The idea of compensating for paralyzed function using electrical stimulation was introduced as early as the 1960s (Liberson et al. 1961). Functional electrical stimulation 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 plantar flexor muscles to assist push-off at the end of stance and enhance the initiation of the swing phase in subjects with incomplete SCI (Bajd et al. 1999). Approaches that focus on swing phase activity are more suitable for less severely disabled individuals 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. 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 individuals 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 electronics 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 positive effects with a BION microstimulator in an incomplete tetraplegic subject 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 stability and efficacy of such implanted systems.