Method and Apparatus for Providing Stimulated Exercise of Paralyzed Limbs

The present invention relates to methods and apparatuses for electrically stimulating muscles and, more particularly, to a computer controlled system and method for exercising paralyzed muscles through programmed electrical stimulation.

Each year thousands of persons are permanently paralyzed as a result of traumas sustained by their spinal cords. Often, the trauma is such that there is no direct damage to most of the muscles in the victim's body, but communication between brain and the lower spinal cord is eliminated, which results in a permanent loss of control of certain muscle groups. This loss of control causes a number of additional problems in the body of the victim. The first of these is the atrophy of the uncontrolled muscle from disuse. Disuse atrophy is distinguishable from other types of atrophy in that the muscle and associated tendons are still functionally healthy, but because of lack of use, the muscle wastes away and loses much of its protein content.

A second problem is demineralization of the bones associated with the uncontrolled muscles due to the lack of muscle activity. After bone demineralization has become advanced, the simple act of passively moving the leg of a victim can break a bone or crack a joint. In addition, deterioration of the tendons and other soft tissue occurs, as well as a severe wasting away of cardiac tissue. The latter results in a compromised cardiovascular system which predisposes the victim's body to secondary cardiovascular disease and secondary renal disease. All of these conditions are commonly found in the pathology of victims of spinal cord injury after chronic paralysis.

Until recently it was believed that a spinal cord injury could not be cured. Consequently very little was done for persons having spinal cord injury aside from trying to maintain their bodily health. However, recent advances in spinal cord research have given new hope to those who are paralyzed due to spinal injury. Furthermore, techniques have now been developed for causing movement of a victim's hands or legs, through functional electrical stimulation. This latter development makes it possible to exercise paralyzed limbs and keep them in good physical condition. Hopefully such conditioning will enable restoration of useful functional activity when a cure for spinal cord injury has been developed. Such conditioning also helps to reverse or eliminate the secondary diseases associated with spinal cord injury. Elimination or reversal of these diseases may greatly reduce medical costs and increase the longevity of the spinal injury victim.

Examples of systems for exercising paralyzed limbs through functional electrical stimulation may be found in Petrofsky et al U.S. Pat. Nos. 4,480,830 and 4,499,900, the disclosures of which are incorporated herein by reference. Such systems can work the muscles to precise levels and increase the metabolic loads as appropriate to produce muscle build-up. With these exercise systems it is possible to bring the muscle back to normal strength and to retrain the cardiovascular system. However, the health of the patient requires that the program be carried out progressively under the direction of an attending physician. During the course of the program it is important that limits be set on the work load and on physiological parameters such as body temperatures, heart rate and blood pressure. The exercise system must include means for measuring such variables, and provision must be made for terminating an exercise routine whenever one of the prescribed limits has been exceeded. If the physician is not present during the exercise, then provision must be made for generating a report of the performance of the patient during the routine, so that the prescribed limits may be appropriately adjusted.

The present invention improves the system disclosed in the aforementioned Petrofsky et al applications by providing an electronic "prescription" system for controlling stimulated exercise of paralyzed limbs. In accordance with the practice of the invention, workload limits for the paralyzed muscles of a subject may be set under the direction of an attending physician. These limits may include the maximum weight to be moved by the limb, maximum heart rate, maximum systolic blood pressure, maximum diastolic blood pressure, maximum body temperature and maximum number of limb movement cycles. Heart rate, blood pressure and body temperature may be specified for resting and for exercise conditions.

The above-noted exercise parameters are stored in an electronically programmable memory cartridge by a computer operated under physician control. This cartridge is given to the patient who takes it to a location equipped with a computer controlled exercise system. This system reads the information on the cartridge and uses it for control of a programmed routine of stimulated exercise. Exercise is automatically terminated whenever any of the prescribed conditions are exceeded. The cartridge may be reused a limited number of times as determined by usage limit information which the physician may cause to be stored within the cartridge.

Upon termination of an exercise session the exercise system records all physiological parameters which have been specified by the physician and stores them in the cartridge. After a cartridge has been used the prescribed number of times, the patient returns it to the physician. The data which is stored on the cartridge is read out by the physician's computer for use in prescribing the further course of the stimulated exercise. In this way the physician can direct a gradual build-up of the muscles under precisely controlled conditions.

Accordingly, it is an object of the invention to provide a method of physician-controlled stimulated exercise of paralyzed limbs.

It is another object of the invention to provide an improved system for carrying out stimulated exercise of paralyzed limbs.

Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

FIG. 1 is a flow chart for a program of stimulated exercise carried out in accordance with this invention;

FIG. 2 is a somewhat schematic side view of an exercise system for conducting stimulated exercise in accordance with the flow chart of FIG. 1;

FIG. 3 is a perspective view of a control console for the exercise system of FIG. 2;

FIGS. 4A-4D are a flow chart for a computer program to control the exercise system of FIG. 2;

FIG. 5 is a flow chart for a computer routine to control leg lifting;

FIGS. 6A and 6B are an electrical schematic diagram for a microprocessor system to carry out the program of FIGS. 4A-4D and 5;

FIG. 7 is a schematic diagram of a parallel input/out port interfaced with the microprocessor system of FIGS. 6A and 6B;

FIG. 8 is a schematic diagram of a muscle stimulator and a digital-to-analog converter connected thereto;

FIGS. 9A and 9B are a schematic diagram of a sensing system for the microprocessor of FIGS. 6A and 6B;

FIG. 10 is a schematic diagram of a computer connected for programming a read only memory cartridge; and

FIGS. 11A-11D are a flow chart for a computer program to store exercise control data in a memory cartridge.

A program of electrically stimulated exercise for a paralyzed person may be established and implemented as generally illustrated by the block diagram of FIG. 1. Accordingly, an attending physician may write a prescription establishing an exercise routine, such as leg lifting or exercycle pedaling, including a set of parameters controlling the nature of the exercise and physiological response limits to be observed. These parameters are stored in a memory cartridge which may comprise an electronically erasable and programmable read only memory, commonly known as EEPROM. Data representing the control parameters may be "burned" into the EEPROM by a suitably programmed computer, as hereinafter described. Preferably, the computer is located in the physician's office and operated by him or personnel under his control.

After the cartridge has been loaded with the appropriate data it is given to the patient, who takes it to a place equipped with apparatus for carrying the prescribed exercise program. This equipment may include an exercycle system of the type generally disclosed in Petrofsky et al U.S. Pat. No. 4,499,900 and an exercise chair as disclosed in Petrofsky et al U.S. Pat. No. 4,480,830. For purposes of this description it will be assumed that the physician has prescribed exercise in an exercise chair. Reference may be made to U.S. Pat. No. 4,499,900 for background information to enable application of the invention to an exercycle. Furthermore, the invention may be used for exercising paralyzed arms through use of a stimulation system of the type described in Petrofsky U.S. Pat. No. 4,558,704.

When prescribing leg exercise the physician may prescribe the weight to be lifted and the maximum number of leg lifts. Other parameters which may be prescribed are the maximum body temperature (at rest and during exercise), maximum diastolic blood pressure (at rest and during exercise), maximum systolic blood pressure (at rest and during exercise) and maximum heart rate (at rest and during exercise). It will be appreciated that only one of the prescribed limits will be reached during an exercise routine. The system affords the physician the flexibility of choosing one parameter as an instrument of exercise control and selecting appropriate values of other parameters to maximize the safety of the procedure.

It has been observed that the endurance of a paralyzed person rapidly increases during the course of a program of stimulated exercise. Accordingly, the physician will want to review the patient's progress from time to time and make appropriate adjustments in the prescribed exercise parameters. For this reason a memory location is reserved in the cartridge for storage of a number indicating the number of cartridge uses. When the EEPROM is programmed at the physician's office, an initializing count (which may be 0000) is stored in that memory location. That number is incremented each time the cartridge is used. After a predetermined maximum count has been reached, further cartridge use is prevented.

Typically, the physician will desire that the patient exercise both legs, so a cartridge will be prepared for each leg. Each cartridge will be loaded with data for controlling the exercise routine as well as the number of cartridge uses. A written prescription may be prepared on an adhesively backed blank and secured on the side of one of the cartridges. The written prescription may document the data loaded into the cartridge and present any additional special instructions such as the muscles to be stimulated, the type of equipment to be used, etc.

When the patient reports to the exercise center the cartridge is installed in the exercise controller for checking of the use count. Thereafter (assuming that leg lifts have been prescribed) stimulated leg lifting is begun on one leg. Prior to and during the course of the exercise the patient's blood pressure (both systolic and diastolic), heart rate and body temperature are monitored automatically. Leg lifting continues until a prescribed physiological parameter has been exceeded or until the predetermined maximum number of leg lifts have been reached. Thereafter the exercise routine for the first leg is terminated, and final measurements of the prescribed physiological parameters are made. Measurement data, together with the leg lift count are stored on the cartridge in specially reserved memory locations. At this time the exercise controller increments the number in that storage location within the cartridge which carries the use count.

After one leg has been exercised, the patient is rested and the entire procedure is repeated for the second leg. The second routine is carried out using the above-mentioned second cartridge, which may, if desired, be programmed differently than the first cartridge. The entire procedure may be repeated on subsequent visits to the exercise center until predetermined maximum usage counts on the cartridges have been reached.

When the cartridges have been used the maximum number of times they are returned by the patient to the programming center, which as noted above, is preferably located at the physician's office. The program which is used for loading exercise control information into the cartridge is provided with a special routine for reading out data stored therein at the exercise center. The data so read out is reported to the physician for use in prescribing the further course of the patient's exercise.

As shown in FIG. 2, an exercise system, generally designated 10, comprises a frame 12 which supports a chair 14 having straps 16 and which is adapted for comfortably supporting a patient 18. The lower portion of the left leg 20 of the patient is enclosed by an adjustable strap 22 attached to a cable 24. The cable 24 is entrained about pulleys 25, 28 and is connected at an opposite end thereof to timing belt 30.

Timing belt 30 is entrained about pulley 32 and pinion gear 34 such that displacement of the timing belt positively drives the gear 34. The pulleys 26, 28 supporting the cable 24, as well as the pulley 32 and gear 34 supporting the timing belt 30, are mounted on the frame 12 by a support structure 36.

An end of the timing belt 30 opposite the end attached to the cable 24 is attached to a load cell 38 of well-known design. A weight 40 is suspended from the load cell 38, and comprises a plurality of plates 42 so that the total resistance provided by the weight 40 can be varied to provide the prescribed weight. Thus, extension of the leg 20 causes the weight to be raised; the weight providing a substantially constant resistance throughout the limited range of movement indicated in FIG. 1.

The gear 34 is attached to a potentiometer 44 which generates a signal proportional to the displacement of the belt 30 (and the extension of leg 20). Electrodes 46a, b, c, are attached to the skin of the leg 20 above the muscles to be stimulated and are connected to a stimulator 48 by wires 50. The potentiometer 44 and load cell 38 are connected to an analog-to-digital converter 52. Also connected to analog-to-digital converter 52 are a ramp generator 64, a body temperature sensor 88, a heart rate sensor 87 and a blood pressure sensor 86.

Ramp generator 64 generates a triangular wave form which is used as a reference signal for feedback control of leg stimulation, as more fully described in Petrofsky et al Ser. No. 417,935. Blood pressure sensor 86 may be a Copal digital sphygmomometer Model UA-251 which provides two outputs utilized as hereinafter described for calculating systolic blood pressure and diastolic blood pressure. Heart rate sensor 87 may be a cardiotachometer Model CT-2 manufactured by Gedco Associates, Inc. Temperature sensor 88 may comprise a thermister 386 mounted in a bridge circuit as generally illustrated in FIG. 9B.

Digitized output signals from A/D converter 52 are provided to computer 56 which may be configured as generally illustrated in FIGS. 6A, 6B and 7. The heart of computer 56 is a Z80 integrated circuit microprocessor designed and manufactured by Zilog, Inc. of Cupertino, Calif. Computer 56 is provided with a connector for reception of a memory cartridge 58 carrying exercise control information stored therein as described above. Computer 56 generates stimulation control signals in a digital format for application to a digital-to-analog converter 60. D/A 60 converts the digital signals to analog format and supplies them to stimulator 48, as illustrated in FIG. 2. A schematic diagram illustrating the details of D/A converters 60 and stimulator 48 is presented in FIG. 8. FIGS. 9A and 9B present schematic details for A/D converter 52, ramp generator 64, blood pressure sensor 86, heart rate sensor 87 and temperature sensor 88. For further description of the control of a muscle stimulation system by a Z80 microprocessor, reference may be made to Ser. No. 561,720.

Computer 56, A/D converter 52, D/A converter 60, ramp generator 64 and stimulator 48 comprise a control system 54 which may be mounted within a console 70 as illustrated in FIG. 3. A receptacle for receipt of cartridge 58 is brought to the surface of console 70, as illustrated.

Console 70 includes a toggle switch 78 which is used to start the exercise program and a push button 76 which stops the program. A first LED 72 is illuminated when the program is active, and a second LED 73 is illuminated after an exercise routine has been completed. The console also has a digital display 68 and a series of switches which are available for implementing embodiments of the invention not described here in detail.

The computer at the physician's programming center may be an APPLE II PLUS personal computer manufactured by Apple Computer, Inc. of Cupertino, Calif. A typical computer as illustrated by the reference numeral 400 on FIG. 10 is provided with an adapter comprising NAND gates 410 through 412 and a flip-flop 413 combined into an assembly which plugs into slot 4 of the computer. Cartridge 58 then plugs into the adapter with pin connections as shown. The pin connections are standard pin connections for the APPLE II PLUS computer and for a 2817 EEPROM (indicated by the reference numeral 102).

When cartridge 58 is taken to the exercise center and plugged into computer 56 it becomes interconnected with a microprocessor 101, as illustrated in FIG. 6A. As noted above, microprocessor 101 is a Z80 microprocessor. FIG. 6A illustrates connections for various standard Z80 terminals. Cartridge 58 (including its EEPROM 102) and microprocessor 101 are interconnected with a 6116 read/write memory 103 as also illustrated in FIG. 6A and a decoder 104 and flip-flop 105 as illustrated in FIG. 6B. Decoder 104 may be a 74LS138 integrated circuit and flip-flop 105 may be a 74LS73 integrated circuit. FIG. 6B also illustrates a crystal oscillator 108 which may operate at a frequency of 2 MHz for use as a system clock.

Computer 56 also includes a Z80 parallel input/output port 113 interconnected to microprocessor 101 as illustrated in FIG. 7. Interconnections between FIG. 7 and FIG. 6A are shown by corresponding reference numerals on lead lines.

In the preferred embodiment exercise control system 54 includes six D/A converters 60 and six stimulators 48, so that the system may be used for stimulated exercise on an exercycle or an exercise chair. One such D/A converter and stimulator are illustrated in FIG. 8. D/A converters 60 may be a DAC0832 integrated circuit. Stimulator 48 comprises integrated circuits 301 through 303, and transistors 304 through 310 connected as illustrated in FIG. 8 and operating as described in detail in Ser. No. 417,935. Stimulator 48 generates pairs of alternately pulsed signals which are applied to electrodes 46A through 46C to stimulate muscular activity in the leg of the patient.

A/D converter 52 may be an ADC0808 integrated circuit connected as illustrated in detail in FIGS. 9A and 9B. Ramp generator 64 includes an integrated circuit 357 and other components as described in detail in Ser. No. 417,935. Ramp generator 64 supplies a triangular wave to input terminal No. 5 of A/D 52.

Load cell 38 includes a strain gage 371 incorporated into a bridge network as illustrated in FIG. 9B. Output signals from the bridge network are amplified by operational amplifiers 365 and 359 and applied to input terminal 0 of A/D converter 52. Temperature sensor 88 includes a thermister 386 connected into a bridge network in a manner similar to the connection for strain gage 371. Temperature sensing signals from temperature sensor 88 are applied to input port 1 of A/D converter 52, while heart rate signals, blood pressure sound signals and blood pressure signals are applied to input ports 2 through 4 respectively. Leg extension indicating signals from potentiometer 44 are applied to input port 6 of A/D converter 52. The output lines from A/D converter 52 are marked on FIG. 9A with reference numerals indicating interconnections with circuitry illustrated on FIGS. 6A and 6B.

As illustrated in FIG. 9B electrosphygmomometer 86 has two input lines 405 and 394. Line 405 carries a sound signal while line 394 carries a pressure signal. The pressure signal is an analog voltage whereas the sound signal is an AC voltage that varies with Korotkoff sounds issued by the artery of the patient during the release of pressure within an inflatable cuff surrounding the patient's arm. The sound signal is converted to a DC voltage by an RMS converter comprising an operational amplifier 402, a diode 403, a resistor 407 and a compacitor 401. The computer system is programmed to determine the systolic blood pressure and the diastolic blood pressure by recording the pressure indicated at input terminal 4 of A/D converter 52 during predetermined values of the sound signal appearing at input port 3.

A suitable program for controlling the physician's computer during storage of exercise control parameters on cartridge 58 is set forth in TABLE I hereto. TABLE II sets forth a listing for a program to control computer 56 in response to data so stored on cartridge 58. The program set forth in TABLE I is written in a version of BASIC suitable for interpretation by a CP/M operating system. The program is stored on a disk for loading into computer 400 prior to cartridge programming. FIGS. 11A through 11D present a flow chart for that program.

The program, as outlined in the flow chart of FIGS. 11A through 11D is generalized for enabling writing data on the cartridge and reading data from the cartridge. Furthermore, the program can be used for setting up a cartridge to control either pedaling on an exercycle or leg lifting as described in the example disclosed herein. The following discussion will be limited to those aspects of the program dealing with leg lifting.

After the program has been loaded into the computer, the physician is directed to enter an input which will select either the cartridge programming routine or the cartridge reading routine. If the programming routine is selected, then the physician is directed to select options indicating those physiological parameters which are to be monitored during the exercise phase. Selection options include heart rate, blood pressure and temperature. The option selections are used later in the program for controlling the input of limits for the selected physiological parameters. The option selections are also used to set the value for a variable stored in hexidecimal location 960F. At a later point the program reads that memory location and burns a corresponding value into cartridge location 060F. In carrying out such storage and retrieval the program utilizes a reference number referred to as "BASE" which has a value of 9000 Hex. This is necessary because the computer utilizes storage location 060F for other purposes and cannot make that location available for temporary storage of exercise control parameters. All other temporary storage for cartridge support is likewise shifted in memory by using the reference number BASE.

Selection of heart rate, blood pressure and temperature options is accomplished by selecting one of the letters Y or N for each of the variables H$, B$ and T$. After this selection has been made, the computer instructs the physician to select an exercise level. In the case of a leg lifting exercise the selected number represents the number of leg lifts being prescribed.

Following selection of the exercise level the physician selects the maximum permitted heart rate, both at rest and during exercise. Then selections are made for maximum permitted values for systolic blood pressure and diastolic blood pressure (rest values and exercise values for each) and maximum permitted body temperature (at rest and during exercise). All values for the above-noted physiological limits are stored in memory at locations shifted above the corresponding cartridge locations by the amount BASE.

The next step in the program is the selection of the type of exercise. For the program listed in TABLE I leg lifting is selected by inputing the letter "A" as the value for a variable called A$. During this same part of the program the physician prescribes the maximum number of cartridge uses, and the computer sets an appropriate corresponding number into the use counter. The program will accept any prescribed number of uses up to a maximum of 16. At this point the program also calculates a number for insertion into location 960F Hex. for indicaton of the physiological parameters which are to be monitored.

If the physician has selected leg lifting as the prescribed exercise, then the computer next instructs selection of a code designating the muscle group to be exercised, a number indicating the weight of the load to be lifted, and a number identifying the patient. It should be noted at this point that the muscle designation code is used only for read out to the physician after the cartridge has been returned. The accompanying written prescription must designate the muscle type for use by operators at the exercise center.

The last step in the cartridge programming routine is the burning of the EEPROM for storage of all exercise control data. As noted above, the cartridge storage locations differ from corresponding storage locations in computer 400 by the value BASE.

If the physician has selected the cartridge read option, then the computer proceeds to read out the cartridge data in the sequence generally illustrated in FIG. 11D. The data so read out includes the limits reached by the designated physiological parameters during each exercise session, together with the number of leg lifts achieved each time. The data so read out is placed in memory within the computer beginning at storage location 9000 Hex. All recorded data is also printed out for use by the physician in prescribing further treatment.

FIGS. 4A through 4D and FIG. 5 illustrate the logical operations carried out by the exercise system in response to the data stored in cartridge 58. As noted in FIG. 4A the first step is to read the load being supported by the load cell 38. This is done before any leg stimulation has been commenced, as a check of the system set up. This is accomplished by calling a weight routine which strobes A/D converter 52 and thereafter reads the data and the memory location assigned to data from load cell 38. The data so read is compared against the weight limits stored in cartridge 58. If the measured weight exceeds the weight limit, then the program branches to a stop routine.

The stop routine is not illustrated in detail in FIGS. 4A through 4D. This routine may be entered from a number of different points in the program. Upon execution, this routine proceeds to store leg the lift count, systolic blood pressure, diastolic blood pressure, heart rate and body temperature in cartridge 58. The stop routine also increments the cartridge use count and stores the incremented count into cartridge 58.

After the load weight has been checked, the system makes an inquiry to determine whether or not a heart rate check has been ordered. If so, then the heart rate is read and recorded and checked against the maximum prescribed resting heart rate. Again, the stop routine is entered if the limit is exceeded.

Following the heart rate check the system checks to determine whether or not the cartridge usage limit has been reached. If so, the system activates an indicator on control console 70 and proceeds no further.

If the cartridge usage limit has not been reached, then the system checks to determine whether or not blood pressure recording has been ordered. If so, the systolic and diastolic blood pressures are read and compared against prescribed resting limits. Systolic blood pressure readings are made by pressurizing a cuff which surrounds the arm of the patient and which forms a part of blood pressure sensor 86. The pressure on the cuff is released, and the computer observes the readings received at input port 3 of A/D converter 52. As noted above, these are converted sound reading from the sound channel of sensor 86. When the sound reading has a value of 50 Hex., then the pressure reading appearing at input port 4 of A/D converter 52 is read and recorded as the systolic blood pressure. The system determines the diastolic blood pressure by reading the pressure at input port 4 of A/D converter 52 when the sound reading at input port 3 has a value of 20 Hex.

Following the blood pressure check, the system performs a temperature check, if ordered, and compares the observed temperature against the prescribed temperature limit.

After the initial checking of the weight and the prescribed physiological parameters has been completed, the program enters its leg lifting routine. That routine commences by incrementing a lift counter and comparing the count in that counter against the maximum prescribed number of leg lifts. If the maximum count is exceeded (which, of course ordinarily will not occur during the first pass through the loop), then leg stimulation commences. During the stimulation phase the system observes the signal from the ramp generator 64 and compares it against the leg position signal received from potentiometer 44. Stimulation commands are applied to D/A converter 60 and stimulator 48 as appropriate to cause the feedback signal to track the ramp signal. This is all indicated by a single block on FIG. 4C. The details of the leg lifting control logic are set forth in FIG. 5. As noted in that figure, the STOP routine is entered if the stimulation voltage exceeds a maximum value of 250 volts. The STOP routine is also entered if the knee becomes hyperextended.

The stimulation command which is generated for stimulator 48 by D/A converter 60 is latched to a constant value and is periodically modified by computer 56. As illustrated in FIG. 5 the stimulation voltage is increased by one unit, if a maximum value of 250 has not been reached and if the knee angle is more than ten units smaller than any ramp angle. The stimulation voltage is decreased by one unit if it has a value greater than ten and if, further, the knee angle is more than ten units greater than the ramp angle.

The system determines that the knee has become hyperextended whenever the knee angle measurement exceeds 127 units, thereby raising the computer's negative status flag. This causes entry into the stop routine as stated above. So long as the knee does not become hyperextended, the stimulation is continued in such a manner as to cause the knee angle to track the ramp angle. The maximum ramp angle is 120 units, as established by the design of ramp generator 64. The computer continually monitors the ramp angle, and when this angle reaches a value of 120 units, then an exit is made from the stimulation routine, and the logic of FIG. 4C continues.

The continuing logic involves heart rate reading and temperature reading, as prescribed, and comparison of these readings against prescribed limits. This reading and limit comparison is followed by a leg lowering routine which is not illustrated in detail, but which is similar to the routine of FIG. 5. Following the leg lowering routine the system checks the systolic and diastolic blood pressures and compares them against prescribed limits, as appropriate. Thereafter, the system again increments the lift counter and proceeds with another lifting cycle.

It will be appreciated that the exercise system 10 produced by the invention herein described is not limited to the use of transcutaneous stimulators, nor is it applicable only to the lower limbs of a human subject. It is believed that comparable devices for exercising neck muscles, arm muscles and stomach muscles, all using stimulators and a strap and pulley arrangement similar to that shown in FIG. 1, may be developed and fall within the scope of the invention. Furthermore, the invention is not confined to systems using closed loop feedback control but may be applied to open loop systems as well.

While the method herein described and the form of apparatus for carrying this method into effect constitute preferred embodiments of this method, it is to be understood that the invention is not limited to this precise form of apparatus and that changes may be made in either without departing from the scope of the invention. ##SPC1##