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.M.R. Popovic, K. Masani and S. Micera, "Chapter 9 – Functional Electrical Stimulation Therapy: Recovery of function following spinal cord injury and stroke," In press, Neurorehabilitation Technology – Second Edition, Z. Rymer, T. Nef and V. Dietz, Ed. Springer Science Publishers in November 2015. FES is sometimes also referred to as neuromuscular electrical stimulation (NMES).M. Claudia et al., (2000), Artificial Grasping System for the Paralyzed Hand, International Society for Artificial Organs, Vol. 24 No. 3
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 an individual's central nervous system re-learn how to execute impaired functions, instead of making them dependent on neuroprostheses for the rest of their life.M.K. Nagai, C. Marquez-Chin, and M.R. Popovic, "Why is functional electrical stimulation therapy capable of restoring motor function following severe injury to the central nervous system?" Translational Neuroscience, Mark Tuszynski, Ed. Springer Science and Business Media LLC, pp: 479-498, 2016. 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.Guyton and Hall Textbook of Medical Physiology, John Hall, 13th edition, Elsevier Health Sciences, 31 May 2015 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.M.R. Popovic and T.A. Thrasher, "Neuroprostheses", in Encyclopedia of Biomaterials and Biomedical Engineering, G.E. Wnek and G.L. Bowlin, Eds.: Marcel Dekker, Inc., vol. 2, pp. 1056–1065, 2004.
FES devices take advantage of this property to electrically activate nerve cells, which then may go on to activate muscles or other nerves.Control of Movement for the Physically Disabled: Control for Rehabilitation Technology, Dejan Popovic and Thomas Sinkjaer, Springer Science & Business Media, 6 December 2012. 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