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Functional Electrical Stimulation (FES) in Spinal Cord Injured patients is a technique that is used to generate a locomotion pattern in patients with spinal cord injury (SCI), head injury, stroke and other neurological disorders using electrical stimulation of major neuronal pathways innervating limb muscles. It is also used to correct gait in patients with neuromuscular disabilities where those affected are unable to coordinate their muscle movements in a pattern required for stepping like motion and balance.

Background
The neuromuscular response to electrical stimulation is nonlinear and any external perturbation might result in unpredictable responses with varying latencies. Stimulation of the lumbar region of the spinal cord at certain frequencies ranging from 2 Hz to 50Hz in known to activate certain locomotor neural pathways that activate muscle synergies responsible for cycling stepping like coordinated movements required for walking. There have been several open loop functional electrical stimulation (FES) systems that have been used to improve motor function in SCI patients during their Locomat-assisted rehabilitation training. These systems however fail to use the bodies sensory feedback system that could stabilize/ correct locomotor pattern in response to variations in external perturbation.

Methodology
The human muscle fibers can be thought of as a mixtrue of slow, Type I, fibers capable of sustaining low levels of contractile activity without fatigue and fast, Type II, fibers which are capable of developing large forces but those which fatigue rapidly. The slow, red, fibers contain myoglobin which gives them their characteristic color and provides them with an extra reserve of oxygen for prolonged contraction while fast, white fibers can only be used intermitently. Slow fibers have relatively less ATPase activity and are therefore slower twitching. All muscle fibers are innervated by motoneurons of the same histochemical type. The motoneurons innervating slow muscle fibers are known to discharge at a low frequency of 10 to 20 Hz and those that innervate fast muscle fbers discharge at a higher frequency of 30 to 60 Hz.

Most SCI candidates for FES aided walking have upper motor neuron lesion which is correlated with a marked increase in the number of white, Type II, fibers in their quadriceps. Also blood flow in th tibial muscle of quadraplegic patients was found to be significantly lower thatn that in the normal biceps of the same patient. Biphasic or monophasic electrical pulses described by an opimal amplitude frequency, duration of a single pulse and duration of a set of pulses called a pulse train are applied to neuromuscular tissue to artificially ellicit muscle contraction. A single pulse usually illicits a short-lived muscle twitch lasting 0.2 sec after which the muscle relaxes. If the frequency of stimulation pulses increases to 10 pulses per secon (10Hz) there will be no time left for the muscle to relac between two consecutinve twitches. At higher stimulation frequencies of 15 to 20Hz this twitching is reduced and the responce is generally smooth, resulting in a tetanic contraction. Increaseing the frequency further between 40 and 100Hz does not seem to affect the responce. However it is know nthat lower stimulation frequencies result in less pronounced fatigue.

Training and Monitoring
FES assisted excercise of limb muscles following SCI have been shown to strengthen them and has been shown to prevent futher muscle attrophy that occurs following SCI. Common muscle training modalities involve isotonic stimulation paradigms. Isometric stimulation where the limb being trained is fixed is less commonly used. If the positon of the extremity can be predetermined by useing a closed loop stimulation system, such types of training systems care classified as isokinetic. Such muscle strenghthening session are often boring to the patient therefore some isokinetic stimulation systems are fitted with bio-feedback systems which provide the patient with a visual or acousic signal that quantifies the patients performance and informs them of area they could improve. Since peripheral nerves have long been known to posses a regenerative property (plasticity), it is predicted that the remaining nerves in the limbs of SCI patients can be re-wired to re-learn locomotion.

Patient Selection
The patients who would benefit from FES systems usually need to meet the following requirements: 1. Lesion level between T-4 TO T-12 2. Upper motor neuron lesion 3. Show positive results in FES restrengthening program 4. No joint contractures like ossification or osteoporosis 5. Satisfactory metal and emotional condition 6. Normal physiological status in regards to heart, lung, circulation and metabolism. 7. Adequate upper extremities and trunk function

Subsequently, patients characteristics that would prevent an SCI patient from benefiting from FES include: 1. Heart, lung, blood circulation or impaired metabolism problems. 2. Inadequate sitting balance 3. Hypersensitivity to electrical currents and lesion in motor neuron of leg muscles 4. Inadequate mental or cognitive function or obesity.

FES- assisted standing
Once the patient has under-gone initial screening and found to be physically capable of benefiting from FES treatment the patient is positioned in the upright position in a standing frame. The patient will be monitored for dizziness, high-blood pressure, cerebral ischaemia and room spinning. Next the patient is introduced to the FES stimulator and muscle strengthening stimulation routines are applied for 1 to 2 weeks. Once the patient can apply adequate initial force with the quadriceps (30 to 50Nm of knee joint torque) proper standing FES stimulation is applied. Because the quadriceps can fatigue rapidly, the initial standing-up trials must not last more than 1 to 2 min. Such standing training must be performed twice a day for 10 to 15 min at a time. After 1 to 2 weeks if the patient is able to execute prolonged periods of standing, the patient can try to independently initiate the standing maneuver using a walker or standing frame.

FES assisted walking in Incomplete and Complete SCI Patients
The patient must have met the requirement for standing and successfully completed the standing training before he/she can qualify for FES assisted walking. Almost all currently used FES systems use open loop control. These control principles can be continuous, discreet or logical algorithms. The open-loop and closed-loop control systems are discussed in greater detail below. The electrical stimulus can be be delivered to the peripheral nervous system as a motoric- efferent or a sensory- afferent. Depending on the severity of incomplete SCI, some scientists have proposed placing electrodes directly in the the CNS or closer to the spinal root nerves exiting the vertebral column to directly stimulate the motor neuron.

Gait training in SCI patients
Gait training can be started once the patient is able to stand independently and safely for at least 3 to 5 minutes. Some of the factors that might exclude the patient from gait training include osteoporosis, skeletal system instability and metabolic disabilities resulting in impaired hart or lung function. Spine and bone integrity must be checked before training can be started. A multichannel, with a minimum of 4 channel, stimulation system is used to differentially and temporally stimulate different muscle sites. A supporting crutch or walker is used along with the FES system to support the patients weight. Most currently available gait training systems provide open loop control or execute a logical algorithm that correct the patient gait. Most of the time the patient's body weight is supported by the gait training device and patients can stabilize themselves using parallel support bars that run along the length of the treadmill apparatus below. Systems like the Lokomat by Hocoma can be controlled by the physical therapist to gradually apply sequentially less correctional force as the patient begins to regain strength in his/her extremities and relearns proper ambulation technique. Initial FES assisted gait training can be jerky and body weight transfer is not fluent and walkers seem to be the preferred training device among most users due to its better sense of stability.

Open-Loop FES systems
Despite the technological progress and persistent research on FES systems, over the past 15 years, most commercial FES systems for restoring standing and walking in paralyzed individuals still use 'open-loop' control systems. The stimulator controller does not receive any direct feedback about the actual state of the system i.e the position of the limb. The neuromuscular stimulator is triggered and adjusted, by the user, using buttons mounted on the crutch or walking frame. The main control buttons on the Parastep system (by Sigmedics Inc.) include ones for standing up, stepping forward, sitting down and increasing or decreasing stimulation intensity. Such systems have been shown to provide an average ambulation of 6-9m with some user ambulating up to 800 m following 20 training sessions. A drawback of this system is that the user has to frequently monitor and press buttons while while simultaneously trying to partially support the persons body weight on the crutch.

Proportional-Integral-Derivative
The Proportional-Integral-Derivative (PID) is a commonly used linear control algorithm due to its simplicity, reliability and its well established tuning methods. However PID controller can only be used to control linear time-invariant systems and does not react well to external disturbances that change the system. A more dynamic version of the PID controller could improve FES performance.

Gain scheduling
The Gain scheduling control involves linearizing the system at a series of operating points representing the operating region. Then a local linear controller is designed for each linearized segment. The scheduling variable is one that can model the non-linearity of the system and change slowly with respect to system dynamics. The scheduling variable is also used to interpolate between local linear models at fixed operating points. This controller works well when operating within the operating region but shows reduced accuracy when outside the operating span. A common scheduling variables chosen in FES walking systems is the knee angle and muscle lengths. However the such variables cannot capture the non-linearities introduced by muscle fatigue, fitness and spasticity. Therefore choosing an ideal scheduling variable and and tuning the controller across an operating region can be lengthy and expensive process.

Sliding mode control
The sliding mode control system involves defining a control rule that causes the state of the system to converge toward a chosen sliding manifold in finite time and evolve along the sliding manifold toward a goal state possibly in infinite time. Such a control algorithm provides good tracking, requires few parameters to be tunes and is adaptive to system dynamics. If an accurate system model is used, the sliding mode system also guarantees a stability. In light of all the aforementioned advantages, the sliding model is note designed to work with time-varying systems. Its performance falls when muscle fatigue is introduced also this control system is susceptible to 'chattering' - rapid back and forth switching between different control actions resulting in controller failure.

Currently available FES systems
Some of the commercially available FES cycling systems for SCI patients include:

+ The RT300 by Restorative Therapies, Inc., Baltimore, MD. + The Ergys 2 by Therapeutic Alliances, Inc., Fairborn, OH. + The NESS H200 FES grasping system by Bioness, Inc., Santa Clarita, CA. + The ODFS Pace FES to correct foot drop by Odstock Medical, Ltd., Wiltshire, U.K. + The WalkAide to correct foot drop by Innovative Neurotronics, Inc., Austin, TX + The L300 Bioness, Inc.