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Description  |
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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.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is representation of a linear electromyogrphic biofeedback system
according to the present invention as is used by a patient.
FIG. 2 is a block diagram of a linear electromyographic biofeedback system
according to the present invention.
FIG. 3A and 3B are a detailed schematic drawing of a linear
electromyographic biofeedback system according to the present invention.
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.
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