Linear electromyographic biofeedback system

A linear electromyographic biofeedback system operates over a dynamic range of at least one thousand to one. That is, the system remains linear from input levels of approximately one microvolt to input levels in excess of one millivolt. The linear electromyographic biofeedback system includes a sensitive transducer which is followed by a protection circuit and connected to a differential amplifier for eliminating common mode noise. The output of the differential amplifier is filtered and amplified to further eliminate unwanted signals. This filtered signal is rectified and averaged in a third order averaging filter to obtain a close approximation of a time averaging without the necessity of discrete timing periods. The output signal from the averaging filter is then used to control a current controlled oscillator which provides a series of audible pulses at a rate which varies linearly with the value of the input voltage detected by the transducer, the repetition rate range being from approximately one hertz to greater than five thousand hertz.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to bioelectronic systems and more 
particularly, to electromyographic biofeedback systems. 
2. Description of the Prior Art: 
There are in the prior art, many systems which measure human muscle 
activity and provide an output signal which corresponds to the muscle 
activity. However, most of these prior art systems are limited in the 
range of operation and are basically digital in nature in that they do not 
provide any response below a preset threshold level. Therefore, the 
patient or the therapist gets no information relative to small muscle 
movements. In some cases, it is the small muscle movements which are most 
important since these are the movements which indicate the earliest signs 
of progress in rehabilitation. 
Examples of the prior are threshold systems are U.S. Pat. No. 3,656,474 to 
Gentry, et al, and U.S. Pat. No. 3,657,646 to Zmyslowski, et al. 
Another prior art electromyograph is shown by Gaarder, et al, in U.S. Pat. 
No. 3,641,993. Gaarder, et al, teaches an electromyograph in which the 
amplification system is non-linear and in fact is responsive to the 
logarithm of the peak value of the input signals representing human muscle 
activity. Gaarder, et al, includes a means to integrate the voltage 
representing muscle activity over a preset time interval. Because of the 
requirement for integration of the input voltage, the teaching of Gaarder, 
et al, is not appropriate for a real time biofeedback system. The patient 
must have an instantaneous response to any muscle activity. A delay which 
is required by an integration system will not give the patient the proper 
kind of feedback information for him to determine what was the cause of 
the signal which he received at any instant of time. Further, the 
logarithmic amplification of the input signal by the Gaarder, et al, 
system prevents a wide dynamic range of operation. The system distorts the 
rate of change of muscle activity in the higher voltage range. For 
example, the output frequency rate in the Gaarder, et al, system changes 
very little in the top portion of the range of muscle activity. 
A biofeedback system, to operate effectively must provide an instantaineous 
signal to the human which can be translated by the brain into an 
indication of small muscle movement. For this reason, it is very important 
that an electromyographic biofeedback system be sensitive over a wide 
range of muscle activity. The response to a single muscle unit, the 
smallest measurable muscle unit, should be such as to provide the patient 
with an appropriate indication of muscle activity. Biofeedback of small 
muscle activity such as an indication of activity by a single muscle unit 
provides a very positive psychological effect on the patient in that the 
patient can hear or see a positive indication of progress in 
rehabilitation. 
Also, it is very important to maintain a uniform sensitivity of the 
electromyographic biofeedback system over a wide range of muscle activity 
to provide the patient and the therapist with an accurate indication of 
progress in the rehabilitation. 
Additionally, since most human systems, such as human hearing are 
logarithmic in nature, it is important that the external biofeedback 
system be linear to prevent distortion of the magnitude of the muscle 
activity signal to the human ear. 
All of the prior art systems discussed have one or more deficiencies which 
are overcome by a linear electromyographic biofeedback system. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an output 
signal from an electromyograph for biofeedback purposes which accurately 
represents instantaneous muscle activity. 
Another object of the present invention is to provide the output set out 
above in a system in which the amplification system from input to output 
is essentially linear. 
A further object of the present invention provides output signals as set 
out above over a dynamic range of inputs in a ratio of one thousand to 
one. 
Yet another object of the present invention to provide a output 
representing muscle activity, the output changing linearly with the 
strength of muscle activity. 
Still another object of the present invention is to provide the output as 
set out above for a patient in a system in which an averaging filter is 
employed to eliminate time integration. 
A further object of the present invention is to provide a patient or a 
therapist with a linear output signal which represents the instantaneous 
value of muscle activity by an electromyographic biofeedback system which 
includes an input transducer having a high input impedance, a differential 
amplifying system to eliminate common mode noise, a band pass filter 
system for elimination of signals outside the band of electromyographic 
activity, a rectifying system for selecting the desired information, an 
averaging filter for providing a signal representative of the 
instantaneous value of muscle activity and a current controlled oscillator 
for generating a sequence of pulses, the frequency of which is directly 
related to the instantaneous value of muscle activity. 
It is a feature of the present invention that the output representing 
muscle activity may be present either in audio or visual form at the 
option of the patient or therapist, or may be presented in both forms so 
that the patient may have an audio output while the therapist has a visual 
representation of muscle activity. 
These and other objects, features, and advantages of the present invention 
will become more apparent by reference to the following description and 
drawing.

A linear electromyographic biofeedback system according to the present 
invention provides the characteristics necessary to feedback learning. 
These characteristics are instantaneous feedback so that the patient will 
immediately know the result of his effort as well as the magnitude of the 
muscle activity resulting therefrom. Second the feedback must be 
proportional, that is a certain change in muscle activity at one extreme 
must provide a change in output signal approximately the same as the same 
change in muscle activity at the other extreme. Third, a wide dynamic 
range of feedback is necessary to allow the patient to recognize the 
results of the patient's efforts in a biofeedback learning situation on a 
single scale. That is frequency increases monotonically with muscle 
activity and the entire range of muscle activity is contained in a single 
scale so that the patient does not have to switch from one range to 
another or any other confusing change which might hinder the feedback 
learning process. 
Referring now to FIG. 1, a patient 8 uses the linear electromyographic 
biofeedback system 10, according to the present invention as follows: 
Input transducer 12, is attached to the patient near the area of the 
muscle which is to be measured for activity. Input transducer 12, has a 
very high impedance generally in excess of 10.sup.10 megohms. 
Referring to FIG. 3A, it can seem that input transducer 12, is a balanced 
transducer. Input transducer 12 is connected to the linear 
electromyographic biofeedback system 10, by lines 14. The output is 
presented to the patient through headset 16, being connected to system 10 
by line 18. In operation, as the patient attempts to move the muscle whose 
activity is being measured, the pulse rate which the patient hears in 
headset 16, will change linearly with the strength of the muscle activity. 
Thus, a very weak muscle movement might produce a very slow pulse rate in 
the order of one Hertz and a very strong muscle movement might produce a 
frequency output in access of 5000 Hertz. 
Although not shown in FIG. 1, a display console for use by a therapist may 
also be connected to the electromyographic biofeedback system 10 so that a 
therapist may monitor in a digital manner the magnitude of muscle 
activity. 
Referring now to FIG. 2, the individual blocks which are connected together 
to form the system according to the present invention will be described. 
The balanced input from transducer 12, is presented at terminals 21 to 
protection circuit 22. Protection circuit 22 protects against voltage 
spikes which could damage the differential amplifier 23. Differential 
amplifier 23 produces an output signal which is proportional to the 
difference between the voltage on the input lines 21. The output signal 
from differential amplifier 23 contains desired signals representing 
muscle activity as well as noise and other information which is not of 
interest in the biofeedback system. Therefore, a band pass filter, 24, is 
connected to the output of differential amplifier, 23, to limit the 
frequency range which will be passed to the remainder of the system. A 
common frequency range for electromyographic activity is in the range of 
50 to 500 Hertz. Block 24 also has amplification built in to maintain 
adequate signal levels within the system. The output of the band pass 
filter is connected to a notch filter 25, which eliminates signals at 60 
Hertz which is the power line frequency and a common frequency for 
undesired signals. The output of notch filter 25, is connected to gain 
adjustment potiometer, 26, which adjusts the gain level input to a second 
band pass filter, 27. Band pass filter, 27, operates in a manner similar 
to band pass filter, 24, but has connected thereto a DC balance adjustment 
potentiometer, 28, to allow for balancing the system due to individual 
component variations. The output of band pass filter, 27, is connected to 
a full wave rectifier, 29, which provides a unidirectional sequence of 
signals as input to third order averaging filter, 30. Averaging filter 30, 
has an overshoot adjustment potentiometer, 31, which is used to balance 
the filter for component variations and to eliminate excessive overshoot 
on pulse, rise, and fall. 
An example of a third order averaging filter which would be used with the 
present invention is shown in a paper published as a Technical Note in 
Medical and Biological Engineering, volume 10, pages 559 and 560, Peter 
Perwgrinus Ltd. 1972. As noted in the introduction of the Technical Note, 
the third order averaging filter provides a very rapid dynamic response 
which is very desirable for instantaneous biofeedback. 
A filter 30 to be used with the present invention must have the following 
characteristics. First, the noise throughput must be small relative to the 
signal from a single muscle unit. Second, the response time must be less 
then 250 milliseconds. Third, the overshoot response must be less then ten 
percent. And fourth, the filter must have a monotonic step function 
response. 
The output signal from averaging filter, 30 is connected to a base line 
control, 32, which is adjusted by the patient to provide a very low or 
zero output pulse rate with no muscle activity. 
The base line control, 32, controls current source, 33, which with current 
mirror, 34, and current controlled oscilator, 35, convert the output of 
averaging filter 30, into a digital output in the form of a series of 
pulses, the repitition rate of which is proportional to the strength of 
the input muscle activity. The output of current controlled oscilator, 35, 
is connected to output buffer, 36, which drives earphone, 16. 
Of course, if desired, both audio and visual outputs can be obtained from 
the system. 
Referring now to FIGS. 3A and 3B, the schematic diagram will be described 
in relation to the block diagram of FIG. 2. 
A balanced high impedance input transducer, 12, is connected at terminals 
1, 2, 3, and 7 to protection circuit, 22, which consists of resistors, R1, 
R2, diodes D1, D2, capacitor C2 and components diodes D9, D10, D11, Q13, 
Q14, Q15 resistors R3, R4, R5, R66, R68, R70, R71 and capacitor C33. These 
latter components which form portion of the power supply, provide a 
current source to supply current to the sources of field effect 
transistors Q1A and Q1B as well as providing bias voltage for cascade 
transistors Q12 A and Q12B to maximize common mode rejection. 
It is important to maintain the input impedance at as high a level as 
possible to provide greatest possible common mode rejection for a very 
small muscle movement. 
The respective collectors of Q12A and Q12B are connected through R9,C5 and 
R10,C6 to the respective minus and plus inputs to differential amplifier 
A1. 
The output of differential amplifier A1 is connected to first band pass 
filter, 24, which consists of operational amplifier A2 and filter 
components R14, C7, R15, C29. 
The band pass filter, 24, is connected through notch filter C9, C10, R17, 
R18, R64 and C1 through emitter follower to Q2, which has an output taken 
from gain adjustment potentiometer, R19, to the second band pass filter, 
27, which includes DC balance adjustment potentiometer, R24. The band pass 
characteristics of second band bass filter, 27, are controlled by 
resistors, R20 and R22 and capacitors, C11 and C12. 
Full wave rectifier 29 shown in FIG. 2 includes resistors R25, R26, R27, 
R28, R29, R30, R31, diodes D3 and D4 and operational amplifiers A4 and A5. 
As with any full wave rectifier, the output of operational amplifier A5 is 
a unidirectional signal which contains the electromyographic information 
representing muscle activity. The output of the full wave rectifier 29 is 
connected to averaging filter 30. 
Averaging filter 30 consists of filter components R32, R33, R34, R35, R36, 
R37 capacitors C14, C15, C16, C17 operational amplifier A6, filter C18, 
C19, R39, R40 and base line control R42, R43, R44 and R45. Variable 
resistor R44 is the base line adjustment potentiometer. A voltage divider 
consisting of resistors R72 and R73 provides a reference point 1 at the 
junction of R72 and R73 which is the return connection for a visual 
display device which may be connected to the DC output from averaging 
filter 30. 
A visual display device such as an analog volt meter or a group of digital 
indicators each of which indicates a successively higher level of muscle 
activity maybe connected between the DC out connection and the junction of 
resistors R72 and R73. 
The output of averaging filter 30 is connected to operationl amplifier A7 
which with transistor Q3 and resistor R47 operate as constant current 
source 33. Variable resistor R47 provides a gain adjustment for current 
source 33. The collector of current source transistor Q3 is connected to 
IC2 which operates as current mirror 34. 
The output of current mirror, 34, is connected to relaxation oscillator Q4, 
Q5, C22, R48, R49, R50, R52, and D5 which provides a pulsed output 
controlled by the voltage present at the positive input to current source, 
33. 
The output of the relaxation oscillator is connected through C23 to 
amplifier Q6 which with Q7 and Q11 provide a buffer amplifier for 
amplifying the audio signal for presentation to headphones, 16. 
While the invention has been described with respect to a preferred 
embodiment thereof, there are many variations in specific circuit 
implementation which may be used to provide the funtional element required 
for applicants invention. 
It will be understood by those skilled in the art that many variations in 
specific implementation may be made without departing from the spirit or 
scope of the invention.