Abstract:
A sensor system for detecting and processing EMG signals including a substrate having a bottom surface adapted for attachment to skin; a plurality of spaced apart electrode arrays projecting from the bottom surface so as to engage the skin and detect EMG signals in muscles located under the substrate; and four differential amplifiers connected to receive EMG signals from four distinct pairs of electrode arrays. The electrode arrays detect the action potentials of the muscle fibers from various orientations so that the shape of an action potential appears substantially dissimilar in each of the four differential pairs.

Description:
[0001]     This application claims priority from U.S. Provisional Application Ser. No. 60/610,435 filed Sep. 16, 2004 entitled SURFACE ELECTRODE FOR SELECTIVE SURFACE EMG SIGNALS. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Medical discipline employs armament for diagnostics, quantitative objective techniques and tests to evaluate degrees of insult or dysfunction. The object of this invention is to utilize such techniques in the field of motor disorders. Each year approximately one million Americans are struck with a debilitating motor disorder or are afflicted with a disease which impairs their ability to move, carry out normal activities of daily living, and in various ways degrade their quality of life. The most common disorders among these are Stroke, Spinal Cord Injuries, Head Injuries, Parkinson&#39;s Disease, Multiple Sclerosis, and various forms of paralysis, such as facial palsy. Although neural lesions associated with upper motoneuron disorders can be imaged with MRI and fMRI magnets to indicate the location and size of the lesion, the images do not provide a diagnostic assessment of the degree of impairment and the degree of recovery.  
         [0003]     In addition to these “upper motoneuron” disorders, there are countless “lower motoneuron” dysfunctions such as myestinea gravis. Peripheral nerve injuries caused by trauma and accident, and an annually increasing number of neuromuscular dysfunctions due to neurotoxins in our environment such as Organophosphate based pesticides and insecticides. These later dysfunctions are typically assessed with procedures that require repeated insertion of needles into muscles and probe the tissues for signs of abnormal action potentials. Although numerous attempts have been made to quantify the parameter of the action potentials the procedure remains essentially subjective and very much dependent on the skill and perseverance of the clinician because the procedure is painful and only one or two action potentials are commonly obtained at each site that is tested. The techniques used for these tests have remained essentially unchanged for the past four decades. Patients find these tests stressful and the collected data is often inconclusive because of the limited size and often poor quality.  
         [0004]     The EMG signal is composed of the action potentials (or electrical pulses) from groups of muscle fibers (grouped into functional units called motor units). Refer to the book  Muscles Alive  (5 Th.Ed, 1985) for details. The signal is detected with electrodes placed on the surface of the skin or with needle or wire electrodes introduced into the muscle tissue. The term decomposition is commonly used to describe the process whereby individual motor unit action potentials (MUAPs) are identified and uniquely classified from a set of superimposed motor unit action potentials which constitute the EMG signal. A decomposed EMG signal provides all the information available in the EMG signal. The timing information provides a complete description of the inter-pulse interval, firing rate and synchronization characteristics. The morphology of the shapes of the MUAPs provides information concerning the anatomy and health of the muscle fibers.  
         [0005]     To date, all techniques that have been able to identify individual action potentials in the superimposed EMG signal and provide useful physiological information have used indwelling electrodes to detect the signal.  
         [0006]     Most recently, a quadrifilar indwelling EMG electrode has been used to collect three channels of EMG signals that could be decomposed, partially automatically, to reveal novel aspects of the behavior of the motor unit control properties. The needle version has the advantage of being repositioned after an insertion or being relocated, thereby increasing the probability of obtaining a quality signal that can be decomposed.  
         [0007]     Recently introduced was a wire-electrode version of the quadrifilar electrode. The wire version possesses two advantages: 1) it may be placed in deep muscles located under an overlying layer of muscle, and 2) it generally provides no sensation of discomfort once inserted. But, it has some disadvantages. Once inserted it cannot be precisely relocated within the muscle. One can pull the wire out fractions of a millimeter, but this procedure can only be done once or twice and with little control over the precise placement of the electrode. Both of these types of electrodes have the inherent limitations that: 
    1. They must be inserted into the muscle. This requires a clinical preparation involving sterilization of the electrodes and the needles, sterilization of the environment where the insertion is to be made.     2. They carry the, albeit low, risk of infection.     3. They cause minor damage to the muscle tissue from which they are detecting the signal.     4. They are not well tolerated by individuals who have needle aversion, such as children.     5. Once these electrodes are inserted, the subject must remain very steady. A minor movement of 0.1 mm may cause the shapes of the motor unit action potentials to change, thus precluding the continued identification of a specific unit and generally incapacitating the decomposition algorithms from identifying actions potentials in the remainder of the contraction.    
 
         [0013]     In addition to these technical limitations, some muscles have not been subjected to investigation because needle insertions would be too dangerous or impractical. For example, the motor unit firing properties of muscles of the lips, eye lids, tongue and most facial muscles have never been investigated.  
         [0014]     The object of this invention, therefore, encompasses a surface array electrode sensor that can detect Electromyographic (EMG) signals consisting of identifiable individual action potentials, the characteristics of which are useful for clinical diagnosis. Additionally, when the electrode array is used in conjunction with special technology and signal processing algorithms it will provide an accurate account of the firing times of each action potential belonging to a motor unit. This information will describe the state of the muscle and the Central Nervous System in a manner that is superior to that currently available by techniques in common practice. Although an important application of the surface sensor would be for clinical use, it has applications in other areas such as: 1) Space Medicine—where it is of interest to understand if the control of muscles is altered during and after prolonged exposures to microgravity, 2) Ergonomics—where it is important to learn how muscles are controlled during sustained and/or repetitive tasks so that they may be protected from damage, and 3) Aging—where it is useful to understand how the control to muscle fibers is altered during the process of aging so that techniques and pharmaceuticals could be developed to counteract the process of aging, and, 4) Physiology—where it will provide a new tool for understanding how muscles are controlled.  
       SUMMARY OF THE INVENTION  
       [0015]     The invention is a sensor system for detecting and processing EMG signals including a substrate having a bottom surface adapted for attachment to skin; a plurality of spaced apart electrode arrays projecting from the bottom surface so as to engage the skin and detect EMG signals in muscles located under the substrate; and four differential amplifiers connected to receive EMG signals from four distinct pairs of electrode arrays. The electrode arrays detect the action potentials of the muscle fibers from various orientations so that the shape of an action potential appears substantially dissimilar in each of the four differential pairs. Because of the greater dissimilarity of the shapes of the same action potential, that the compound electrical signal detected from the arrays can be decomposed into individual action potentials.  
         [0016]     According to one feature of the invention, the distinct pairs of electrode arrays are spaced apart in different directions on the substrate. This feature provides particularly valuable test data.  
         [0017]     According to another feature, the distinct pairs of electrode arrays are arranged in an orthogonal pattern. This arrangement provides two orthogonal perspectives of the action potential emanating from fibers that traverse the interior of the array perimeter. The different media in these two directions provides substantially different filtering effects on the action potential, resulting in desirable wave shapes that have different spectral and time dependent characteristics. According to yet another feature, the substrate is elongated in one of the orthogonal directions of said pattern. This feature assists in properly aligning the substrate over muscle being tested.  
         [0018]     According to a further feature, the distinct pairs of electrode arrays are arranged in a radial pattern. This arrangement provides two 45 degrees shifted orthogonal perspectives of the action potential emanating from fibers that traverse the interior of the array perimeter to accommodate the orientation of muscle fibers that are not orthogonal to the perimeter of the array.  
         [0019]     According to another feature, the distinct pairs of electrode arrays include two pairs spaced apart in first aligned directions and two pairs spaced apart in second aligned directions substantially parallel to the first directions. This array arrangement is sensitive to the varying electrical properties of the tissues surrounding the muscle fibers along their length which will have different filtering effects on the action potential.  
         [0020]     According to other important features of the invention, the electrode arrays comprise pins with rounded tips, a uniform diameter in the range between 0.3 mm and 1 mm and a projection length of approximately 2 mm, the system includes decomposition circuitry connected to receive the four channel signal output from the amplifiers; and the substrate is flexible to accommodate flexing over the skin. These features assist further in providing valuable test data.  
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0021]     These and other objects and features of the invention will become more apparent upon a perusal of the following description taken in conjunction with the accompanying drawings wherein:  
         [0022]      FIG. 1  is a block diagram illustrating one embodiment of the invention;  
         [0023]      FIG. 2  is a schematic top view of an electrode array used in the embodiment of  FIG. 1 ;  
         [0024]      FIG. 3  is a side view of the electrode array shown in  FIG. 2 ;  
         [0025]      FIG. 4  is a block diagram of another embodiment of the invention;  
         [0026]      FIG. 5  is a schematic top view of an electrode array used in the embodiment of  FIG. 4 ;  
         [0027]      FIG. 6  is a side view of the electrode array shown in  FIG. 5 ;  
         [0028]      FIG. 7  is a block diagram of another embodiment of the invention;  
         [0029]      FIG. 8  is a schematic top view of an electrode array used in the embodiment of  FIG. 7 ;  
         [0030]      FIG. 9  is a side view of the electrode array shown in  FIG. 8 ; and  
         [0031]      FIG. 10  illustrates four channels of differential signal pairs provided by the embodiment of  FIGS. 1-3 .  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]     A system  11  for detecting and processing EMG signals is shown in the block diagram of  FIG. 1  and includes an electrode section  12 , an amplifier section  13 , a filtering section  14  and a decomposing section  15 . Included in the section  12  is an electrode array ( FIG. 2 ) composed of electrodes A 1 , B 1 , C 1 , and D 1  uniformly spaced apart on a rectangular substrate  16 . Connected by a cable  18  to the electrodes A 1 , B 1 , E 1  and D 1  are, respectively, output terminals TA, TB, TC and TD. As shown in  FIG. 3 , the electrodes are pins having rounded ends and projecting a length l of 2 mm from a bottom surface  19  of the substrate  16 . The terminals TA-TD are connected to section  13  with terminals TA and TB connected to a differential amplifier  21 , terminals TB and TC connected to a differential amplifier  22 , terminals TC and TD connected to a differential amplifier  23 , and terminals TD and TA connected to a differential amplifier  24 . Preferably, the pin electrodes A 1 -D 1  are uniformly spaced apart, as shown, in an orthogonal array with a uniform spacing of between 1.5 mm and 5 mm, preferably a distance of about 3.6 mm. Also, all of the pin electrodes have a diameter of between 0.3 mm and 1 mm.  
         [0033]      FIG. 4  depicts another sensor system  31  in which another electrode section embodiment  32  is connected to an amplifier section  30 , a filtering section  33  and a decomposing section  34 . Included in the section  32  is an electrode array ( FIG. 5 ) composed of electrodes A 4 , B 4 , C 4 , D 4  and E 4  spaced apart on a rectangular substrate  35 . Connected by a cable  36  to the electrodes A 4 -E 4  are, respectively, output terminals TA, TB, TC, TD and TE. As shown in  FIG. 6 , the electrodes are pins having rounded ends and projecting a length l of 2 mm from a bottom surface  35  of the substrate  33 . The terminals TA-TE are connected to the amplifier section  30  a section  13  with terminals TA and TE connected to a differential amplifier  37 , terminals TB and TE connected to a differential amplifier  38 , terminals TC and TE connected to a differential amplifier  39  and terminals TD and TE connected to a differential amplifier  41 . Preferably, the electrode pins A 4 -D 4  are uniformly spaced from the electrode pin E 4  in a radial array and with a uniform spacing of between 1.5 mm and 5 mm and preferably a distance d of about 3.6 mm. Also, all of the pins have a diameter of between 0.3 mm and 1 mm.  
         [0034]      FIG. 7  illustrates another sensor system  51  in which an electrode section embodiment  52  is connected to an amplifier section  50 , a filtering section  53 , and a decomposing section  54 . Included in the section  52  is an electrode array ( FIG. 8 ) composed of electrodes A 7 , B 7 , C 7 , D 7 , E 7  and F 7  spaced apart on a rectangular substrate  55 . Connected by a cable  56  to the electrodes A 4 -F 7  are, respectively, output terminals TA, TB, TC, TD, TE and TF. As shown in  FIG. 6 , the electrodes are pins having rounded ends and projecting a length l of 2 mm from a bottom surface  60  of the substrate  33 . The terminals TA-TF are connected to amplifier section  50  with terminals TA and TB connected to a differential amplifier  57 , terminals TB and TC connected to a differential amplifier  58 , terminals TF and TE connected to a differential amplifier  59  and terminals TD and TE connected to a differential amplifier  61 . Preferably, the electrode pin pairs A 7  and B 7 , B 7  and C 7 , E 7  and F 7 , and D 7  and E 7  are uniformly spaced apart with a spacing of between 1.5 mm and 5 mm and preferably by a distance L of about 2.54 mm. Also, all of the pins again have a diameter of between 0.3 mm and 1 mm.  
         [0035]     In use, one of the surface array electrodes  12 ,  32  or  52  is placed on the skin above the muscle of interest. The electrode selected is determined by both the muscle characteristics to be tested and the particular muscle under test. For example, the electrode array  32  of  FIGS. 4-6  is especially effective when used over muscles with non-parallel fibers such as panate muscle, or the electrode array  52  is especially effective when tests of movement of action potentials along parallel muscle fibers f ( FIGS. 4 and 7 ) are being made. Sufficient pressure is provided to establish good electrical contact as evidenced by the best signal-to-noise ratio of the detected signals. Good electrical contact is accomplished by viewing the detected signal on a computer screen in real time. However, if the signal to noise ratio is poor, it can be improved by applying conductive gel to the tip of the pins. In a typical test, for example, the leads from the electrode pins A 1 -D 1  are connected to the inputs of the differential amplifiers  21 - 24 . A subject or patient is then asked to contract a muscle of interest and over which the substrate  13  is placed. The signals from the surface electrode array  12  are then stored. Next the signals are conditioned by bandpass filtering, usually from 250 Hz to 2 kHz in the section  14  in order to remove any movement artifact at the low end of the spectrum and any excessively long tail that some action potentials have. Depending on the configuration of the electrode array that is used and on the particular muscle being tested, the bandwidth may vary from 100 to 2,000 Hz.  
         [0036]      FIG. 10  illustrates four channels of differential EMG signals detected by the electrode array  12  of  FIG. 1 . The four channels are differential signals provided by the amplifiers  21 - 24  from the electrode pairs A 1 -B 1 , B 1 -C 1 , C 1 -D 2 , and D 1 -A 2 . The signals were detected from the First Dorsal Interosseous muscle in the hand of a male subject. Note that the individual action potentials (pulses) derived from the muscle are clearly visible and identifiable. Some superposition of action potentials from different motor units (having different shapes) does occur. These superpositions, as well as other alterations in the signal, such as gradual modifications in the shape of the action potentials from a particular train, similarities in the shapes of motor units from different motor units, etc. are resolved via special decomposition algorithms.  
         [0037]     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is to be understood, therefore, that the invention can be practiced otherwise than as specifically described.