Patent Publication Number: US-2001000187-A1

Title: Functional neuromuscular stimulation system

Description:
BACKGROUND OF THE INVENTION  
       1. The present invention relates to the art of functional neuromuscular stimulation. It finds particular application in providing hand control functions in central nervous system (CNS) disabilities such as quadraplegia and stroke victims and will be described with particular reference thereto. However, it is to be appreciated that the invention is also applicable to providing locomotive and control of other lower body functions in CNS disabled victims and to providing control of other muscles over which the patient has lost partial or full voluntary control.  
       2. In healthy humans, electrical signals originate in the brain and travel through the spinal cord and subsequently to peripheral nerves to a muscle which is to be contracted. More accurately, the signals travel to two or more muscles whose contractions apply forces antagonistically to a joint structure. The relative forces determine the degree and speed of movement. By appropriately applying the electrical stimulation to various muscles, a wide degree of voluntary movement can be achieved. In injuries to the CNS, the passage of electrical signals through the injured area may be disrupted. Commonly, lower spinal cord injuries will terminate the transmission of electrical control signals to muscles in the lower part of the body. Damage to the upper part of the spinal cord may block the flow of voluntary muscular control signals to upper and lower body regions. For example, in an upper spinal column injury at the C6 vertebrae, which is frequently injured in accident victims, muscular control below the elbows is commonly lost.  
       3. As early as 1791, Luigi Galvani produced artifical contractions in the muscle of frogs&#39; legs by the application of electrical potentials. In the ensuing years, electrical stimulation therapy has been greatly refined. Cardiac pacemakers, for example, have become commonplace.  
       4. Several different groups of researchers have enabled paraplegic patients to stand and walk with walkers or crutches by applying preselected sequences to surface electrodes over their leg muscles. Surface stimulation is satisfactory for some walking and other less detailed movements. However, with surface electrodes, it is difficult to make an accurate selection of the muscle to be stimulated or an accurate prediction of the strength of the stimulus signal reaching the muscle.  
       5. Surgically implanted electrodes provide accurate selection of the muscle to be stimulated. Further, the stimulation remains more consistent over a long period of time. This renders implanted electrodes advantageous for the more delicate and complex motion associated with the hands.  
       6. Numerous experimental systems have been devised and implemented to provide computer controlled electrical stimulation to the muscles of the legs, arms, and hands of patients. These experimental systems are commonly large and bulky. Frequently, the patient must be connected with a personal computer or other small computer by a cable or tether. Although smaller, dedicated computer systems could be designed, the larger programmable computer systems are generally preferred for experimental flexibility. The response to a given stimulus varies widely among patients and over time within each patient. The larger programmable computer facilitates customizing for different patients and changes in a given patient.  
       7. The present invention provides a new and improved functional neuromuscular stimulation system which increases patient independence and performance.  
       SUMMARY OF THE INVENTION  
       8. In accordance with one aspect of the present invention, a functional neuromuscular stimulation system is provided. An input command means provides a command control signal which is indicative of a selected physiological movement or group of movements. A first parameter processing means derives the parameters of a first stimulus pulse train from the control signal. The first parameter processing means includes an amplitude means for selecting an amplitude of each stimulation pulse of the pulse train, an interval means selects an interpulse interval between pulses of the pulse train and a pulse width means selects a pulse width for each pulse of the pulse train, each in accordance with the control signal. A pulse train generator generates a pulse train with the selected amplitude, interpulse interval, and pulse width. An electrode is connected with the pulse train generator for applying the pulse train to a muscle to be stimulated.  
       9. In accordance with a more limited aspect of the present invention, a plurality of similar parameter processing means are provided for uniquely deriving additional stimulation of pulse trains from the control signal(s) for application to additional electrodes implanted at other locations in the same or other muscles.  
       10. In accordance with another more limited aspect of the present invention, a physiological parameter monitor is provided for monitoring a preselected parameter of physiological movement, such as position or force. A parameter comparing means compares the monitored parameter with a parameter value retrieved from a preprogrammed look-up table. Any difference between the monitored and retrieved parameters is determined. At least one of the amplitude, interpulse interval, and the pulse width of the stimulus pulse train are adjusted such that the difference is minimized.  
       11. In accordance with another aspect of the present invention, a Hall-effect command control signal generator is provided. A permanent magnet is mounted in a ball member, such as in an externally worn device or surgically implanted, e.g. in the clavical of the patient. A first pair of Hall-effect plates are mounted in a socket member, such as external device or the sternum of the patient to define an axis. At least one additional Hall-effect plate is mounted in the socket member to define a second axis. A power supply provides a current flow in one direction across each of the Hall-effect plates. A potential difference monitoring means monitors the potential difference generally transverse to the first direction across each Hall-effect plate to provide an output signal indicative of the change of potential thereacross. In this manner, as the permanent magnet moves relative to the Hall-effect plates, the change in their relative proximity causing corresponding changes in the magnetic flux density across each plate which causes corresponding changes in the path of current flow along said one direction, hence the potential difference across the Hall-effect plates. In this manner, the output signals from the potential difference monitoring means are indicative of the angular position of the ball and socket member relative to the first and second axes.  
       12. In accordance with another aspect of the invention, a joystick includes a ferrite core mounted in a ball member. The ball member is rotatably mounted in a socket member. A driving coil is connected with the socket member encircling at least a portion of the ferrite core. A plurality of sensing coils are mounted to the socket member adjacent the ferrite core such that the transfer of an input signal from the driving coil to each of the sensing coils is controlled by the relative proximity between the ferrite core and the sensing coils.  
       13. In accordance with another aspect of the invention an implanted telemetry system is provided. An antenna receives a radio frequency signal which is converted into electromotive power by a power supply. An encoding means encodes an electrical signal which controls a gate means. The gate means selectively connects a load across the antenna to modulate a characteristic thereof such that a monitorable characteristic of the radio frequency signal is also modulated by the load.  
       14. In accordance with another aspect of the present invention, a laboratory system customizes electrical stimulus pulses to the patient. The system includes a command processing means for providing control parameters indicative of selected command functions and degrees of movement. A movement planning means derives movement parameters indicative of preselected movement, force, or other motion related parameters of the controlled limb in response to each control parameter. A coordination and regulation means derives appropriate stimulus parameters from the motion parameters. A stimulus generator assembles an appropriate electrical stimulus pulse train in accordance with the stimulus parameters.  
       15. In accordance with a more limited aspect of the present invention, a comparing means is provided for comparing actual physical motion parameters achieved by the patient&#39;s limb being controlled and the selected motion parameters of the movement planning means. The stimulus parameters selected by the coordination regulation means are automatically adjusted in order to bring the actual and selected motion parameters into optimal coincidence.  
       16. In accordance with another aspect of the present invention, a multichannel implanted stimulator system is provided. The stimulator system includes an antenna for receiving a carrier signal which is modulated with channel, pulse width, and pulse amplitude information for one or more of the channels. A power supply means derives operating voltage for other system components from the carrier signal. A decoding means decodes at least selected channel, pulse width, and pulse amplitude information from the modulations. For each channel, an energy storage means is provided for providing energy for a current pulse from the power supply through the muscle tissue between a stimulating electrode and a reference electrode. A channel selection means selects the appropriate channel and corresponding stimulating electrode to which an electrical pulse of the decoded pulse width is to applied. A current regulating means regulates the amplitude of the pulse in accordance with the decoded amplitude.  
       17. In accordance with another aspect of the invention, the implanted stimulus system includes a metal capsule which defines a hermetically sealed chamber therein. A receiving antenna receives signals indicative of the stimuli to be applied to electrodes. Electrical circuitry is mounted in the capsule for converting received radio frequency signals into stimulus pulses. A plurality of electrical leads are electrically connected with the circuitry and the electrodes and mechanically connected with the capsule.  
       18. In accordance with another aspect of the invention, an electrical lead construction for implanted electrodes is provided. First and second lengths of multi-strand wire are wrapped helically around a longitudinal axis of the lead. A flexible polymeric insulator material encapsulates the helically wound wires.  
       19. In accordance with another aspect of the present invention, a shield assembly is provided for protecting a percutaneous interface. A shield member includes a peripheral lip portion extending peripherally around a central shield member portion. The central shield member portion is constructed of a resilient elastomeric material with a low profile. An aperture is defined through the central shield member for alignment with a point at which electrical wires pass through the patient&#39;s skin. An electrical connector which is operatively connected with the electrical lead wires passing through the patient&#39;s skin is mounted to the shield member central section. An overlay member having an aperture which conforms with the shield member central portion overlays the shield member and is adhesively adhered to the shield member peripheral lip portion and to the patient&#39;s skin around the shield member.  
       20. One advantage of the present invention is that it is readily customized to an individual patient. Moreover, the customization can be altered and refined as the patient becomes more proficient with the apparatus, as the patient&#39;s muscles become stronger, and the like.  
       21. Another advantage of the present invention resides in its portability.  
       22. Yet, another advantage of the present invention resides in the ease with which operators can adapt it to an individual patient.  
       23. Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     24. The invention may take form in various steps and arrangements of steps and in various parts and arrangements of parts. The drawings are only for purposes of illustrating a preferred embodiment of the invention and are not to be construed as limiting it.  
     25.FIG. 1 is a diagrammatic illustration of the present invention in combination with a user;  
     26.FIG. 2 is a block diagram of a functional neuromuscular stimulation system in accordance with the present invention;  
     27.FIG. 3 is a side sectional view of a Hall-effect joystick in accordance with the present invention;  
     28.FIG. 4 is a view of the socket of FIG. 2 through section  4 — 4 ;  
     29.FIG. 5 is a circuit diagram of the Hall-effect joystick and a transmitter for transmitting joystick position information to the patient exterior;  
     30.FIG. 6 is a block diagram of the interaction between a patient-carried unit and a laboratory system;  
     31.FIG. 7 is a hardware diagram of a patient-carried microprocessor base control unit of the stimulator system of FIG. 1;  
     32.FIG. 8 is a diagrammatic illustration of an exemplary muscle stimulation electrical pulse sequence in accordance with the present invention;  
     33.FIG. 9 is a further block diagram of the patient-carried unit of FIG. 7;  
     34.FIG. 10 is diagrammatic illustration of thumb and finger extension, flextion, and force as a function a proportional command signal during a stimulated thumb and forefinger gripping motion;  
     35.FIG. 11 is a diagrammatic illustration of the data handling stages of the laboratory system;  
     36.FIG. 12 is a hardware configuration of a laboratory system which interfaces with the patient-carried stimulator;  
     37.FIGS. 13,  14 , and  15  are diagrams of data processing in the laboratory system of FIG. 12;  
     38.FIG. 16 is a diagrammatic illustration of an implanted stimulator for stimulating implanted electrodes;  
     39.FIG. 17 is a detailed diagram of the power supply of the implanted stimulator;  
     40.FIG. 18 is a detailed illustration of the circuitry for applying electrical pulses through muscle tissue between a stimulus and a reference electrode;  
     41.FIG. 19 is a side sectional view of the implanted stimulator illustrating the mechanical encapsulation thereof;  
     42.FIG. 20 illustrates electrode lead wire construction;  
     43.FIG. 21 is an expanded view of a lead wire connector;  
     44.FIG. 22 is a side sectional view of an alternate embodiment of a joystick in accordance with the present invention;  
     45.FIG. 23 is a sectional view of the joystick of FIG. 22 taken through section  23 — 23 ; and,  
     46.FIG. 24 is an expanded, perspective view of a percutaneous interface.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     47. With reference to FIGS.  1  and  2 , an input command control means A produces electrical command control signals for controlling the limb or muscles in question. The input commands are derived from remaining voluntary functions of the patient, e.g. shoulder. In the preferred embodiment, the input control means provides both function selection or logic signals and proportional signals in response to shoulder movement of the patient. The function selection signal selects the motor function to be performed, such as turning the system on or off, freezing the stimulus parameters applied to the patient&#39;s hand, selecting among a preselected group of gripping or other hand motions, and the like. The proportional signal indicates a selected degree of the physical movement or force. In this manner, the patient can accurately control the progress of the selected movement, the position of the hand, arm or other limb, the strength of a grip, and the like.  
     48. A portable, patient-carried control system or means B receives the function selection and proportional signals from the input command means A. From the received signals, the portable control system selects the appropriate electrodes to receive electrical stimulation and the appropriate electrical stimulation signals for each electrode. More specifically, the portable control means B selects the pulse width, interpulse interval, amplitude, or other characteristics of an electrical stimulation signal or pulse train in accordance with the proportional signal. The portable system selects appropriate electrodes, algorithms or conversion factors between the proportional signal and stimulation signal parameters, internal control functions, and the like in response to the received function selection signals.  
     49. A central or laboratory reprogramming means or system C selectively reprograms the portable system B. The reprogramming adjusts the relationship between the proportional signal and the electrical stimulus signals, alters the internal control functions, and otherwise customizes the portable system to the patient. As the patient&#39;s muscle tone and strength improve with the continued use, the operating parameters of the portable control system B are reprogrammed and refined. Further, the central means C analyzes the performance of the portable system for potential failures or defects, accumulates and analyses historical data, provides physical therapy instructions, derives data of therapeutic value to the operator, provides training routines, and the like.  
     50. In the preferred embodiment, the portable control means B only selects the appropriate electrical stimulation pulse train parameters. A first implanted stimulator means D under the control of the portable system B applies the electrical signals to first implanted electrodes E to control a first body function, such as hand movement. A second implanted stimulator D′ selectively receives control signals from the portable system B and applies electrical signals to one or more second implanted electrodes E′ to control a second body function, such as bladder control. Additional implants may also be provided. Preferably, each implant has an interrogatable identification which is interrogated by the portable unit B. The portable unit correlates the transmitting channels with the corresponding implanted unit. This self correcting feature saves the patient the inconvenience of matching a dedicated transmitting antenna with a specific implant. Alternately, the portable system may be connected directly with the electrodes E through a percutaneous interface.  
     51. With continuing reference to FIG. 1 and particular reference to FIGS.  3  and  4 , one of the preferred embodiments of the input command control means A is provided. The input command control means is mounted to be controlled by the shoulder of the patient opposite to the hand which is to be controlled. To control the right hand, the input control is mounted for movement by the left shoulder. A permanent magnet  10  is imbedded in a ball member, preferably surgically implanted in the clavical  12  of the patient. In a matching socket joint  14 , preferably in the sternum, four Hall-effect transducer plates  16 ,  18 ,  20 , and  22  are surgically mounted. Hall plates  16  and  20  are mounted along a first axis which is orthogonal to a second axis along which Hall elements  18  and  22  are mounted. Preferably, the Hall elements are mounted in coordination with the axes along which the patient has the greatest, most controllable shoulder motion. It is to be appreciated that other numbers of plates may be used. For example, three plates can define two axes. Even two plates can define the relative position of the ball member and socket, but with an ambiguity. In some applications, proper placement of the plates and signal processing circuitry may be able to resolve the ambiguity adequately.  
     52. With reference to FIG. 5, each Hall element is a conductive plate across which a current is induced flowing from a power regulator  24  to ground. In accordance with the Hall-effect, the current passing through the conductive plate is deviated toward the side of the plate in the presence of magnetic flux from the permanent magnetic  10 . This deviation causes a change in the potential difference between the sides of the plate which is proportional to the magnetic flux density through the element which, in turn, varies with the proximity of the permanent magnetic  10  thereto. Differential amplifiers  26 ,  28 ,  30 , and  32  are each associated with one of the Hall-effect plates to measure the potential change thereacross.  
     53. A first axis differential amplifier  34  differentially combines the output of differential amplifiers  26  and  30  to produce a first axis analog signal which is proportional to motion of the permanent magnetic relative to the first axis. A second axis differential amplifier  36  is connected with the Hall elements to provide a second axis analog output signal which varies in proportion to the position of the permanent magnetic along the second axis.  
     54. In the preferred embodiment, the output signal from the Hall elements along the axis along which the patient has the greatest range of movement provides the proportional signal and the output signal from along the other axis provides the function selection or logic signal. In the preferred embodiment, the function is changed in response to the function selection signal making a sudden change in amplitude of at least a preselected duration.  
     55. Optionally, an accelerometer may be mounted in the patient&#39;s shoulder and the output of the accelerometer may provide the function selection signal. Proportional control signals may then be provided corresponding to two axes. As yet another option, the Hall-effect elements and the permanent magnet may be mounted in a ball and socket joint of man-made construction. The two portions of the man-made ball and socket joint are selectively connected with portions of the shoulder, either externally or implanted.  
     56. With continuing reference to FIG. 5, the power supply  24  is connected with a receiving antenna  40  which is irradiated with a radio frequency signal applied external to the patient by a power transmitting cord  42  of the portable unit B. The received signal, in the preferred embodiment about 10 MHz., induces currents in receiving antenna  40  which are converted to motive power by the power supply  24 .  
     57. A telemetry unit  50  receives the first and second analog outputs of the Hall-effect joystick for transmission to the portable processor B. The telemetry unit includes an encoding means  52  which encodes the first axis signal from amplifier  34  and the second axis signal from amplifier  36  into a preselected digital format. Optionally, analog formats may also be implemented. The ones and zeroes of the encoded digital signal control a gate means  54  which selectively applies a load  56  across the receiving coil  40 . The applied load changes the characteristics in a manner which can be sensed by the power transmitting coil  42  and the portable unit B. Alternately, the encoded signal from the telemetry unit  50  may be transmitted on a carrier frequency for reception by a receiving coil of the portable unit B. As another alternative, direct electrical connection can be utilized, particularly if the joystick is mounted to the patient externally.  
     58. In a typical functional interactive system operation, electrical communication is established between the input command controller A and the portable unit B. In the preferred embodiment, making this connection first prevents the unit from going into the exercise mode. Second, an electrical interconnection is established between the portable unit B and the implanted electrodes E. The system powers up to an idle mode which is a non-stimulating low power consumption state.  
     59. In the preferred embodiment, the patient depresses a switch mounted adjacent to the shoulder position transducer to commence operation. The system goes into a grasp mode selection scan in which feedback cues indicate which grasp mode is indicated. Releasing the chest switch or performing another preselected operation, such as shifting the shoulder vertically, stops the scanning in the desired grasp mode.  
     60. During a short delay, the patient positions his shoulder forward and aft at a desired zero set point. The portable unit sets itself with the selected position as the zero or null set point between plus and minus ranges of motion. It is to be appreciated that if the operator does not select the center of his physical movement range, significantly greater control will be provided in one direction of movement than in the other. After the set point selection delay, the system turns on in a functional mode with the defined set point representing a zero level of command. Shoulder movement from the set point, in turn, proportionally controls the selected grasp.  
     61. Rapid movement along the axis orthogonal to the proportional axis initiates a hold or lock mode which maintains a constant stimulus output independent of shoulder position. To exit the hold mode, another rapid movement allows the user to regain proportional control after realigning the shoulder to the position it was in along the proportional control axis when the hold was initiated. This provides a smooth transition from the hold mode to the proportional control mode without disrupting the grasp. Feedback cues, such as audio tones, indicate the state of operation to the user. To place the system in idle mode, the user depresses and releases the chest mounted switch.  
     62. With particular reference to FIG. 6, the signals from the input command controller A are operated on by control algorithms  60  which operate on the proportional signals with algorithms which select muscles to be stimulated and electrical pulse stimulation characterisitics for each muscle. A formating means  62  formats the stimulation pulse train characterisitics to an appropriate format to be transmitted by radio frequency transmitter  64  to the implanted stimulator D. A portable percutaneous stimulator  66  enables the formated control signals to be applied directly to the electrodes E through a percutaneous interface  68 .  
     63. Referring again to FIG. 1, the patient-carried control B includes a receiver  70  for receiving the function selection and proportional signals, i.e. the first and second axis signals. When the input control is externally mounted or when the interconnection is by way of a percutaneous interface, electrical wires directly connect the patient carried control B and the input control means A. When the input control means is implanted and the patient-carried control system is externally carried, they are interconnected by the telemetric interface  50 .  
     64. With reference to FIG. 7, a programmable gain and offset means  80  selectively adjusts the gain and an offset for an analog proportional signal to bring it into the appropriate range for an analog to digital converter  82 . The function selection signal is conveyed directly to selected channels of the analog to digital converter  82 . With digital received signals, the gain and offset means and the analog to digital converter are eliminated.  
     65. An feedback means  84  provides the patient with feedback regarding the state of the portable unit, e.g. selected grapsing mode, on/off state, locking mode, the position of the null set point, or the like. The feedback means  84  may be a tone generator, an electrocutaneous system such as a shoulder mounted electrodes  84  of FIG. 1, or the like. The electrocutaneous feedback is advantageous in public environments in which the audio tones may prove indiscernable or embarressing to the user. Microprocessors  86  and  88  select electrical stimulation signal parameters in accordance with the input proportional and functional selection signals and in accordance with patient parameters retrieved from memories  90 ,  92 ,  94 , and  96 . The microprocessors  86  and  88  are interconnected by a processor bus to perform distributed processing operations. This enables the microprocessors to share responsibility, handle high level math, accommodate additional microprocessors for more sophisticated control functions. Output lines from the processor bus are connectable with the laboratory system C to provide an operator viewable display showing the command signals, the shape and characteristics of the signals that are being sent to the nerves, and the like. The appropriate stimulation electrical signal is conveyed through an input-output port  98  to an output means  100 . The output means  100  includes a plurality of radio frequency transmitters in the preferred embodiment. In another embodiment, the output means applies the selected stimulus signal directly to the electrodes through a percutaneous interface.  
     66. A cable identification means  102  determines which cables are interconnected with the portable unit. In the preferred embodiment. The portable unit B serves as both an exercise system as well as a functional system. Electrically induced exercise enables the muscle strength and fatigue resistance to be increased. In the exercise mode, the input command controller A is disconnected or disabled. In the preferred embodiment, the exercise mode is selected without external switches by merely disconnecting the cable to the input command controller and completing the connection with the implanted electrodes. The exercise algorithm allows the grasp to be ramped open, closed, and held at particular values. Alternately, the exercise algorithm can cycle between two or more grasping patterns and turn on and off in a preset time cycle. A typical exercise regime, which is applied throughout the night while the patient is sleeping, provides a 50 minute period of alternating grasp modes and a 10 minute rest period.  
     67. A power management means  104  controls the power sources which power the portable unit. In particular, the power management means monitors whether the portable unit is connected with line power, the level of charge in rechargeable batteries, and the presence of servicable batteries. The power management system selects which of the available power sources are to be utilized. If rechargeable batteries and line power are both available, the power management initiates the recharging of the rechargeable batteries. If a power cable should be pulled out or if a battery should run down, the power management system automatically changes to another power supply.  
     68. A watchdog timer  106  monitors for system problems and shuts the portable unit off if a problem arises. In particular, the microprocessors cycle through the program at predictable intervals. The watchdog timer monitors the cycles and if a cycle fails to come in the appropriate period, a software problem is assumed and the system is shut off.  
     69. With reference to FIG. 8, the stimulus pulse train signal which is applied to the electrodes E includes a series of biphasic pulses  110 . Each pulse has a pulse width  112  and an amplitude  114 . The leading edges of adjacent pulses are separated by an interpulse interval  116 . A short interpulse delay after each stimulus pulse, an opposite polarity pulse  118  is applied to the electrodes. The delay prohibits repolarization of the active nerve fibers. The amplitude and duration of the opposite polarity pulse are selected such that the net charge transfer of the reverse polarity pulse is some proportion of the stimulation pulse, usually zero. Zeroing the net charge transfer helps prevent tissue damage with long term usage.  
     70. With reference to FIG. 9, the functional interrelationship of the parts of FIG. 7, particularly the function of the microprocessors and other software are explained in greater detail. A selected motor function decoder  120  determines the selected motor function indicated by the function selection signal and enables one or more of a plurality of electrode stimulation signal parameter selection means or channels  122 . For example, a selected motor function may require the stimulation of a preselected subset of the implanted electrodes. The selection of a freeze or hold function may be implemented by holding or freezing the command signal such that the signals controlling the positions of the patient&#39;s hand or arm remain fixed.  
     71. In the preferred embodiment, each of the stimulation parameter selection means or processing path is the same construction. Specifically, each stimulation parameter selection means includes an amplitude algorithm  124  which selects an appropriate amplitude  114  of the stimulation pulse in accordance with the proportional signal. In the preferred embodiment, the amplitude algorithm means  124  is a 1 byte×256 memory or look-up table. Each of the 256 memory positions are preprogrammed to be retrieved by a corresponding one of 256 processed proportional signal levels. An amplitude index means  126  addresses the corresponding input of the amplitude look-up table.  
     72. An interpulse interval algorithm means  128  including an interval index means  130  provides an appropriate interpulse width for each level of the proportional signal. The interval algorithm means  128  is again preferably a 1 byte×256 memory or look-up table. A pulse width algorithm means  132  including a pulse width index  134  select an appropriate pulse width  112  in correspondence with the proportional signal. The pulse width algorithm is again preferably a 1 byte×256 memory or look-up table. The relationship between the proportional signal and the selected amplitude, interpulse interval, and pulse width vary from patient to patient. Further, these relationships vary as the patient develops increased muscle tone and strength through increased exercise of the stimulated muscles. Accordingly, the values in each of the look-up memories are loaded and readjusted by the central control system C for each patient and fine tuned for each patient periodically.  
     73. When the stimulation system D is implanted, the amplitude, interpulse interval, and pulse width parameters are conveyed to a radio frequency encoder  136  which encodes a radio frequency carrier signal with the selected electrode number, the amplitude, the interpulse interval, and the pulse width information. The transmitter  100  transmits the encoded radio frequency signal to the implanted stimulator system D. In the preferred embodiment, the radio frequency encoding scheme includes both digital and analog encoding. The electrode number is digitally encoded by periodically blanking the radio frequency signal to provide a digital representation of the electrode number to which the current is to be applied. The amplitude is also encoded digitally. In the preferred embodiment, two digital pulse spaces provide an encoding scheme to select one of 32 amplitude levels. The pulse width is encoded with an analog encoding scheme in which the width of an off portion of the RF carrier signal is indicative of the pulse width. The interpulse interval is selected by the frequency or periodicity with which the parameters are transmitted. That is, the interpulse interval is controlled by the frequency with which the RF carrier is encoded. If the stimulation pulses are channelled directly to the electrodes, the stimulus or a pulse train generator D may be carried with the portable unit B. The stimulus generator assembles a pulse train with the selected amplitude, interpulse interval, and pulse width.  
     74. The system may be operated in an open loop mode as described above. Alternately, closed loop operation may also be provided. A position or movement monitor or transducer  140  monitors the movement, position, or degree of extension or flextion of the limb or digit to be moved. Analogously, a force monitor or transducer  142  monitors the force with which the fingers or other limbs or digits are contracted or extended. It is to be appreciated that even the simplest limb movement involves the operation of two antagonistically operated muscles. A first muscle or group of muscles operates to move the skeleton in one direction while a second muscle or group of muscles provides an antagonistic or counter force. When the forces balance in three dimensions, the limb is held stationary. When one force exceeds the other, the limb moves in the direction of the predominant force vector. The stationary position or motion is controlled by the difference between these antagonistically applied forces. Although the relative forces applied by the antagonistic muscles may be relatively high or relatively low, only the difference in the forces is observed by the position or motion transducer.  
     75. With reference to FIG. 10, an exemplary position and force diagram is presented for gripping an object between the thumb and the knuckle of the forefinger. The proportional signal starts at one extreme indicating the hand is fully open or extended, generally in a handshake position, on the left side of FIG. 10. As the proportional signal progresses to the other extreme on the right side of FIG. 10, the position of the fingers contracts generally along curve  144 . That is, the fingers start with no flextion and progressively flex until a fist position is reached at position  146 . Thereafter, the fingers cease becoming more flexed. The thumb starts fully raised or fully flexed. At a point  150 , the thumb commences becoming less flexed, i.e. approaches the forefinger. At a point  152 , the thumb contacts the forefinger and stops flexing. The force with which this thumb moves is illustrated by curve  154 . In the illustrated embodiment, the thumb moves toward the forefinger with relatively little force until the thumb and forefinger contact point  156 . Thereafter, the force is increased by causing the appropriate muscle to contract more strongly until a maximum force or grip is reached at point  158 . In the illustrated embodiment, the force with which the fingers contract is illustrated by curve  160 . In the illustrated embodiment, the fingers contract with relatively little force until the thumb contacts the forefinger. Thereafter, the finger or squeezing force is increased to a higher level. Other relationships between thumb and finger force and position may, analogously, be plotted. Similarly, relationships of position and force between the fingers and thumb when performing other functions or for other limbs may be plotted.  
     76. With reference again to FIG. 9, in the closed loop system, a force and position look-up table  162  is preprogrammed with the selected relationships between the proportional command level and various finger or thumb positions and forces. For example, the look-up table  162  may be programmed in accordance with the graphs of FIG. 10. A force comparing means  164  and a position comparing means  166  compare the actual position and force monitored by position and force monitors  140  and  142  with the preselected position and force values retrieved from look-up table  162 . A force index adjusting means  168  and a position difference index adjusting means  170  adjust the index means  126 ,  130 , and  134  of the active channels until the difference between the selected and actual position and forces are optimized. The position and force difference adjusting means may simply step the appropriate index or indices up or down as may be required to bring the actual and selected force or position into coincidence. Alternately, programming logic may be provided to bring the force or position into coincidence more precisely. For example, large differences and small differences may be programmed at different rates to prevent overshoot or oscillating about the preselected position or force.  
     77. As yet another option, a sequence control means  172  may be provided for causing a preselected sequence of muscular movement and forces. For example, the preselected forces and positions of FIG. 10 may be progressively addressed out to the force and position comparing means  164  and  166 . The proportional signal may be used to control the rate at which the addressing out progresses. It is to be appreciated, that the sequence may be used with the open loop system as well as with the closed loop system.  
     78. With reference to FIG. 11, the instrumentation and processing required for functional neuromuscular stimulation orthoses can be separated into several conceptual stages. A first stage  180  is to transduce and process commands to provide parameters suitable for planning a desired movement. These parameters specify the type of movement to be executed as well as movement parameters such as the magnitude or velocity. The first stage of processing may range from simple gain or offset changes to accessing transformations, signal filtering, and quantitization of continuous commands.  
     79. A second stage  182  is the planning of movement based on the control parameters. The second stage specifies the joint angle trajectories and applied torques. These movement parameters are used by a third stage  184  which coordinates and regulates the process to specify the stimulus parameters to be applied by the stimulus generator D to the muscles.  
     80. If a closed loop control sequence is implemented, a force and position monitoring stage  186  monitors the forces and positions achieved by the user. A feedback stage  188  converts the sensed force and position information into a map of actual physical movement for comparison with the planned movement parameters. Deviations between magnitude of movement, velocity of movement, trajectory, end position, and other movement parameters are used to adjust the planned movement parameters and the stimulus.  
     81. With reference to FIG. 12, the hardware for the laboratory system C includes a central processing unit  190 . An analog to digital converter  192  converts the analog output of potentiometers  194  of a joystick, such as the joystick of the input control means A to digits. The potentiometers  194  may be attached to the patient or may be available to the operator. For example, the amplitude of the stimulus pulses can be set manually by the operator on potentiometers  194 . Force and position monitoring transducers  156  also produce analog output signals indicative of patient motion and force. The position and force analog signals are converted with the analog to digital converter  192  and a digital input means  198  to an appropriate input for the central processing unit.  
     82. A digital output device  200 , a microprocessor based pulse width and interpulse interval modulator  202 , and output stages  204  provide a biphasic current pulse train to the electrodes to stimulate the patient. The stimulus pulse train, as illustrated in FIG. 8, has a rectangular cathodic phase followed by an anodic phase generated by capacitive discharge through the tissue. An interphase delay on the order of of 0 to 100 microseconds between cathodic and anodic phases has been found to be a value which allows an action potential to develop but which reduces potential tissue damage. If the delay between the two phases is too small, the nerve may repolarize prior to developing an action potential. If the delay is too long, the biproducts and discharge transfer at the electrode surface may diffuse away from the electrode. The biphasic stimulus insures charged neutrality for minimal tissue damage.  
     83. The microprocessor based modulator  202  stores stimulus information descriptive of the stimulus to be applied to the electrodes. The same stimulus or pattern is repeatedly applied to the electrodes until the central processing unit  190  reprograms the modulator memories. In this manner, only changes need be communicated to the modulator. More specific to the preferred embodiment, the modulator allows the flexible formation of stimulus groups, i.e. one or more stimulus channels that operate at the same interpulse interval. The modulator stores the number of stimulus groups within the stimulation system, the stimulus channels belonging to each group, the interpulse interval for each stimulus group, and the stimulus pulse duration for each stimulus channel. In this manner, stimulus pulse trains may be applied by each electrode at a faster rate than would otherwise be permitted by the speed of the central processing unit.  
     84. The pulse width, current amplitude, and the interpulse interval modulation can be controlled independently for each electrode. This allows modulation of the muscle force by recruitment (pulse width or amplitude) and by temporal summation (interpulse interval). In a preferred intramuscular stimulation embodiment, pulse widths on the order of 0 to 255 microseconds may be selected with a resolution of one microsecond. For other applications such as direct nerve stimulation, surface stimulation, and the like, other appropriate pulse widths ranges, interpulse intervals, amplitudes, and resolutions may be selected. The stimulus timing is controlled by the software which is discussed below.  
     85. Other peripheral hardware includes a feedback generator  206  for providing audio, electrocutaneous, or other feedback to the patient regarding the operation of the system, e.g. whether the system is active, etc. A digital plotter  208  and a printer  210  provide a hard copy of the data and parameters. A graphics storage oscilloscope  212  and a video terminal  214  provide the operator with appropriate information, such as stimulus signal strength and parameters, patient position and response, system functioning and parameters, and the like.  
     86. The software provides the intelligent decision making capability of the stimulation system. The software may be divided into four main sections. The first section provides the operator with methods to examine and specify the operation and configuration of the system. The remaining three sections are real time processes that convert the input command signals to control parameters, process the control parameters to specify stimulus parameters, and activate the external hardware to generate the stimuli. The operator interaction system is streamlined for ease of use with many different uses or subjects. The operator may specify the channels of stimulation. The stimulation channels may be organized into groups for sequential stimulation. Channels within a group are activated in a fixed sequence. For a constant interpulse interval, the phasing of one channel with respect to the next may be determined by dividing 360° by the number of channels in the group. Optionally, the channels may have selected non-uniform relative phases. The group organization also allows sequential stimulation in which portions of single muscle or muscle synergists are activated at a low frequency, out of phase with each other. Because the forces ellicited by the individual channels sum at a joint, a fused response can be maintained at a lower stimulus frequency on each channel than would be possible with a single channel scheme. This reduces fatigue. Channels may also be activated pseudo-simultaneously by putting them in separate groups with the same input control signal and the same relationship between the control signal and the interpulse interval.  
     87. The relationship between the control signals and the stimulus parameters may be specified for each channel. The system allows a non-linear pulse width and interpulse interval modulation to correct for non-linear modulation of muscle force by recruitment. Piecewise linear relationships can be specified between a single continuous control signal and the interpulse interval and the pulse width of each channel. The coordination of different muscles is achieved by specifying stimulus modulation of stimulus parameters in different channels by the same control signal.  
     88. The piecewise linear relationships may be specified by the end points of individual linear segments. These end points can also be specified or altered while stimulation is taking place by assigning the control of the individual channels to specific potentiometers on the analog to digital interface. A separate command channel can control the interpulse interval modulation and another command signal can be assigned to control pulse width modulation for each channel. One or more channels can be controlled independently of the others so that its contribution to the coordinated movement can be assessed or altered. When the stimulus parameters for that channel are appropriate, as assessed by visual monitoring or measurement of the movement or force, that combination of stimulus parameters can be entered automatically as one of the end points of a linear segment.  
     89. Command input information, stimulus parameters, patient information, data about the test and muscle being stimulated, electrode information, and general comments can be entered and stored in a secondary storage medium  216 . This enables the system to be used as a notebook. The notebook information may be printed out or recalled automatically to facilitate set up in subsequent tests with the same patient.  
     90. The operator can display graphically the relationships between the command signals and the stimulus parameters in several ways. These relationships can be plotted on the storage graphics oscilloscope  212  or plotted as hard copy on the digital plotter  208 . A less detailed display is available continuously on the video terminal  214 . The range of pulse width and interpulse interval modulation is displayed as a function of the command input for each channel. This display allows the operator to see the relationship between pulse width and interpulse interval modulation on one channel as well as with respect to other channels. This information enables the operator to assess which muscles are coactivated.  
     91. With reference again to FIG. 11, the command processing section  180  of the software is a real time process which converts one or more input command sources into control parameters. The purpose of this process is to translate external command signals from their raw form into an internal digital parameter suitable for specifying stimulus parameters. The command processor has been designed to accept one or more analog input signals as the command storage. Accordingly, most any command source may be made compatible with the system. Suitable command sources include joint positions, myoelectric signals, or contact information.  
     92. The assignment of command inputs to the control of the individual channels or groups of channels can be accomplished as described above. However, more accurate inputs can be obtained than the command input as received from the transducer  196 . The processing provided by this section of the program converts the information to the proper form. Several operations may be performed on the input command. The processing of the preferred embodiment converts command information derived from the shoulder position of the patient obtained from transducing elevation-depression and protraction-retraction movements of the patient. The position command of one axis is used as the proportional control parameter and the velocity movement of an orthogonal axis is used to initiate a logic function. First, the program provides a transformation for linearizing the output of the transducer by projecting its spherical image into x,y coordinates.  
     93. The signal is further processed to translate the transducer axes into perceived patient axes. This allows for compensation for the patient&#39;s actual shoulder movements and also may allow for the use of axes which are not truly orthogonal. The signal is scaled to match the full range of the patient&#39;s shoulder movement to the internal control parameters in order to maximize resolution in the command process. Re-zeroing or nulling specifies an arbitrary level of a command that the patient wants to use as a reference for movement. This allows the patient to select any value in the command range as the null or zero point. In the subsequent section of the program, this null point may be set to correspond to a specific point in the range of control parameters. For example, the null point may be set to correspond to the middle of the control parameter range so that movements in one direction can be used to perform a function different from movements in the opposite direction.  
     94. Hold processing enables the present control parameter level to be maintained despite subsequent changes in the command on the proportional axis. In the preferred embodiment, the velocity on the logic axis is compared with a preselected level to determine whether the control output should be held at a constant value. In this manner, the patient may move his shoulder suddenly to initiate the constant value mode. The patient may regain control by again exceeding the velocity threshold and returning command to the proportional command axis. A time delay in the proportional axis creates a lag between the logical axis and the proportional axis to insure that inadvertent movement does not alter the control output of the proportional axis prior to the hold command. The time delay is a software adjustable parameter which is a function of the ability of the subject to separate the proportional control axis movements and the logical signal axis movements from one another.  
     95. The movement planning and coordination section  182  translates the control parameter(s) that is produced by the command processor into a set of stimulus parameters that correspond to each control parameter level. The piece-wise linear modulation process is simplified by the use of look-up tables. In the preferred embodiment, the input control parameter is treated as having eight bit resolution and one 256-element integer array as allocated for the pulse width modulation for each channel and one 256-element interger array as allocated for the interpulse interval modulation of each group. The contents of each pulse array are filled during the parameter setting procedure and the values are loaded into the microprocessor based modulator to produce a desired pulse width corresponding to each possible value of the command. The contents of each interpulse interval array are the actual interpulse intervals to be set to the stimulus timing process of the stimulus generator D. The contents of these arrays, when finally adjusted, are loaded in the look-up table  124 ,  128 , and  132  of the portable unit B.  
     96. The movement coordination and regulation process  184  runs in a continuous loop which runs whenever the command process and stimulus timing process are not being serviced. Each time through the loop, the input control signal level is used as an index to the look-up table for each of the channels and groups in use. The contents of the pulse width look-up tables at that entry are then loaded into the microprocessor based modulator. The movement planning and coordination process also checks for instructions that are entered at the terminal by the operator and updates the display of stimulus parameters on the terminal.  
     97. The fourth or stimulus processing stage controls the stimulus timing for each of the groups. The timing can be communicated to the electrodes either with an implanted stimulator or a percutaneous system implemented with output stage modules. Communication of stimulus information from the computer is carried over a parallel interface. One or more stimulus channels are provided, each of which operate at the same interpulse interval. The coordination and regulation stage  184  indicates the electrodes which are within each stimulus group, the channels which belong to each group, the interpulse interval for each group, and the stimulus pulse durations for each channel. The stimulus generator stage stores the received information and repeatedly stimulates the electrodes in accordance with the stored information. The coordination and regulation stage  184  as necessary changes the stored information to change the stimulation parameters. Stimulus information is updated as needed, allowing complete modulation of all group interpulse intervals and individual channel stimulus pulse durations.  
     98. With reference to FIGS.  6  and  13 , the software creates a model of the position and force for a selected movement and sets appropriate stimuli. As the patient practices the motion, the patient&#39;s muscle tone improves and the response of the muscles to a given stimuli changes. To this end, the laboratory system is periodically used to adjust the portable system of the patient for desired performance. A function selection means  220  selects an appropriate motion of the patient to be fine tuned. A motion module  222  selects the appropriate force and position for each muscle while performing the movement, as shown for example in FIG. 13. A stimulus selection means  224  formats an appropriate stimulus to achieve the selected motion. In particular, the stimulus selection means  224  selects the amplitude, interpulse interval, and pulse width to be stored in the portable, patient carried unit B. A stimulus generator D applies the selected stimulation pulse train.  
     99. The actual position and force achieved by the patient as the movement is monitored by empirical observation or by a position monitor  228  and a force monitor  230 . A comparing means  232  compares the actual position and force from the monitors with the select position and force from the motion model module  222 . Any differences between the position and force alter the selected stimulus pulse train parameters accordingly. This process is iteratively repeated readjusting the control algorithms until an optimum match is achieved. The reoptimized control algorithms are loaded by the microprocessor based stimulus selecting means  224  into the control algorithm memory  60  of the portable unit B.  
     100. This match reoptimize is repeated periodically to maintain the patient operating at the best possible mode.  
     101. With reference to FIG. 14, a preferred upper body control input command to control processing schemes is illustrated. The command input means A is mounted to the patient and connected with the laboratory unit. As the patient moves his shoulder or other portion of the anatomy to which the input command means is attached, an axis resolving means  240  determines and resolves the proportional instruction axis and the function selection or logic axis. As described above, it is advantageous to select the proportional control along an axis over which the patient has relatively large and relatively accurately controllable range of motion. Because the logic or function selections are carried out in the preferred embodiment by sudden movements, it is advantageous to select the function or logic selection axis as one over which the patient can move his shoulder rapidly a significant distance. As also indicated above, it is advantageous for the axes to be othogonal to avoid cross-talk. However, limited amounts of cross-talk may be satisfactorily removed with appropriate filtering, signal analysis, and the like.  
     102. A range of movement measuring means or step  242  measures the patient&#39;s range of movement along the proportional axes resolved by the axes resolving means  240 . A filter selecting means or step  244  monitors the smoothness or degree of accuracy with which the patient moves along the proportional axis. A filter function is selected which removes unevenness or lack of coordination or control by the patient as he moves along the proportional axis. An amplitude selection means or step  246  selects an appropriate output signal amplitude for each position along the range. The amplitudes are selected in the preferred embodiment to provide a linear relationship between the output and motion. However, other relationships may be provided as is appropriate. For example, for some applications, it may be advantageous to have more precise control at one end of the range. To achieve more precise control, a greater range of movement may be required for a corresponding change in the signal.  
     103. A velocity and time measuring means or step  248  measures the velocity and duration over which the patient can move his shoulder along the logic axis. A filter selection means or step  250  selects an appropriate filter to remove incidental movements which are smaller than the readily obtained velocity and time movements in order to inhibit false signals. An amplitude selecting means or step  252  selects appropriate on/off amplitudes to indicate that the patient has selected a change in the command function. Again, the amplitude and filter functions are periodically re-evaluated as the patient becomes more adept. The selected amplification, velocity threshold and axes, and the like are recorded in the portable patient carried system B.  
     104. With reference to FIG. 15, the data collection/system evaluation portion of the system determines whether the system is working properly and if not, diagnoses what is wrong. A diagnostic algorithm  260  monitors and compares the input command signals from the input means A with the measured position and force of the patient. When the two become inconsistent, an appropriate diagnostic correction is determined for display on the printer  210  or video terminal  214 . For example, the diagnostic algorithm looks for intermittent, large differences between the measured and commanded positions and forces. As another example, the diagnostic algorithm looks for a gradual shift in the two over time which would be indicative of muscle tone improvements by the patient which show that recalibration is required.  
     105. An electrode impedence monitoring means or step  262  monitors the impedence across each electrode. Changes in the wave form of the impedence are indicative of system failures. For example, a sudden jump in the impedence may indicate a break in the electrode or lead wires thereto.  
     106. With reference again to FIG. 13, a memory means  270  periodically stores the differences between the motion model and the actual motion and force achieved. An improvement algorithm  272  analyses the differences stored over a long period of time to determine whether the patient is becoming more proficient. The improvement algorithm determines monitoring whether the patient and the system are able to work together to achieve repeatable and stable results. The improvement algorithm determines from this information whether the system needs adjustments and refinements and how well the patient is performing over time.  
     107. With further reference to FIGS.  6  and  12 , the central processing unit  190  further performs motoric and neurological assessment procedures. These procedures determine whether a person is a candidate for the program. In this procedure, an analysis of the nerves which are still intact and functioning in the affected limb to be controlled are determined. Surface stimulation is applied to cooberate which nerves are intact. The range of motion over which the limb can be articulated are measured and evaluated. A sensory evaluation determines the extent of sensory feedback or feeling in the limb. Commonly, patients with a damaged spinal column are spared the loss of some sensation in the limb providing the patient with a limited amount of feedback. This system also determines the level of voluntary control of musculature. That is, it is determined how much the patient can do compared to a scale of a normal individual. The laboratory system evaluates this data and determines whether or not the patient is a likely candidate for the present invention.  
     108. With reference to FIG. 16, the implantable stimulator D includes an electronic circuit  290  which receives and decodes incoming stimulus information, provides output stimulus pulses to the electrodes, provides immunity from external disturbances, and maintains safe operating conditions. The electronic circuitry is packaged in a hermetic incapsulation constructed of biocompatible materials. The physical size of the packaging and the electronic circuitry is minimized to increase the flexibility in selecting implantation sites in the patient. The stimulus electrodes E each include a narrow, flexible conductive lead  292  for conducting the stimulus pulse train from the implanted electronics to the appropriate muscle group. A terminal stimulus electrode  294  provides direct tissue interface to the muscles for stimulus charged injection and subsequent charge recovery. A reference electrode  256  completes the circuit.  
     109. With particular reference to FIGS.  16  and  17 , the implanted stimulator obtains its electromotive power through radio frequency electromagnetic induction. In particular, the stimulus signal parameters are encoded on a 10 MHz radio frequency carrier. A receiving coil  300  is connected, analogous to a secondary coil of a transformer, with a full wave rectifier  302 , a voltage limiting zener diode  304 , a filtering capacitor  306 , and a voltage regulator  308 .  
     110. Because the efficiency of power transmission through the patient&#39;s skin is only about 30%, the power consumption requirements of the implanted circuitry are kept to a minimum. To minimize the power consumption, the circuitry  290  utilizes CMOS technology. Further, the CMOS circuitry is custom designed to achieve high density integration with a relatively small number of system components. This results in versatile circuit design with high reliability, a reduced number of fabrication procedures, and a small circuit size.  
     111. As set forth above, the modulation of the carrier pulse in the preferred embodiment is achieved by gating the carrier frequency on and off. Optionally, other conventional frequency and amplitude modulation techniques may be utilized. The control signal includes two parts, a digitally encoded portion and an analog encoded portion. Optionally, all digital and all analog coding schemes may be advantageously implemented. The digitally encoded portion carries a digital indication of which the electrode channel is to carry pulses in accordance therewith. The amplitude of the pulses is also digitally encoded. In the preferred embodiment, the pulse width is encoded with an analog encoding scheme in which the width of an off portion of the RF carrier signal is indicative of the pulse width. The frequency with which the modulated pulse packets are transmitted is indicative of the interpulse interval. In this manner, channel selection, stimulus pulse width, stimulus pulse amplitude, and stimulus pulse interpulse interval are all under external control.  
     112. A control signal recovery means  310  separates the coding pulses from the carrier signal. The digital channel number encoding is decoded by a channel decoder  312 . The digital amplitude designation is decoded by an amplitude decoding means  314 . The pulse width encoding is decoded with a pulse width decoder  316 . An interpulse interval decoder  318  sets the interpulse interval. With the interpulse interval encoded in the repetition frequency of the control signal, the interpulse interval decoder may be a trigger circuit for triggering a new stimulus pulse in response to a preselected portion of the signal. The channel selection, amplitude, pulse width, and interpulse interval decoders are connected with an output stage  320  which creates a stimulus pulse train of the selected characteristics on the selected channel.  
     113. A voltage monitor  322  monitors the voltage of the power supply and disables the logic circuitry if the voltage should fall below a preselected level. The low voltage may be due to various factors such as antenna misalignment or low transmitted power. When the voltage returns to the preselected level, the voltage monitor  322  again enables the logic circuitry.  
     114. The power supply includes an energy storage means  330  which stores potential for applying current pulses to electrodes in each channel. Because a current pulse is transmitted for a relatively short duration of each cycle, the charge may be accumulated during the non-tramsmitting portions of each cycle. The charge from the energy storage means  330  is selectively conveyed to the electrodes  294  by a channel selection section  332 . Current flows from the electrodes  294  to the grounded reference anode  296 . A switch  334  is closed when no current is flowing between the electrodes to recharge the energy storage means  330  and is opened by the output circuit  320  during current discharge across the electrodes.  
     115. With particular reference to FIG. 18, each of the output stages provides a regulated current output for the excitation of muscle tissue followed by a current reversal to recover injected charge necessary to minimize tissue damage. During stimulation, the output circuit  320  provides a stimulus pulse to the base of a switching transistor  340  in the channel selection means  332 . When the transistor turns on, a stimulus current  342  flows from an energy storage capacitor  344  through the collector to the emitter controlled by a stimulus current regulator  346  and through the muscle tissue between the electrodes. The stimulus current regulator is set by the output circuit  320  to provide the selected one of a plurality of current amplitudes. For example, a typical amplitude may be 20 milliamps drawn from the capacitor  344  of the energy storage device  330 . The stimulus pulse occurs concurrently with the duration of the control command, i.e. the pulse width. At the end of the stimulus pulse, the transistor  340  is turned off halting the stimulus current. The charge storage capacitor  344  now recharges back up to the power supply voltage. A recharging current flows through the switch  334 , an isolation diode  348 , and a charge regulator  350  and a reverse current  352  flows in the reverse direction from the stimulus electrode  294  to the anode  296  providing the charge recovery and completing the biphasic stimulus pulse.  
     116. The output capacitor  344  serves three functions. First, it provides a reservoir of energy from which relatively large currents can be drawn for short periods of time. Second, it provides a charge reversal and insures complete charge recovery. Third, it provides AC coupling for the stimulating electrode blocking DC current flow between the stimulating electrode and anode whether the circuit is active or dormant. The DC current blocking coupled with a maximum capacitor leakage current of 1 microamp helps prevent possible galvanic electrochemical corrosion when dissimilar metals are used for the stimulus electrodes and the anode.  
     117. Recharging current to the energy storage capacitors is limited to 0.5 milliamps for two reasons. First, it places only a relatively small demand on the RF power circuit, even when several channels are recharging simultaneously. Second, during recharge current direction is such that the stimulating electrode would undergo anodic electrochemical corrosion. The low level of the recharge current helps prevent the potential delivered to the electrode during the anodic phase from exceeding the potential at which the electrode materials may corrode.  
     118. A zener diode  354  on the base of the switching transistor  340  prevents erroneous stimulus output during powering up and powering down of the stimulator circuitry. During removal or replacement of the external powering antenna, the integrity of the control logic cannot be guaranteed as the logic supply voltage rises and falls. The zener diode prevents transistor switching until the control logic is stable.  
     119. With reference again to FIG. 16, the stimulus current regulator  346  operates on a current mirroring principle. One of a plurality of selectable reference currents is set up using one of a plurality of reference mirror CMOS transistors  360 . Due to the uniformity of device characteristics on the same integrated circuit die, this reference can be used to mirror the reference current into other discrete mirror current transistors  362 . By selectively grouping different numbers and geometry types of the current mirror transistors together with each reference mirror transistor, one can select a regulated current that is one of a wide range of multiples of the reference current. By selectively gating different numbers of the mirror transistors conductive, different amplitudes of the stimulus currents may be selected. In the preferred embodiment, stimulus currents in the range of 0 to 32 milliamps may be selected. To conserve power, the reference current is applied to reference mirror transistors  360  only during the output of a stimulus pulse. If all of the output stages share the use of the same current regulator, simultaneous outputs from two or more of the channels may not be obtained at their full amplitude.  
     120. When using the portable system to control a plurality of implanted stimulators, an interrogation system  370  is provided to enable the portable unit to ascertain which implanted stimulator is interconnected with each transmitting coil or aerial. On initial set up, the portable unit interrogates the implanted stimulator which is interconnected with each transmitting coil and receives an implanted stimulator indicating signal back. The portable unit switches the appropriate control circuits for each implanted stimulator into interconnection with the appropriate transmitting coil.  
     121. In the preferred embodiment, the implanted stimulator interrogation system includes an identification signal