 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates generally to systems and methods which utilize
signals obtained from a living body and more particularly, to systems and
methods of analyzing a human body and controlling prosthetic devices using
acoustic signals obtained from skeletal muscles alone and in combination
with myoelectric signals.
Considerable effort has been expended in prior years toward the extraction
and analysis of electrical signals generated within living bodies. Of
particular interest here are myoelectric signals which are understood to
be representive of electrical excitation in skeletal muscles. It is now
understood that myoelectric signals originate with the depolarization of
the membranes of cells of individual muscle fibers during contraction.
Such depolarization causes the generation of electrical potentials and
currents which are detectable at remote locations, such as the surface of
the skin. Thus, noninvasive techniques can be used to obtain the
myoelectric signals, and therefore, such signals have been useful in
controlling elementary prosthetic devices.
Ordinarily, myoelectric signals are obtained by placing an electrode, which
may be made of a conductive, noncorrosive metal, such as silver or gold,
on the surface of the skin of a living being. It is now well known that
the placement of the electrode on the surface of the skin is a critical
maneuver since precise placement of the electrode on the skin is required
if a satisfactory signal detection is to be achieved. Generally, any
slippage of the electrode from its initial location will degrade signal
transmission.
In addition to the foregoing, myoelectric signal detection is adversely
affected by variations in skin condition. For example, the impedance of
the electrical communication between the electrode and the skin is altered
substantially by the presence of perspiration. Thus, the electrical
characteristics of the coupling to the skin of the electrode vary with
skin condition. This is a substantial disadvantage of systems which rely
upon myoelectric signals, in view of the very small amplitude of such
signals.
In addition to requiring direct contact with the skin, myoelectric systems
are subject to disruption by the presence of stray electrical fields.
Accordingly, substantial electrical shielding is required, thereby
increasing the cost and complexity of such systems.
It is a further problem with myoelectric signals that they do not contain
within them complete information which characterizes muscular activity. In
other words, the myoelectric signals are not representative of muscle
activity, particularly after the onset of fatigue. During fatigue,
excitation-contraction coupling is substantially reduced, and may in fact
be near zero. Under such conditions, electrical activity of a muscle, as
evidenced by the characteristics of a myoelectric signal, may appear to be
normal, but little or no muscle contraction may be present. Thus, there is
a need for a system which can assist in the determining of the onset of
fatigue.
It has been known at least since the early nineteenth century that a
rumbling-type of noise is produced when muscles are contracted. This
noise-making capacity of skeletal muscles was publicized in the
publication Philosophical Transactions of the Royal Society, pages 1-5
(1810). In this early lecture, Doctor William Hyde Wollaston describes a
noise produced by contracting musculature having a frequency generally
between 20 and 30 cycles per second, and amplitude which varies with the
degree of force exerted by the muscle.
Much more recently, Doctors Oster and Jaffe reported in the Biophysical
Journal, Vol 30, April 1980, pp. 119-128, in a paper entitled "Low
Frequency Sounds from Sustained Contraction of Human Skeletal Muscle",
that the sound produced by a muscle grows louder with the increased
loading. The sound is quite loud at the commencement of the loading, but
rather quickly settles to a steady volume. Such a sound is further
reported as arising in the muscles themselves, and is not of vascular
origin.
The acoustic signals generated by muscles, in the form of a relatively low
frequency rumbling noise, can be detected by a transducer, such as a
microphone, which need not be placed in direct communication with the
surface of the skin. In fact, the skin can be covered by a sock. Such a
covering may be particularly useful in situations where skin conditions,
such as those requiring dressing or ointment, render direct communication
between the microphone transducer and the skin undesirable. However, the
amplitude of the acoustic signal received by the transducer decreases
substantially absent direct communication between the transducer and the
skin, and of course, with distance from the skin.
It is, therefore, an object of this invention to provide a system for
producing signals responsive to muscle activity.
In is another object of this invention to provide a system for detecting
muscle fatique.
It is a further object of this invention to provide an arrangement for
monitoring signals pertaining to muscular activity in a living being
without being affected by skin condition, or changes in skin condition
over time, such as impedance changes which occur as a result of
perspiration.
It is also an object of this invention to provide a system for producing a
signal responsive to muscle activity without requiring contact with the
skin.
It is yet another object of this invention to provide a system for
evaluating postural problems.
It is yet a further object of this invention to provide a system for
evaluating and diagnosing dynamic muscular problems, such as those which
result in improper gait.
It is also another object of this invention to provide a noncontact system
for facilitating biofeedback training.
It is still another object of this invention to provide a muscle activity
detection system for analyzing fatique time for the muscles of patients,
particularly patients on respirators.
It is still a further object of this invention to provide a system for
providing signals for controlling prosthetic devices.
It is yet still another object of this invention to provide an arrangement
for detecting muscle activity utilizing a transducer which is not as
location sensitive as myoelectric electrodes.
It is an additional object of this invention to provide a system for
producing signals responsive to muscle activity, the system being
generally unaffected by nearby electrical fields.
Additionally, it is an object of this invention to provide a muscle
monitoring system which does not require substantial shielding against
electrical fields.
It is a further additional object of this invention to provide a system for
producing a signal coresponding to muscle activity, the system having high
signal-to-noise ratio.
It is yet an additional object of this invention to provide a muscle
activity monitoring and detection system which is simple, inexpensive, and
which can utilize circuitry of the type used to analyse myoelectric
signals.
SUMMARY OF THE INVENTION
The foregoing and other objects are achieved by this invention which
provides in its various embodiments the methods and systems which utilize
acoustic signals generated by muscles during contraction. The acoustic
signals may be used to monitor muscular response, prognosticate the
fatique characteristics of muscles and control prosthetic devices.
In accordance with a method aspect of the invention, a system is provided
for determining fatigue, or excitation-contraction coupling, of a muscle
or group of muscles in a living being. A first signal which is responsive
to the electrical activity in a muscle or muscle group is received and
compared against a second signal which is responsive to the contraction
activity in the muscle or muscle group. The first signal corresponds to
the electrical excitation of the muscle, while the second signal, which in
a preferred embodiment corresponds to an acoustic signal generated within
the living being, is responsive to muscle contraction. The comparison of
the excitation characteristic and the responsive contraction of a muscle,
viewed either at a point in time or over a predetermined period, enables
determination of instantaneous and dynamic excitation-contraction coupling
parameters.
The signal-to-noise ratio of the acoustic signal can be improved by using a
differential circuit arrangement wherein acoustic signals on either side
of a muscle under consideration are combined in a manner which eliminates
noise common to both such signals. In one highly advantageous embodiment
of the invention, the first and second signals are each subjected to the
subtractive combination of differential circuitry, the respective
transducers being coupled to similar, and preferrably identical,
electrical circuitry. Thus, a myoelectric electrode which produces the
first signal, and a microphonic transducer which produces the acoustically
derived signal, may be interchangeable without requiring circuit
modifications.
In accordance with a further method aspect of the invention, dynamic muscle
situations, such as situations which result in poor posture or incorrect
gait can be analyzed by receiving a plurality of acoustically derived
signals from a living body and recording the signals. In a preferred
embodiment, the signals, which correspond to muscular activity at various
body locations, are recorded on a multichannel recorder which produces a
visible representation of the respective waveforms on synchronized time
scales.
In addition to analyzing muscle difficulties of the type noted hereinabove,
an acoustic transducer may be inserted into a living body through a
catheter so as to produce signals corresponding to internal muscle
activity. This is useful in situations such as bladder spasms. The patient
in such a situation can be catherized such that a transducer is inserted
into the bladder, the fluid in the bladder performing as an acoustical
transmission medium, and therefore the particular location and orientation
of the transducer within the bladder is not critical. Conventional
myoelectric techniques cannnot achieve any of these advantages.
Irrespective of the location of the acoustic transducer, the resulting
electrical signals, which are typically analog signals, can be digitized
by circuitry and supplied to a processor for analysis. The analysis of
such myoacoustical signals provides significant advantages over, and
different items of information from, conventional myoelectric signals.
The invention further includes within its scope an arrangement for
monitoring a muscular function in a living body. Such monitoring is
achieved by utilizing an acoustic transducer which converts an acoustic
signal corresponding to a muscle contraction within a living being into a
corresponding electrical signal. An indicator responsive to the electrical
signal produces a perceptible indication responsive to a predetermined
characteristic parameter of the electrical signal. In one embodiment, the
predetermined characteristic parameter which is indicated corresponds to
the amplitude of the electrical signal. Alternatively, the indicated
parameter may correspond to a frequency component of the electrical
signal. Of course, such amplitude and frequency characteristics can be
combined in some embodiments of the invention.
The indicator noted herein may be arranged to be perceptible to the living
being itself. Thus, the present arrangement is suitable for assisting in
biofeedback training of an individual whereby control of a predetermined
muscle function can be learned.
It should be noted that the present invention provides the advantage that
there need not be direct communication between the transducer and the skin
of the living being. This is particularly useful in situations where the
skin has been damaged, such as by fire, and direct contact therewith is
neither appropriate nor desirable. In addition, skin contact can be
avoided where ointments or bandages have been applied to the skin. It is a
highly significant advantage of the present invention that the
myoacoustical signals can be detected even though a fluid is interposed
between the transducer and the skin of the living being, and even if the
living being is immersed in a fluid. In such a situation, it may be
desirable to utilize differential circuitry for cancelling an
objectionable noise propagated through the fluid. It should be noted that
when contact between the transducer and the skin of the living being is to
be avoided, the acoustical signals propagate more readily through a fluid,
than through air. The high difference in density between the skin and the
air causes the acoustical signals to be reflected back into living being
at the interface of the skin and air. However, since a fluid approximates
the density of skin more closely, significantly more signal energy is
propagated through the skin-fluid interface.
It is a highly significant aspect of the present invention that the
acoustical signals generated by contracting muscles can be utilized to
control cybernetic and servo systems, such as prosthetic devices. Of
course, the electrical signals produced by the acoustical transducers can
be utilized to control electromechanical equipment located at a distance
from the living being.
BRIEF DESCRIPTION OF THE DRAWINGS
Comprehension of the invention is facilitated by reading the following
detailed description in conjunction with the annexed drawings, in which:
FIG. 1 is a function block diagram of a system for detecting and processing
myoacoustical signals to produce a signal for driving a prosthesis;
FIG. 2 illustrates waveforms of signals taken at terminals A and B of FIG.
1 during various stages of muscular activity for wrist extensors and
flexors respectively;
FIGS. 3A to 3F illustrate acoustical signal waveforms produced by muscles
contracting against different weights;
FIG. 4 is a graph illustrating the RMS amplitude of myoacoustic signals
plotted against load;
FIG. 5 is a plot illustrating increases in the amplitude of an acoustic
signal in response to the onset of fatigue;
FIGS. 6A and 6B are waveforms of myoacoustic and electromyographic signals
respectively, recorded simultaneously; and
FIG. 7 is a waveform representation of a digitized sample of acoustic data
showing resolution of a single motor unit.
DETAILED DESCRIPTION
FIG. 1 is a function block diagram of a circuit system for producing an
electrical signal responsive to acoustical signals produced by contraction
of muscles in a living being. As shown on this figure, a human arm 10 has
coupled thereto a transducer 12, which may be a microphone, connected to a
preamplifier 13. Preamplifier 13 is connected to a rectifier 14 which
produces at its output a DC signal corresponding to a rectified version of
the acoustic signal. Transducer 12 is arranged in the vicinity of wrist
extensor muscles, while a transducer 12' is arranged in the vicinity of
wrist flexor muscles.
In this particular embodiment, which is directed to driving a prosthetic or
cybernetic device, a high signal-to-noise ratio is achieved by arranging
transducer 12' opposite to transducer 12. Transducer 12' is connected to a
respectively associated preamplifier and a rectifier, 13' and 14'
respectively.
Rectifiers 14 and 14' are coupled to noninverting and inverting inputs,
respectively, of a differential amplifier 16. The differential amplifier
produces at its output a signal which corresponds to the difference
between the outputs of rectifiers 14 and 14'. This signal is conducted to
a bandpass filter which in one embodiment, may be tuned in the vicinity of
20 HZ to 30 HZ. The filtered output signal is conducted to an integrating
circuit 18 which has a relatively long RC time constant, illustratively on
the order of 0.2 to 1 second. The amplitude of the acoustic signals
produced in arm 10 is sufficient for transducers 12 and 12' to produce an
output signal of 50 millivolts.
In the specific illustrative embodiment of FIG. 1, the electric signal at
the output of integrating circuit 18 is conducted to a controller 19 which
may be responsive to the amplitude or the frequency thereof. The resulting
drive signal is conducted to an actuator arrangement 20 which produces a
mechanical force responsive to the selected one of the amplitude or
frequency component of the electric signal. In this manner, a mechanical
drive system responsive to myoacoustically derived information is
achieved.
FIG. 2 illustrates a continuous pair of waveforms taken at terminals A and
B in FIG. 1. Waveforms A and B are obtained via simultaneous acoustic
recording from opposite sides of the forearm of an untrained, normal
subject. The figure illustrates the waveforms produced during the
positions of wrist flexion, wrist extension, and then neutral wrist
position. Moreover, this illustration shows that the acoustic signal has
sufficient signal-to-noise ratio and dynamic range to drive a powered
upper-extremity prosthesis.
FIGS. 3A to 3F show the waveforms of acoustic signals which are produced at
various levels of load. FIG. 3A shows an illustrative waveform with a
muscle at rest. FIG. 3B shows a slight increase in the amplitude of the
waveform as a result of loading the muscle with five pounds. FIGS. 3C to
3F show the waveforms produced with respective loadings of 10 lbs., 12.5
lbs., 15 lbs. and 20 lbs. Clearly, acoustic signal intensity, or
amplitude, increases as loading is applied.
The foregoing is illustrated in the graphical plot of FIG. 4 which shows
the manner in which the RMS magnitude of the acoustic signal increases
with the loading weight.
FIG. 5 is similar to the plot of FIG. 3 but shows the effects of loading
over a period of time. In FIG. 5, five trials were taken, and it is
evident that the RMS amplitude of the acoustic signals increases for each
subsequent trial, as a result of an increasing level of fatigue.
FIGS. 6A and 6B are simultaneously recorded waveforms or myoacoustic and
myoelectric signals, respectively. It is sometimes possible to isolate
individual motor units simultaneously using myoelectric and myoacoustic
signals.
FIG. 7 shows a digitized sample of acoustic data demonstrating the
resolution of a single motor unit. This is provided by flexing a muscle
only very slightly.
Although the invention has been disclosed in terms of specific embodiments
and applications, persons skilled in the art, in light of this teaching,
can generate additional embodiments without exceeding the scope or
departing from the spirit of the claimed invention. Accordingly, it is to
be understood that the drawings and descriptions in this disclosure are
proffered to facilitate comprehension of the invention and should not be
construed to limit the scope thereof.
* * * * *
|
|
 |
|
 |
Description |
|
