Functional Electrical Stimulation (FES) for Standing Balance
The idea of compensating for paralyzed function using electrical stimulation was introduced as early as the 1960s (Liberson et al. 1961). FES of the common peroneal nerve was found to be effective in assisting foot clearance during the swing phase (Liberson et al. 1961). There has also been a report of attempts to stimulate the ankle plantarflexor muscles to assist push-off at the end of stance and enhance the initiation of the swing phase in participants with incomplete SCI (Bajd et al. 1999). Approaches that focus on swing phase activity are more suitable for less severely disabled persons who have adequate balance to support their stance leg during gait. There are also more complex systems that involve several channels of stimulation that support proper extension as well as foot clearance during swing (e.g., Sigmedics 2000). These are more suitable for patients who require assistance in standing as well as gait, such as those with neurologically complete SCI. FES systems such as the Parastep or ALT-2 provide stimulation of thigh extensor muscles (quadriceps, gluteal muscles) to support extension and standing, as well as stimulation of the common peroneal nerve to assist with swing phase movements. FES may also be combined with bracing to counter trunk and hip instability (Solomonow et al. 1997). FES to assist with foot clearance during swing (drop-foot) has been studied more extensively in the stroke population (Bosch et al. 2014) and may provide some evidence for people with incomplete SCI who present with hemiparesis similar to stroke.
One of the limitations of surface FES is possible skin irritation, discomfort under the electrodes, or difficulties with proper positioning of the electrodes. With improvements in electronic technology, FES systems have become smaller and more practical for everyday use. In addition, some patients have opted for implanted FES systems that may be inserted without surgery. These systems offer a more precise delivery of stimulation, enabling greater muscle selectivity and the ability to access deeper muscles, such as the hip flexors (Kobetic et al. 1997). Percutaneous electrodes, which are inserted through the skin with a hypodermic needle, offer one possibility to circumvent complications with surface electrodes (Kobetic et al. 1997; Marsolais & Kobetic 1986). However, there may be complications due to infection or irritation at the site of insertion, and electrode movement or breakage (Agarwal et al. 2003). More recently, there was a case study reporting the positive effects with a BION microstimulator in a participant with incomplete tetraplegia with drop-foot (Weber et al. 2004). Thus, preliminary reports of the use of such innovative FES technology are promising, but further study is warranted to determine the long-term efficacy of such implanted systems.
Discussion
To date, there are few studies assessing different training protocols which include some type of FES intervention for balance outcomes in people with chronic SCI (Galea et al. 2018; Houston et al. 2020, 2021; Kapadia et al. 2014; Kuhn et al. 2014).
In an RCT, Galea et al. (2018) assessed 116 patients with chronic SCI who were randomly assigned to receive a multimodal exercise training program (comprising BWSTT, FES cycling and trunk and upper and lower limb exercises) or an upper body exercise program for 12 weeks. The findings of the study do not support the hypothesis that an intensive full-body exercise program might lead to neurological recovery, walking improvement, or falls concern-balance in participants with chronic SCI (Galea et al. 2018). The authors stated that the heterogeneity of their sample, which included participants with a range of injury levels and severity (AIS A and B, n=74; AIS C and D, n=42), may have contributed to these results (Galea et al. 2018). Kapadia et al. (2014) found in participants with chronic and incomplete SCI that FES-assisted BWSTT (intervention group) and an aerobic and resistance training program (control group) significantly improved standing balance (TUG) function after 16 weeks of training. However, these improvements were not statistically different between groups (Kapadia et al. 2014).
Other lower-quality studies were carried out for FES-cycling interventions, such as the cohort study of Kuhn et al. (2014), which assessed 30 participants with complete and incomplete SCI during their rehabilitation in a special acute care unit (median time since injury: 2 months). After 4 weeks of 20-min FES-cycling program for 2 days per week, only the five participants with partial walking ability at the start of the study showed a non-significant improvement in TUG (p=0.5) (Kuhn et al. 2014).
The two pre-posts of Houston et al. (2020) and Houston et al. (2021) included three patients with tetraplegia and two with paraplegia who received three 1h training sessions per week for four weeks, consisting of FES applied bilaterally to the ankle plantarflexors and dorsiflexors while they performed visual feedback balance training (VFBT) exercises. After the intervention, improvements were seen in four of five participants on at least one balance scale (BBS and/or Mini-BESTest), with less impact on balance confidence (ABC scale) (Houston et al. 2020). Additionally, some of the biomechanical parameters (area of maximal CoP excursion) increased for all participants, while there was little effect on quiet stance assessments (Houston et al. 2020). In semi-structured interviews assessed at the end of the intervention, participants perceived that the risk of falling was slightly reduced or unchanged, but they felt that their balance confidence had increased (Houston et al. 2021). The authors highlighted the fact that FES+VFBT was able to elicit improvements in balance ability despite a small training dosage, so this intervention could be a promising intervention for standing balance rehabilitation among people with incomplete SCI, which has to be confirmed in larger and higher-quality studies (Houston et al. 2020).
Conclusions
There is level 1 evidence (from 1 RCT: Galea et al. 2018) that a multimodal exercise training program (comprising BWSTT, FES cycling, and trunk and upper and lower limb exercises) does not provide better balance (Spinal Cord Injury – Falls Concern Scale [SCI-FCS]DCS), neurological or walking improvements than an upper body exercise program in patients with chronic SCI.
There is level 2 evidence (from 1 RCT: Kapadia et al. 2014) that a program consisting of FES-assisted BWSTT compared with another of aerobic and resistance training, for 16 weeks, provides significant, but similar improvements, in standing balance (TUG) function in participants with chronic and incomplete SCI.
There is level 2 evidence (from 1 cohort study: Khun et al. 2014) that two days per week of 20-minute FES-cycling for four weeks in a special acute care unit does not provide significant improvement in standing balance (TUG) in participants with partial walking ability due to acute SCI.
