functional electrical stimulation functional electrical stimulation functional electrical stimulation (fes) is a technique that uses low-energy electrical pulses to artificially generate body movements in individuals who have been paralyzed due to injury to the central nervous system. more specifically, fes can be used to generate muscle contraction in otherwise paralyzed limbs to produce functions such as grasping, walking, bladder voiding and standing. this technology was originally used to develop neuroprostheses that were implemented to permanently substitute impaired functions in individuals with spinal cord injury (sci), head injury, stroke and other neurological disorders. in other words, a person would use the device each time he or she wanted to generate a desired function. fes is sometimes also referred to as neuromuscular electrical stimulation (nmes). functional electrical stimulation – schematic representation: illustration of motor neuron stimulation. (a) the cell nucleus is responsible for synthesizing input from dendrites and deciding whether or not to generate signals. following a stroke or spinal cord injury in mahnoor's muscles are impaired because motor neurons no longer receive sufficient input from the central nervous system. (b) a functional electrical stimulation system injects electrical current into the cell. (c) the intact but dormant axon receives the stimulus and propagates an action potential to (d) the neuromuscular junction. (e) the corresponding muscle fibers contract and generate (f) muscle force. (g) a train of negative pulses is produced. (h) depolarization occurs where negative current enters the axon at the "active" electrode indicated. fes technology has been used to deliver therapies to retrain voluntary motor functions such as grasping, reaching and walking. in this embodiment, fes is used as a short-term therapy, the objective of which is restoration of voluntary function and not lifelong dependence on the fes device, hence the name functional electrical stimulation therapy, fes therapy (fet or fest). in other words, the fest is used as a short-term intervention to help the central nervous system of the person to re-learn how to execute impaired functions, instead of making the person dependent on neuroprostheses for the rest of her or his life. initial phase ii clinical trials conducted with fest for reaching and grasping, and walking were carried out at kite, the research arm of the toronto rehabilitation institute. principles neurons are electrically active cells. in neurons, information is coded and transmitted as a series of electrical impulses called action potentials, which represent a brief change in cell electric potential of approximately 80–90 mv. nerve signals are frequency modulated; i.e. the number of action potentials that occur in a unit of time is proportional to the intensity of the transmitted signal. typical action potential frequency is between 4 and 12 hz. an electrical stimulation can artificially elicit this action potential by changing the electric potential across a nerve cell membrane (this also includes the nerve axon) by inducing electrical charge in the immediate vicinity of the outer membrane of the cell. fes devices take advantage of this property to electrically activate nerve cells, which then may go on to activate muscles or other nerves. however, special care must be taken in designing safe fes devices, as electric current through tissue can lead to adverse effects such as decrease in excitability or cell death. this may be due to thermal damage, electroporation of the cell membrane, toxic products from electrochemical reactions at the electrode surface, or over-excitation of the targeted neurons or muscles. typically fes is concerned with stimulation of neurons and nerves. in some applications, fes can be used to directly stimulate muscles, if their peripheral nerves have been severed or damaged (i.e., denervated muscles). however, the majority of the fes systems used today stimulate the nerves or the points where the junction occurs between the nerve and the muscle. the stimulated nerve bundle includes motor nerves (efferent nerves—descending nerves from the central nervous system to muscles) and sensory nerves (afferent nerves—ascending nerves from sensory organs to the central nervous system). the electrical charge can stimulate both motor and sensory nerves. in some applications, the nerves are stimulated to generate localized muscle activity, i.e., the stimulation is aimed at generating direct muscle contraction. in other applications, stimulation is used to activate simple or complex reflexes. in other words, the afferent nerves are stimulated to evoke a reflex, which is typically expressed as a coordinated contraction of one or more muscles in response to the sensory nerve stimulation. when a nerve is stimulated, i.e., when sufficient electrical charge is provided to a nerve cell, a localized depolarization of the cell wall occurs resulting in an action potential that propagates toward both ends of the axon. typically, one "wave" of action potentials will propagate along the axon towards the muscle (orthodromic propagation) and concurrently, as the other "wave" of action potentials will propagate towards the cell body in the central nervous system (antidromic propagation). while the direction of propagation in case of the antidromic stimulation and the sensory nerve stimulation is the same, i.e., towards the central nervous system, their end effects are very different. the antidromic stimulus has been considered an irrelevant side effect of fes. however, in recent years a hypothesis has been presented suggesting the potential role of the antidromic stimulation in neurorehabilitation. typically, fes is concerned with orthodromic stimulation and uses it to generate coordinated muscle contractions. in the case where sensory nerves are stimulated, the reflex arcs are triggered by the stimulation on sensory nerve axons at specific peripheral sites. one example of such a reflex is the flexor withdrawal reflex. the flexor withdrawal reflex occurs naturally when a sudden, painful sensation is applied to the sole of the foot. it results in flexion of the hip, knee and ankle of the affected leg, and extension of the contralateral leg in order to get the foot away from the painful stimulus as quickly as possible. the sensory nerve stimulation can be used to generate desired motor tasks, such as evoking flexor withdrawal reflex to facilitate walking in individuals following stroke, or they can be used to alter reflexes or the function of the central nervous system. in the later case, the electrical stimulation is commonly described by the term neuromodulation. nerves can be stimulated using either surface (transcutaneous) or subcutaneous (percutaneous or implanted) electrodes. the surface electrodes are placed on the skin surface above the nerve or muscle that needs to be "activated". they are noninvasive, easy to apply, and generally inexpensive. until recently the common belief in the fes field has been that due to the electrode-skin contact impedance, skin and tissue impedance, and current dispersion during stimulation, much higher-intensity pulses are required to stimulate nerves using surface stimulation electrodes as compared to the subcutaneous electrodes. (this statement is correct for all commercially available stimulators except myndmove stimulator (developed my milos r. popovic), which has implemented a new stimulation pulse that allows the stimulator to generate muscle contractions without causing discomfort during stimulation, which is a common problem with commercially available transcutaneous electrical stimulation systems, based on us patents 8,880,178 (2014), 9,440,077 (2016), and 9,592,380 (2016) and related foreign patents.) a major limitation of the transcutaneous electrical stimulation is that some nerves, for example those innervating the hip flexors, are too profound to be stimulated using surface electrodes. this limitation can be partly addressed by using arrays of electrodes, which can use several electrical contacts to increase selectivity. subcutaneous electrodes can be divided into percutaneous and implanted electrodes. the percutaneous electrodes consist of thin wires inserted through the skin and into muscular tissue close to the targeted nerve. these electrodes typically remain in place for a short period of time and are only considered for short-term fes interventions. however, it is worth mentioning that some groups, such as cleveland fes center, have been able to safely use percutaneous electrodes with individual patients for months and years at a time. one of the drawbacks of using the percutaneous electrodes is that they are prone to infection and special care has to be taken to prevent such events. the other class of subcutaneous electrodes is implanted electrodes. these are permanently implanted in the consumer's body and remain in the body for the remainder of the consumer's life. compared to surface stimulation electrodes, implanted and percutaneous electrodes potentially have higher stimulation selectivity, which is a desired characteristics of fes systems. to achieve higher selectivity while applying lower stimulation amplitudes, it is recommended that both cathode and anode are in the vicinity of the nerve that is stimulated. the drawbacks of the implanted electrodes are they require an invasive surgical procedure to install, and, as is the case with every surgical intervention, there exists a possibility of infection following implantation. typical stimulation protocols used in clinical fes involves trains of electric pulses. biphasic, charged balanced pulses are employed as they improve the safety of electrical stimulation and minimize some of the adverse effects. pulse duration, pulse amplitude and pulse frequency are the key parameters that are regulated by the fes devices. the fes devices can be current or voltage regulated. current regulated fes systems always deliver the same charge to the tissue regardless of the skin/tissue resistance. because of that, the current regulated fes systems do not require frequent adjustments of the stimulation intensity. the voltage regulated devices may require more frequent adjustments of the stimulation intensity as the charge that they deliver changes as the skin/tissue resistance changes. the properties of the stimulation pulse trains and how many channels are used during stimulation define how complex and sophisticated fes-induced function is. the system can be as simple such as fes systems for muscle strengthening or they can be complex such as fes systems used to deliver simultaneous reaching and grasping, or bipedal locomotion. note: this paragraph was developed in part using material from the following reference. for more information on fes please consult that and other references provided in the paragraph. history electrical stimulation had been utilized as far back as ancient egypt, when it was believed that placing torpedo fish in a pool of water with a human was therapeutic. fes - which involves stimulating the target organ during a functional movement (e.g., walking, reaching for an item) - was initially referred to as functional electrotherapy by liberson. it was not until 1967 that the term functional electrical stimulation was coined by moe and post, and used in a patent entitled, "electrical stimulation of muscle deprived of nervous control with a view of providing muscular contraction and producing a functionally useful moment". offner's patent described a system used to treat foot drop. the first commercially available fes devices treated foot drop by stimulating the peroneal nerve during gait. in this case, a switch, located in the heel end of a user's shoe, would activate a stimulator worn by the user. common applications spinal cord injury injuries to the spinal cord interfere with electrical signals between the brain and the muscles, resulting in paralysis below the level of injury. restoration of limb function as well as regulation of organ function are the main application of fes, although fes is also used for treatment of pain, pressure, sore prevention, etc. some examples of fes applications involve the use of neuroprostheses that allow the people with paraplegia to walk, stand, restore hand grasp function in people with quadriplegia, or restore bowel and bladder function. high intensity fes of the quadriceps muscles allows patients with complete lower motor neuron lesion to increase their muscle mass, muscle fiber diameter, improve ultrastructural organization of contractile material, increase of force output during electrical stimulation and perform fes assisted stand-up exercises. walking in spinal cord injury kralj and his colleagues described a technique for paraplegic gait using surface stimulation, which remains the most popular method in use today. electrodes are placed over the quadriceps muscles and peroneal nerves bilaterally. the user controls the neuroprosthesis with two pushbuttons attached to the left and right handles of a walking frame, or on canes or crutches. when the neuroprosthesis is turned on, both quadriceps muscles are stimulated to provide a standing posture. electrodes are placed over the quadriceps muscles and peroneal nerves bilaterally. the user controls the neuroprosthesis with two pushbuttons attached to the left and right handles of a walking frame, or on canes or crutches. when the neuroprosthesis is turned on, both quadriceps muscles