% are not uncommon (Faghri et al. 1992). Hakansson et al. (2012) tested new electrical stimulation timing patterns (Stim3, designed using a forward dynamic simulation to minimize the muscle stress-time integral) to determine whether SCI participants could increase work and metabolic responses when pedalling a commercial FES ergometer, and found that subjects performed 11% more work pedalling with Stim3 than with existing stimulation patterns.

Investigations with FES training have also shown an improvement in musculoskeletal fitness. Similar to arm exercise training, limited investigations have shown an improvement in cardiac function after FES training. An investigation has also revealed that the degree of muscular adaptation that can be achieved via FES exercise is dependent upon the load that is applied to the paralyzed muscle (Crameri et al. 2004).

Researchers have also shown that hybrid exercise training (FES leg cycling combined with arm ergometry) may elicit greater changes in peak work rates and VO₂peak/VO₂max than FES leg- cycling exercise alone (Krauss et al. 1993; Mutton et al. 1997). Moreover, it appears that the physiological adaptations to combined FES leg cycling and arm ergometry training are partially maintained after eight weeks of detraining (Gurney et al. 1998). Other interventions (Table 8) that make use of hybrid FES training have also been shown to improve the exercise capacity and cardiovascular health of persons with SCI. It would appear that the potential adaptations with hybrid exercise may be greater than FES alone; however, further research is required to test this hypothesis.

A series of intrinsic muscle adaptations can also occur after FES training that enhance the ability for oxidative metabolism at the cellular level, which in turn facilitate improved endurance, exercise tolerance and functional capacity. Key intrinsic muscle adaptations that have been observed include an increase in the proportion of type 1 fibres, an enhancement in cross-sectional fibre area, an increase in capillary-to-fibre ratio, a shift towards more fatigue resistant contractile proteins, and an increase in citrate synthase activity (an enzyme important for metabolism). Given the importance of musculoskeletal fitness for health and functional status (Warburton et al. 2001a,b; Warburton et al. 2006; Warburton et al. 2010), further research is clearly warranted with persons with SCI. Accordingly, randomized, controlled exercise interventions (both arm and/or FES training) that evaluate concurrent changes in musculoskeletal fitness and health status are particularly needed.

Conclusion

There is level 4 evidence (from multiple pre-post studies: Berry et al. 2012; Griffin et al. 2009; Zbogar et al. 2008; Crameri et al. 2004; Hjeltnes et al. 1997; Mohr et al. 1997; Barstow et al. 1996; Faghri et al. 1992; Hooker et al. 1992) that FES training performed for a minimum of three days per week for two months may be effective for improving musculoskeletal fitness, the oxidative potential of muscle, exercise tolerance, and cardiovascular fitness.

There is level 2 evidence (Fornusek et al. 2014) that there is no difference in cardiorespiratory responses or peak values between ES leg isometric exercise compared to FES leg cycling.

There is level 4 evidence (from multiple pre-post studies: Hopman et al. 2002; Gerrits et al. 2001; Ragnarsson et al. 1988) that FES training may be effective in improving exercise cardiac function in persons with SCI.

There is level 1b evidence (Bakkum et al. 2015) that there there is no difference in metabolic components between the hybrid cycle group and hand cycle group. Both groups experienced beneficial effects on metabolic syndrome components, inflammatory status and visceral adiposity. Conversely, there is level 5 evidence (Bakkum et al. 2014) that metabolic rate, heart rate, and ventilation levels are higher during hybrid cycling than during hand cycling.

There is level 4 evidence (Taylor et al. 2011) that arm-cranking exercise assisted by FES increases peak power output, and may increase oxygen uptake.

There is level 4 evidence (Kahn et al. 2010) that FES leg cycle ergometry decreases platelet aggregation and blood coagulation in persons with SCI.

There is level 4 evidence (Hakansson et al. 2012) that the use of patterns that minimize the muscle stress-time integral can prolong FES pedaling.