## 3 Treatments for Spinal Cord Injury
### Functional Electrical Simulation (FES) Systems  
Functional electrical stimulation is a method of neuroprostheses. FES applies an electrical stimulus to the damaged nerve fibre in the spinal cord, in turn activating the paralysed nerve muscle and restoring the function of the dormant limbs [11]. The application of electrical stimulus, in turn, produces a contraction in the muscle. This contraction, however, is different from a natural physiological contraction in two main ways. The first difference is that in an electrically driven contraction, the action potential generated will be anterograde when travelling to the neuromuscular junction and retrograde when travelling to the anterior horn cell. Secondly, electrical stimulation is unable to process motor neurons from small to large, as is performed in voluntary contraction. Instead, they process them from large to small, as the large diameter of the motor units is easier to activate electrically. This results in a lack of smooth conversion between active and inactive motor units and recruits motor units judging by their size and the proximity to the electrode attached to the spinal cord. In turn, this produces a staggered response in the form of movement in the limb instead of a smooth and gradual movement. The most effective way to use electrical stimulation is to use it in combination with the natural voluntary response so that the electrical stimulation can recruit motor units when the voluntary contraction does not. FES systems have been used in clinical studies to treat patients with cervical spinal cord injury. A trial performed at the Cleveland Functional Electrical Stimulation Centre in 1992 on implanted FES systems was studied in the paper 'Functional Electrical Stimulation and Spinal Cord Injury' by [PERSON] et al., 2014 [12]. This trial was one of the largest clinical studies of neuroprostheses of the upper extremities and was called the Freehand trial. During this trial, a receiver-stimulator that consisted of eight channels was implanted into the cervical spinal cord at the damaged position. The implant worked using the level of elevation in the patient's shoulder to gain control of the opening and closing of the patient's grasp. Following the limitations of only eight channels in this trial, the authors proceeded to produce a second-generation implant called the 'Implantable Stimulator Telemeter Twelve-channel System' (IST-12). This allowed the patients to move both their hands and use myoelectric signals emitted from the wrist to improve grasp. Though this trial was able to increase the movement in the patient's hands with the second-generation implant, it was still not a fully implanted device. This meant that there were still wires that protruded from the patient, affecting their quality of life. The use of myoelectric control algorithms could have had a more major role in this implant to allow for the patient's limbs to be multifunctional [12]. Other treatments with functional electrical stimulation include FES cycling. This is a piece of equipment that aids in the stimulation of motor neurons and muscle fibres that have been damaged using electrical pulses. The electrical stimulation from every single pulse activates all the muscles in the leg, which will be recorded on the FES bicycle. It records the muscle's power output and allows the patients to keep track of which muscles are weaker. Consistently using the FES bicycle 2-3 times a week is proven to improve the motor and sensory neurons in the lower limbs, relative to the ASIA Impairment Scale Score [13]. The frequency the FES cycle uses for effective simulation of muscles is 30 Hz; however, for optimal activation, 50-60 Hz is used for 30 min per session [14]. The electrodes used for FES cycling can be externally attached to the skin as well as be implanted into the body; however, implanting this type of implant can be very costly [15]. Additional limitations to FES cycling may be the cost of the equipment itself [16].