Patent Publication Number: US-2018043154-A1

Title: Muscle training device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The contents of the following Japanese patent application are incorporated herein by reference. 
     Japanese Patent Application No. 2016-158006 filed on Aug. 10, 2016. 
     FIELD 
     The present invention relates to a muscle training device utilizing electrical muscle stimulation (EMS) for applying an electrical stimulation signal to a body surface above a muscle to expand and contract the muscle. More specifically, the present invention relates to a muscle training device configured to apply an optimal electrical stimulation signal according to a muscle condition to effectively train a muscle. 
     BACKGROUND 
     A muscle training device configured to apply an electrical stimulation signal to a muscle to expand and contract the muscle for training for the purpose of muscle strength enhancement and dieting has been typically known (JP-A-2005-348859).  FIGS. 8 and 9  illustrate a muscle training device  100  described in JP-A-2005-348859. For training of an abdominal muscle, four stimulation electrodes  101   a,    101   b,    101   c ,  101   d  closely contact a body surface of the abdomen. 
     An electrical stimulation output part  102  is configured to generate an electrical stimulation signal output to each of the four stimulation electrodes  101   a,    101   b,    101   c ,  101   d  based on an instruction from a controller  103 . For example, the electrical stimulation output part  102  outputs an electrical stimulation signal of 1000 Hz to between the stimulation electrodes  101   a,    101   c  arranged on one diagonal line and included in the four stimulation electrodes  101   a,    101   b,    101   c,    101   d,  and outputs an electrical stimulation signal of 900 Hz to between the stimulation electrodes  101   b,    101   d  arranged on the other diagonal line. This generates an interference wave of 10 Hz between adjacent ones of the stimulation electrodes  101 . Accordingly, an abdominal muscle of such a portion is strengthened by expansion and contraction thereof 
     In a storage part  104  connected to the controller  103 , correlation data between the frequency and current value of the electrical stimulation signal is stored. The controller  103  instructs the electrical stimulation output part  102  to generate the electrical stimulation signal according to the correlation data stored in the storage part  104 . Moreover, a display  105  displays the degree of stimulation by the electrical stimulation signal output to each of the stimulation electrodes  101   a,    101   b,    101   c,    101   d . While viewing such a displayed indication, a user operates, when feeling painful due to an electrical stimulation, an operation part  106  to lower an output level such as the current value of the electrical stimulation signal generated by the electrical stimulation output part  102  via the controller  103 . 
     Moreover, a muscle training device  110  configured to apply an electrical stimulation signal to a muscle for training and to obtain a muscle composition in an objective manner has been also known (JP-A-2016-49241). As illustrated in  FIG. 10 , the muscle training device  110  is configured such that a pair of stimulation electrodes  111   a,    111   b  and a photoelectric sensor  112  between the stimulation electrodes  111   a,    111   b  are arranged in close contact with a body surface above a muscle to be trained. The photoelectric sensor  112  can measure the concentration of oxygen in the muscle under the body surface to which the photoelectric sensor  112  closely contact. The concentration of oxygen in the muscle changes in response to an electrical stimulation applied from the pair of stimulation electrodes  111   a,    111   b  to the muscle. Thus, information on a muscle composition (e.g., a ratio between a slow muscle and a fast muscle) is obtained from a relationship between the strength of the electrical stimulation and the measured oxygen concentration. 
     Thus, the muscle training device  110  is operable between the following two types of modes: a training mode in which the electrical stimulation signal is output from a device body  113  to between the stimulation electrodes  111   a,    111   b,  and a load due to the electrical stimulation is provided to the muscle under the body surface to perform training; and a measurement mode in which while the strength of electrical stimulation of the muscle by the pair of stimulation electrodes  111   a,    111   b  is being changed, the concentration of oxygen in the muscle is measured by the photoelectric sensor  112  to obtain the information on the muscle composition in the device body  113 . For example, according to the ratio between the slow muscle and the fast muscle in the measurement mode, the strength of the electrical stimulation provided to the muscle in the training mode is adjusted. 
     SUMMARY 
     Technical Problem 
     In the typical muscle training device  100  utilizing the electrical muscle stimulation EMS, the stimulation electrodes  101  closely contact the body surface above the muscle to be trained. However, the position of the muscle covered with the body surface cannot be accurately determined. For this reason, the position of the muscle and the electrical stimulation position shift from each other, and the electrical stimulation cannot be effectively applied to the muscle. Particularly for outputting an electrical stimulation signal to between stimulation electrodes as a pair of positive and negative electrodes to effectively expand and contract a muscle, the positive and negative electrodes need to closely contact a body surface along the longitudinal direction of the muscle. Thus, the device needs to rely on a skilled user&#39;s experience to determine such a close contact position. 
     Moreover, an action state such as the degree of fatigue of the muscle to be trained cannot be determined in an objective manner, and for this reason, the strength of the electrical stimulation and a stimulation time cannot be optimally adjusted according to the state of action of the muscle. 
     According to the muscle training device  110 , the strength of electrical stimulation to the muscle can be optimally set according to the composition of the muscle to be trained. However, for evaluation of the muscle composition, the photoelectric sensor  112  needs to be separately prepared and to closely contact the body surface. In addition, the photoelectric sensor  112  and the pair of stimulation electrodes  111   a,    111   b  closely contact different positions of the body surface. Thus, unlike information on the composition of the muscle to which the electrical stimulation is actually provided, an optimal strength of electrical stimulation might not be able to be applied based on the muscle composition information obtained from the photoelectric sensor  112 . 
     Further, the photoelectric sensor  112  only obtains the muscle composition information from the relationship between the electrical stimulation strength and the muscle oxygen concentration. For this reason, the degree of fatigue of the muscle due to the electrical stimulation and positional shift between the electrical stimulation position of the stimulation electrode and the muscle position cannot be determined, for example. Thus, the electrical stimulation cannot be applied to the muscle in an optimal manner considering the above. 
     The present invention has been made in view of the above-described typical problems, and intends to provide a muscle training device configured to provide an electrical stimulation to a muscle in an optimal manner according to the state of action of the muscle without the need for closely contacting a separate photoelectric sensor to a body surface above the muscle. 
     Moreover, the present invention intends to provide a muscle training device being able to closely contact a stimulation electrode to a proper position of a body surface to effectively apply an electrical stimulation to a muscle. 
     Further, the present invention intends to provide a muscle training device configured to provide an electrical stimulation to a muscle with an optimal strength for an optimal time according to the degree of fatigue of the muscle. 
     Solution to Problem 
     For solving the above-described problems, the muscle training device according to a first aspect is a muscle training device for applying a first electrical stimulation signal for training to a body surface above a muscle to expand and contract the muscle by the first electrical stimulation signal. Such a muscle training device includes an insulating sheet configured such that a plurality of electrodes are exposed on an inner surface closely contacting the body surface above the muscle, a first stimulation signal output unit configured to output the first electrical stimulation signal to a stimulation electrode which is at least any of the plurality of electrodes, a myoelectric signal detection unit configured to detect, while output of the first electrical stimulation signal is stopped, a muscle action potential shown at a myoelectric detection electrode which is at least any of the plurality of electrodes, and a muscle condition determination unit configured to determine the state of action of the muscle from the muscle action potential of the myoelectric detection electrode. 
     The plurality of electrodes provided on the insulating sheet can be used in combination as the stimulation electrode for training and the myoelectric detection electrode for determination of the muscle action state. 
     In the muscle training device according to a second aspect, the stimulation electrode to which the first electrical stimulation signal is output and/or the method for outputting the first electrical stimulation signal are/is adjusted according to the state of action of the muscle determined by the muscle condition determination unit. 
     The stimulation electrode to which the first electrical stimulation signal is output and/or the method for outputting the first electrical stimulation signal are/is adjusted according to the state of action of the muscle without the need for providing a separate myoelectric detection electrode in addition to the stimulation electrode for training. 
     In the muscle training device according to a third aspect, the plurality of electrodes dispersedly exposed on a two-dimensional plane along the inner surface of the insulating sheet serve as myoelectric detection electrodes, and the stimulation electrode to which the first electrical stimulation signal is output is selected from ones of the plurality of myoelectric detection electrodes from which a relatively-high muscle action potential has been detected. 
     Since the myoelectric detection electrodes are dispersedly exposed on the two-dimensional plane along the inner surface of the insulating sheet, any of the myoelectric detection electrodes closely contacts the body surface near the muscle. Since the muscle generates the muscle action potential in association with expansion and contraction thereof, a relatively-high muscle action potential is detected from the myoelectric detection electrode closely contacting the body surface near the muscle. Such a myoelectric detection electrode serves as the stimulation electrode, and therefore, the first electrical stimulation signal is output to the body surface near the muscle regardless of the bonded position of the insulating sheet to the body surface. 
     The muscle action potential is a potential formed in such a manner that action potentials generated at a plurality of muscle fibers overlap with each other, and is transmitted at 3 to 5 m/sec along the longitudinal direction of the muscle formed of the plurality of muscle fibers. Such a transmission velocity decreases as fatigue of the muscle increases. Any of the myoelectric detection electrodes from which a relatively-high muscle action potential has been detected closely contacts the body surface along the muscle. The transmission velocity of the muscle action potential is obtained from a time difference in detection of the muscle action potential by each myoelectric detection electrode and an already-provided distance between the myoelectric detection electrodes. Then, the muscle condition determination unit determines the degree of fatigue of the muscle in a quantitative manner. 
     In the muscle training device according to a fourth aspect, the plurality of electrodes dispersedly exposed at positions in a grid pattern on the inner surface of the insulating sheet serve as the myoelectric detection electrodes. 
     Since the plurality of myoelectric detection electrodes are dispersedly exposed at the positions in the grid pattern, it is ensured that any of the myoelectric detection electrodes closely contacts the body surface along the longitudinal direction of the muscle and serves as the myoelectric detection electrode from which a relatively-high muscle action potential is detected. 
     In the muscle training device according to a fifth aspect, the plurality of electrodes dispersedly exposed at positions in a concentric pattern on the inner surface of the insulating sheet serve as the myoelectric detection electrodes. 
     The plurality of electrodes are dispersedly exposed on the two-dimensional plane of the bottom surface. Thus, even when the body surface near the muscle to be trained is curved, at least any of the electrodes closely contacts the body surface near the muscle. 
     In the muscle training device according to a sixth aspect, a stimulation sub-electrode including at least one electrode closely contacts the body surface along the muscle in the vicinity of the body surface to which the insulating sheet closely contacts. The first electrical stimulation signal output unit causes one of the stimulation electrode or the stimulation sub-electrode on the insulating sheet to serve as a positive electrode and causes the other one of the stimulation electrode or the stimulation sub-electrode on the insulating sheet to serve as a negative electrode, thereby outputting the first electrical stimulation signal to between the positive and negative electrodes. 
     The stimulation electrode and the stimulation sub-electrode on the insulating sheet can closely contact distant positions of the body surface along the muscle, and therefore, the first electrical stimulation signal flows between the stimulation electrode and the stimulation sub-electrode along the direction of the muscle fibers of the muscle. 
     In the muscle training device according to a seventh aspect, a pair of insulating sheets closely contact two body surface portions along the muscle, and any of a plurality of electrodes exposed on an inner surface of one of the insulating sheets serves as a stimulation sub-electrode. 
     The electrodes exposed on the inner surface of each insulating sheet serve as the myoelectric detection electrodes, and the state of action of the muscle at each position of the body surface closely contacting the insulating sheet can be determined. 
     Since any of the plurality of electrodes exposed on the inner surface of one of the insulating sheets serves as the stimulation sub-electrode, the first electrical stimulation signal flows along the direction of the muscle fibers of the muscle between the stimulation electrode and the sub-electrode apart from each other on the body surface above the muscle. 
     The muscle training device according to an eighth aspect further includes a second stimulation signal output unit configured to apply, while output of the first electrical stimulation signal is stopped, a second electrical stimulation signal for generating an induced muscle action potential to the body surface along the muscle in the vicinity of the body surface to which the insulating sheet closely contacts. The myoelectric signal detection unit detects the induced muscle action potential shown at the myoelectric detection electrode by application of the second electrical stimulation signal. 
     The M-wave of the induced muscle action potential induced by the second electrical stimulation signal is transmitted along the direction of the muscle fibers of the muscle from the body surface to which the second electrical stimulation signal is applied, and is detected by each of the myoelectric detection electrodes closely contacting different positions of the body surface along the muscle. 
     The transmission velocity of the induced muscle action potential for evaluation of the degree of fatigue of the muscle in a quantitative manner is obtained from a time at which the M-wave reaches each myoelectric detection electrode. 
     According to the first aspect of the invention, the state of action of the muscle to be trained can be determined without the need for bonding a separate device such as a photoelectric sensor to the body surface. 
     According to the second aspect of the invention, stimulation strength and a stimulation time by the first electrical stimulation signal can be adjusted such that the muscle is trained in an optimal manner according to the state of action of the muscle determined by the muscle condition determination unit. 
     According to the third aspect of the invention, the first electrical stimulation signal can be applied to the body surface above the muscle without time and effort for closely contacting the stimulation electrode to the body surface along the muscle. 
     Moreover, the myoelectric detection electrode from which a relatively-high muscle action potential has been detected closely contacts the vicinity of the muscle. Thus, the relative transmission velocity of the muscle action potential indicating the degree of fatigue of the muscle is obtained from the time difference in detection of the muscle action potential by the myoelectric detection electrode, and the strength and output time of the first electrical stimulation signal can be adjusted according to the degree of fatigue of the muscle determined by the muscle condition determination unit. 
     According to the fourth aspect of the invention, the first electrical stimulation signal can be reliably output to the stimulation electrode closely contacting the body surface near the muscle regardless of the direction of the insulating sheet bonded to the body surface. 
     According to the fifth aspect of the invention, at least any of the electrodes closely contacts the body surface near the muscle. Thus, even when the body surface near the muscle to be trained is curved, a relatively-high muscle action potential can be detected from the closely-contacting electrode, and a training stimulation signal can be effectively applied to the muscle. 
     According to the sixth aspect of the invention, even when the muscle has such a size that the insulating sheet cannot cover the muscle, the first electrical stimulation signal flows in the direction of the muscle fibers of the muscle such that the muscle expands and contracts. 
     According to the seventh aspect of the invention, the state of action of the muscle at each position closely contacting the pair of insulating sheets can be determined. Moreover, the first electrical stimulation signal flows in the direction of the muscle fibers of the muscle through the muscle having such a size that the single insulating sheet cannot cover the muscle, and therefore, the muscle can expand and contract. 
     According to the eighth aspect of the invention, the induced muscle action potential is generated at the muscle by the second electrical stimulation signal. Thus, the muscle action potential can be reliably detected by the myoelectric detection electrode distinctively from noise of an electric signal of an instruction from the brain and an electric signal generated from a muscle due to an exercise load. 
     Moreover, the transmission velocity of the induced muscle action potential induced by the second electrical stimulation signal only reflects a change in a peripheral state, and delays due to fatigue of the muscle. Thus, the muscle can be trained by output of the first electrical stimulation signal in an optimal manner according to fatigue of the muscle for accurate quantitative evaluation. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a reference diagram of a use state of a muscle training device  1  of a first embodiment of the invention of the present application; 
         FIG. 2  is a block diagram of the muscle training device  1 ; 
         FIG. 3  is a reference diagram of a use state of a muscle training device  20  of a second embodiment; 
         FIG. 4  is a reference diagram of a use state of a muscle training device  30  of a third embodiment; 
         FIG. 5  is a block diagram of a muscle training device  40  of a fourth embodiment; 
         FIG. 6  is a reference diagram of a use state of the muscle training device  40 ; 
         FIG. 7  is a bottom view of an insulating sheet  5 ; 
         FIG. 8  is a block diagram of a typical muscle training device  100 ; 
         FIG. 9  is a view of a use state of the muscle training device  100 ; and 
         FIG. 10  is a view of a configuration of a typical muscle training device  110 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A muscle training device  1  of a first embodiment of the present invention will be described below with reference to  FIGS. 1 and 2 . As illustrated in  FIG. 1 , the muscle training device  1  is a device configured to expand and contract a muscle  50  by application of an electrical stimulation signal to the muscle  50 , thereby training the muscle  50 . In the present specification, training of the muscle  50  means, regardless of purposes, electrical muscle stimulation (EMS) for expanding and contracting the muscle  50  by application of a later-described training stimulation signal as an electrical stimulation signal to a body surface above the muscle  50 . Thus, the muscle training device  1  may be any of the following devices configured to expand and contract the muscle  50  for various purposes: a muscle training device for the purpose of dieting for surrounding fat combustion by expansion and contraction of a muscle  50 ; a muscle training device for the purpose of enhancing muscle strength; and a medical muscle training device configured to regenerate a damaged muscle  50 . 
     The muscle training device  1  includes an insulating sheet  4  to which four electrodes  2   a,    2   b,    2   c,    2   d  for providing an electrical stimulation to the muscle  50  under the body surface are attached in an insulated state, and a training device body  10  configured to output, to any of the four electrodes  2   a,    2   b,    2   c,    2   d,  a training stimulation signal as an electrical stimulation signal for training via a connection cable  7 . 
     The outer shape of the insulating sheet  4  is defined by an elongated band-shaped flexible printed circuit board (FPC) so that the insulating sheet  4  can be positioned on the body surface along a muscle fiber direction of the muscle  50  to be trained. The four electrodes  2   a,    2   b,    2   c,    2   d  are, in the insulated state, formed integrally with the flexible insulating sheet  4  made of PET etc. The four electrodes  2   a,    2   b,    2   c,    2   d  are exposed at regular intervals on a bottom surface of the insulating sheet  4 , i.e., an inner surface of the insulating sheet  4  facing the body surface, and are each connected to a corresponding common terminal of a later-described switch  13  via a wiring pattern of the FPC and the connection cable  7 . A not-shown double-faced tape is bonded to the substantially entire bottom surface of the insulating sheet  4  other than the portion where the four electrodes  2   a,    2   b,    2   c,    2   d  are exposed. An adhesive layer exposed by detachment of release paper of the double-faced tape is bonded to the body surface above the muscle  50  to be trained, and in this manner, the insulating sheet  4  is positioned. As described above, the four electrodes  2   a,    2   b,    2   c,    2   d  closely contact the body surface facing these electrodes. 
     As illustrated in  FIG. 2 , the training device body  10  includes the switch  13  configured to switch, according to a mode switch signal output from a controller  3 , each common terminal between a first selection terminal  13   a  connected to an output of an electrical stimulation output part  14  and a second selection terminal  13   b  connected to an input of a multiplexer  15 , each common terminal being connected to a corresponding one of the four electrodes  2   a,    2   b,    2   c,    2   d.  When receiving the mode switch signal for selecting a particular electrode  2  to make transition to a training mode, the switch  13  switchably connects a common terminal connected to such an electrode  2  to the first selection terminal  13   a,  and the selected electrode  2  serves as a stimulation electrode to which the training stimulation signal is output from the electrical stimulation output part  14 . For example, when the electrodes  2   a,    2   b  are selected by the mode switch signal for making transition to the training mode, the common terminals connected respectively to the electrodes  2   a,    2   b  are each switched to the first selection terminal  13   a , and then, the training stimulation signal is output to each of the electrodes  2   a,    2   b . When the mode switch signal for selecting a particular electrode  2  to make transition to a muscle condition determination mode is output from the controller  3  to the switch  13 , a common terminal connected to the selected electrode  2  is switched to the second selection terminal  13   b,  and such an electrode  2  serves as a myoelectric detection electrode configured to detect a muscle action potential. 
     In response to a control signal input from the controller  3 , the multiplexer  15  alternately connects ones of the four types of second selection terminals  13   b  to an amplification part  11  connected to a later stage of the multiplexer  15 , the ones of the second selection terminals  13   b  being connected respectively to the electrodes (the myoelectric detection electrodes)  2  selected by the mode switch signal for making transition to the muscle condition determination mode. Then, the multiplexer  15  outputs a muscle action potential of a myoelectric signal detected at each myoelectric detection electrode  2  to a muscle condition determination part  12  via the amplification part  11  and an A/D conversion part  16 . That is, the muscle action potential of the myoelectric signal detected from the myoelectric detection electrode  2  closely contacting the body surface is a minute value. For this reason, after amplification at the amplification part  11 , such a potential is converted into a digital value at the A/D conversion part  16  so that the muscle condition determination part  12  can make determination. 
     The muscle condition determination part  12  is configured to determine, from the level (the muscle action potential) and amplitude of the myoelectric signal detected by each myoelectric detection electrode  2 , the position of the muscle  50  to be trained under the body surface and the state of fatigue of the muscle  50  due to training. 
     The position of the muscle  50  to be trained is estimated from the position of the body surface closely contacting the myoelectric detection electrode  2  from which a relatively-high muscle action potential has been detected. That is, among muscles, a skeletal muscle being able to voluntarily act under the control of brain nerves generates, in response to a stimulation from motor nerves, the muscle action potential at a plurality of muscle fibers forming the muscle, and then, contracts to generate tension. Thus, in the case where the muscle  50  to be trained is the skeletal muscle, a high muscle action potential is detected from the myoelectric detection electrode  2  closely contacting the body surface near the muscle  50 , and the muscle condition determination part  12  estimates from many myoelectric detection electrodes  2  that the muscle  50  is present near the myoelectric detection electrode  2  from which a relatively-high muscle action potential has been detected. 
     When fatigue of the muscle  50  increases due to training, the number of mobilization activities and a firing frequency per motor unit for making up for contractility of the muscle  50  increase, and the amplitude of the myoelectric signal increases. Thus, a change in the amplitude of the myoelectric signal detected from the myoelectric detection electrode  2  having estimated as closely contacting the vicinity of the muscle  50  in the above-described manner is monitored, and the degree of fatigue of the muscle  50  is determined from an amplitude increase. 
     The velocity of transmission of the muscle action potential transmitted along the longitudinal direction (the muscle fiber direction) of the muscle  50  becomes lower as fatigue of the muscle  50  increases. Thus, the transmission velocity along the muscle  50  may be detected from a time difference in detection of the myoelectric signals having the same waveform by the plurality of myoelectric detection electrodes  2  closely contacting the vicinity of the muscle  50 , and in this manner, the degree of fatigue of the muscle  50  may be determined. 
     The muscle condition determination part  12  is connected to the controller  3  to output a determination result of the muscle  50  by the muscle condition determination part  12  to the controller  3 , and the controller  3  inputs, to the muscle condition determination part  12 , information on the position of each myoelectric detection electrode  2  closely contacting the vicinity of the muscle  50  for determination of the degree of fatigue of the muscle  50  by the muscle condition determination part  12 . 
     The controller  3  operates in any of the training mode and the muscle condition determination mode. The controller  3  is connected to a storage part  17  configured to store the determination result of the muscle condition determination part  12  and a series of operation program of the controller  3 , an operation part  18  allowing a user to start the muscle training device  1  and to perform input operation for stopping operation, a display  19  configured to display, e.g., the progress of training and the degree of fatigue detected by the muscle condition determination part  12 , and the electrical stimulation output part  14 . When receiving a start-up instruction from the operation part  18 , the controller  3  performs sequence control of each part of the muscle training device  1  such that the muscle training device  1  executes a series of operation from the muscle condition determination mode to the training mode. 
     Under the control of the controller  3  in the training mode, the electrical stimulation output part  14  generates the training stimulation signal to output the generated training stimulation signal to each first selection terminal  13   a  of the switch  13 . The training stimulation signal has an AC signal waveform with a current value of about 10 to 80 mA providing no feeling of electrical stimulation to the user and with a frequency of 10 to 30 Hz, for example. The waveform and frequency of the training stimulation signal may vary according to the progress of the training mode to obtain an optimal training effect. 
     The method for training the skeletal muscle  50  by using the muscle training device  1  will be described below together with advantageous effects of the muscle training device  1 . In the case where the user trains the skeletal muscle  50 , the adhesive layer exposed by detachment of the release paper from the bottom surface of the insulating sheet  4  is bonded to the body surface in the vicinity of the muscle  50  to be trained, and in this manner, the four electrodes  2   a,    2   b,    2   c,    2   d  closely contact the body surface. Subsequently, input operation for starting the muscle training device  1  is performed for the operation part  18 . 
     After the muscle training device  1  has been started, the controller  3  first operates in the muscle condition determination mode, and then, outputs, to the switch  13 , the mode switch signal for selecting all of the four electrodes  2   a,    2   b,    2   c,    2   d  to make transition to the muscle condition determination mode. When receiving such a mode switch signal, the switch  13  switchably connects, to the second selection terminal  13   b , each common terminal connected to a corresponding one of the four selected electrodes  2   a,    2   b,    2   c,    2   d.  Then, each of the electrodes  2   a,    2   b,    2   c,    2   d  serves as the myoelectric detection electrode configured to detect the muscle action potential. 
     Subsequently, when the user voluntarily expands and contracts the skeletal muscle  50 , the muscle action potential is generated at the plurality of muscle fibers forming the muscle  50 , and a relatively-high muscle action potential is detected from each myoelectric detection electrode  2  closely contacting the vicinity of these muscle fibers. The controller  3  outputs, to the switch  13 , the mode switch signal for making transition to the above-described muscle condition determination mode, and serially connects, to the amplification part  11 , the second selection terminals  13   b  connected respectively to the electrodes  2   a,    2   b,    2   c,    2   d  selected by such a mode switch signal. Thus, after the muscle action potential detected from each of the electrodes  2   a,    2   b,    2   c ,  2   d  has been amplified in the amplification part  11 , the resultant is converted into the digital value in the A/D conversion part  16 , and then, is output to the muscle condition determination part  12 . Then, relative comparison is made for the resultant. The muscle action potential detected from each of the myoelectric detection electrodes  2   a ,  2   b,    2   c  is a relatively-higher muscle action potential than that of the myoelectric detection electrode  2   d,  supposing that the myoelectric detection electrodes  2   a,    2   b,    2   c  closely contact the vicinity of the muscle  50  and that the myoelectric detection electrode  2   d  closely contacts the body surface apart from the muscle  50 . Thus, the muscle condition determination part  12  estimates, from such a comparison result, that the muscle  50  is along the electrodes  2   a,    2   b,    2   c  under the body surface. Then, the determination result including the amplitude of the myoelectric signal detected from each of the myoelectric detection electrodes  2   a,    2   b,    2   c  is output to the controller  3 . In comparison of the muscle action potential detected from each myoelectric detection electrode  2 , a value obtained by subtracting an abnormal value from a muscle action potential detected multiple times may be targeted for comparison, or the myoelectric detection electrode  2  having detected a muscle action potential equal to or higher than a predetermined potential may serve as the myoelectric detection electrode  2  having detected a relatively-high muscle action potential. 
     After the controller  3  has output, to the switch  13 , the mode switch signal for making transition to the muscle condition determination mode, when the determination result of the position of the muscle  50  is input from the muscle condition determination part  12 , such a determination result is primarily stored in the storage part  17 , and the electrodes  2   a,    2   b,    2   c  estimated from the determination result as closely contacting the vicinity of the muscle  50  are selected as the stimulation electrodes. Then, the controller  3  outputs, to the switch  13 , the mode switch signal for selecting the stimulation electrodes  2   a,    2   b,    2   c  to make transition to the training mode. After or before this point, the controller  3  controls the electrical stimulation output part  14  to generate and output the training stimulation signal. 
     The switch  13  having received the mode switch signal switchably connects, to the first selection terminal  13   a,  each common terminal connected to a corresponding one of the selected electrodes  2   a,    2   b,    2   c,  and the electrodes  2   a,    2   b,    2   c  serve as the stimulation electrodes  2   a,    2   b,    2   c  to which the training stimulation signal is output from the electrical stimulation output part  14 . Thus, the training stimulation signal is output to the electrodes  2  closely contacting the vicinity of the muscle  50  so that the electrical stimulation can be effectively applied to the muscle  50  without the need for considering such bonded position and orientation of the insulating sheet  4  that each electrode  2  closely contacts the vicinity of the muscle  50  to be trained. 
     Note that all of the myoelectric detection electrodes  2  having detected a relatively-high muscle action potential in the muscle condition determination mode do not necessarily serve as the stimulation electrodes  2  to which the training stimulation signal is output. Of the myoelectric detection electrodes  2   a,    2   b,    2   c  having detected a high muscle action potential, one of the myoelectric detection electrodes  2   a,    2   c  closely contacting distant positions may serve as a positive electrode, and the other one of the myoelectric detection electrodes  2   a,    2   c  may serve as a negative electrode. The training stimulation signal may be output to between these electrodes, for example. 
     The controller  3  may terminate the training mode after a predetermined training time has been elapsed. However, in the present embodiment, the training mode is temporarily switched to the muscle condition determination mode in the process of the training mode, and then, the state of fatigue of the muscle  50  due to training is checked. Such mode switching is performed as follows: the indication of transition to the muscle condition determination mode is displayed on the display  19 , and information on the electrodes  2   a,    2   b,    2   c  estimated from the determination result stored in the storage part  17  as closely contacting the vicinity of the muscle  50  is read; and then, the mode switch signal for selecting these electrodes  2   a,    2   b,    2   c  to make transition to the muscle condition determination mode is output to the switch  13 . 
     When the user having checked the indication on the display  19  voluntarily expands and contracts the muscle  50 , the muscle action potential is detected from each of the myoelectric detection electrodes  2   a,    2   b,    2   c  near such a muscle. After each potential value has been amplified in the amplification part  11 , the resultant is converted into the digital value in the A/D conversion part  16 , and the digital value is input to the muscle condition determination part  12 . The muscle condition determination part  12  compares the amplitude of the myoelectric signal of a series of muscle action potentials detected from one or more of the myoelectric detection electrodes  2   a,    2   b,    2   c  with the amplitude before the training mode. Then, the muscle condition determination part  12  determines the degree of fatigue of the muscle  50  from an amplitude increase state, and outputs such a determination result to the controller  3 . Note that the relative positions of the myoelectric detection electrodes  2   a,    2   b,    2   c  closely contacting the vicinity of the muscle  50  have been already provided, and therefore, the degree of fatigue of each portion of the muscle  50  can be more accurately determined from these relative positions and a change in the amplitude of the myoelectric signal shown at each of the myoelectric detection electrodes  2   a,    2   b,    2   c.    
     The controller  3  displays, on the display  19 , the determination result indicating the degree of fatigue of the muscle  50  to notify the user of the determination result. Moreover, when the determination result shows that fatigue of the muscle  50  does not reach a target value in training, the controller  3  further switches the muscle condition determination mode to the training mode to repeat electrical stimulation to the above-described muscle  50 . When the degree of fatigue of the muscle  50  reaches the target value in training, the controller  3  displays such a state on the display  19  to terminate operation of the muscle training device  1 . 
     A muscle training device  20  of a second embodiment of the present invention is configured such that a plurality of electrodes  21  are dispersedly exposed on a two-dimensional plane of a bottom surface of an insulating sheet  22 , and will be described below with reference to  FIG. 3 . The same reference numerals as those of the above-described muscle training device  1  are used to represent equivalent elements of the muscle training device  20  functioning in a manner identical or similar to that of the muscle training device  1 , and description thereof will not be repeated. 
     The insulating sheet  22  used for the muscle training device  20  has a horizontally-elongated rectangular profile as viewed in the figure, and the plurality of electrodes  21  (m, n) are dispersedly attached to the insulating sheet  22  in the state in which these electrodes are exposed and positioned in a matrix of four columns and six rows on the bottom surface of the insulating sheet  22 . The same number of common terminals and the same number of pairs of first and second selection terminals  13   a,    13   b  switchably connected to a corresponding one of the common terminals as the number of electrodes  21  (m, n) attached to the insulating sheet  22 , i.e.,  24  common terminals and  24  pairs of first and second selection terminals  13   a,    13   b,  are arranged at a switch  13  of a training device body  10 , and each common terminal is connected to a corresponding one of the electrodes  21  (m, n) via a connection cable  7 . 
     In the case where a muscle  50  is trained by the muscle training device  20 , the insulating sheet  22  is first bonded to a body surface near the muscle  50 . Then, a user operates an operation part  18  to start the device  20  and to voluntarily expand and contract the muscle  50  to be trained. Right after start-up of the muscle training device  20 , a controller  3  operates in a muscle condition determination mode to output, to the switch  13 , a mode switch signal for selecting all of the electrodes  21  (m, n) to make transition to the muscle condition determination mode. As a result, all of the electrodes  21  (m, n) serve as myoelectric detection electrodes  21 , and muscle action potentials detected from all of the electrodes  21  (m, n) are serially input to a muscle condition determination part  12 . 
     When each muscle action potential detected from ones of the electrodes  21  (m, n) indicated by cross marks in  FIG. 3  is a relatively-higher muscle action potential as compared to those of the other ones of the electrodes  21  (m, n), the muscle condition determination part  12  estimates that the electrodes  21  (m, n) indicated by the cross marks closely contact the body surface near the muscle  50 , and determines that the muscle  50  is present in a muscle direction  50   c  illustrated in the figure. Then, the muscle condition determination part  12  outputs such a determination result to the controller  3 . 
     After having received the determination result of the position of the muscle  50  from the muscle condition determination part  12 , the controller  3  transitions to a training mode. Then, the controller  3  outputs, to the switch  13 , a mode switch signal for selecting each electrode  21  (m, n) (m=1 to 2, n=4 to 6) surrounded by a dashed line  21 A in the figure as a positive stimulation electrode and selecting each electrode  21  (m, n) (m=3 to 4, n=1 to 3) surrounded by a dashed line  21 B in the figure as a negative stimulation electrode to make transition to the training mode. Accordingly, a training stimulation signal flows along the muscle direction  50   c  indicated by the determination result. Thus, the training stimulation signal for expanding and contracting the muscle  50  flows, from an electrical stimulation output part  14 , between the group of the six positive electrodes  21  (m, n) surrounded by the dashed line  21 A in the figure and the group of the six negative electrodes  21  (m, n) surrounded by the dashed line  21 B in the figure. 
     As in the first embodiment, for checking the state of fatigue of the muscle  50  due to training in the process of the training mode, the controller  3  is temporarily switched to the muscle condition determination mode so that the muscle action potentials of the electrodes  21  (m, n) serving as the myoelectric detection electrodes can be detected. Then, the muscle condition determination part  12  determines the state of fatigue of the muscle  50  based on comparison of the amplitude of a myoelectric signal of a series of muscle action potentials with the amplitude right after start-up. At this point, connection of the switch  13  is switchably connected to each second selection terminal  13   b,  and the electrodes  21  (m, n) serving as the myoelectric detection electrodes, such as the electrodes  21  (m, n) surrounded by the dashed line  21 A,  21 B in the figure or the electrodes  21  (m, n) indicated by the cross marks, can be optionally selected from all of the  24  electrodes  21  (m, n). 
     As described above, the myoelectric detection electrodes and the stimulation electrodes are not necessarily the same myoelectric detection electrodes  21  (m, n), and can be optionally selected from all of the electrodes  21  (m, n) according to applications or purposes. 
     In the case where the entirety of the muscle  50  to be trained cannot be covered with the single insulating sheet  22 , a plurality of insulating sheets are bonded to different positions of the body surface above the muscle  50  so that the muscle  50  can be trained by application of the training stimulation signal across a large area. A muscle training device  30  of a third embodiment using two insulating sheets  32 A,  32 B will be described below with reference to  FIG. 4 . Note that the same reference numerals as those of the above-described muscle training devices  1 ,  20  are used to represent equivalent elements of the muscle training device  30  functioning in a manner identical or similar to those of the muscle training devices  1 ,  20 , and description thereof will not be repeated. 
     As illustrated in  FIG. 4 , the muscle training device  30  includes the two insulating sheets  32 A,  32 B, i.e., the insulating sheet  32 A on which nine electrodes  31 A (m, n) are dispersedly exposed and positioned in a matrix of three columns and three rows on a bottom surface, and the insulating sheet  32 B on which nine electrodes  31 B (m, n) are dispersedly exposed and positioned in a matrix of three columns and three rows on a bottom surface. Each of the electrodes  31 A (m, n) and the electrodes  31 B (m, n) is connected to a corresponding one of common terminals of a switch  13  via a connection cable  7 , and is selectively connectable to any of a first selection terminal  13   a  and a second selection terminal  13   b.  Each of these electrodes can serve as a myoelectric detection electrode and a stimulation electrode according to a mode switch signal output from a controller  3  to the switch  13 . 
     In the present embodiment, the controller  3  operates in a training mode right after the muscle training device  30  has been started. The mode switch signal for selecting all of the electrodes  31 A (m, n) exposed on the insulating sheet  32 A as positive stimulation electrodes and selecting all of the electrodes  31 B (m, n) exposed on the insulating sheet  32 B as negative stimulation electrodes to make transition to the training mode is output to the switch  13  such that a training stimulation signal flows across a large area of a muscle  50 . Accordingly, the training stimulation signal for extending/contacting the muscle  50  flows, from an electrical stimulation output part  14 , between the group of the nine positive stimulation electrodes  31 A (m, n) exposed on the insulating sheet  32 A and the group of the nine negative stimulation electrodes  31 B (m, n) exposed on the insulating sheet  32 B. Thus, an electrical stimulation can be, for training, applied to a large area of the muscle  50  which cannot be covered with a single insulating sheet  32 . 
     In the process of the training mode, the controller  3  may be switched to a muscle condition determination mode, and some electrodes  31  of the stimulation electrodes  31 A (m, n),  31 B (m, n) may serve as the myoelectric detection electrodes. The amplitude of each myoelectric signal detected from these myoelectric detection electrodes may be monitored, and the degree of fatigue of the muscle  50  may be determined from a change in the amplitude of the myoelectric signal. 
     In each of the above-described embodiments, when the user voluntarily expands and contracts the skeletal muscle  50 , the position of the muscle  50  and the degree of fatigue of the muscle  50  are, in the muscle condition determination mode, determined from the level of the muscle action potential generated at the muscle  50  and the amplitude of the myoelectric signal of a series of muscle action potentials. Thus, the muscle  50  to be trained is limited to the skeletal muscle, and the user needs to voluntarily expand and contract the muscle  50  every time the state of action of the muscle  50  is evaluated. For this reason, a muscle stimulation induction signal different from the training stimulation signal may be output to the vicinity of the body surface closely contacting the myoelectric detection electrode, and an induced muscle action potential generated at the muscle  50  by an electrical stimulation of the muscle stimulation induction signal may be detected by the myoelectric detection electrode. Then, the state of action of the muscle  50  may be evaluated from the waveform (the M-wave) of the induced muscle action potential. 
     A muscle training device  40  configured to output a muscle stimulation induction signal in a muscle condition determination mode according to a fourth embodiment of the present invention will be described below with reference to  FIGS. 5 and 6 . The same reference numerals as those of the above-described muscle training devices  1 ,  20 ,  30  are used to represent equivalent elements of the muscle training device  40  functioning in a manner identical or similar to those of the muscle training devices  1 ,  20 ,  30 , and description thereof will not be repeated. 
     An electrical stimulation output part  41  of the muscle training device  40  is configured to generate two types of electrical stimulation signals, i.e., a training stimulation signal and a muscle stimulation induction signal. The electrical stimulation output part  41  outputs the training stimulation signal to a first selection terminal  13   a  of a switch  13  when a controller  3  operates in a training mode, and outputs the muscle stimulation induction signal to the first selection terminal  13   a  of the switch  13  when the controller  3  operates in a muscle condition determination mode. The training stimulation signal generated in the electrical stimulation output part  41  is an electrical stimulation signal having an AC signal waveform with a current value of about 10 to 80 mA and a frequency of 10 to 30 Hz as in the training stimulation signal output from the electrical stimulation output part  14 . Moreover, the muscle stimulation induction signal generated in the electrical stimulation output part  41  to generate an induced muscle action potential is an electrical stimulation signal having a rectangular wave with a current value of equal to or greater than 5 mA, a voltage of 100 V, and a pulse width of 0.5 msec, for example. 
     After start-up of the muscle training device  40 , the controller  3  first operates in the training mode to control output of the training stimulation signal from the electrical stimulation output part  41 . Moreover, a mode switch signal for selecting all of electrodes  31 A (m, n) exposed on an insulating sheet  32 A as positive stimulation electrodes and selecting all of electrodes  31 B (m, n) exposed on an insulating sheet  32 B as negative stimulation electrodes to make transition to the training mode is output to the switch  13  such that the training stimulation signal flows along a muscle  50 . Accordingly, the training stimulation signal for expanding and contracting the muscle  50  for training thereof flows, from the electrical stimulation output part  41 , between the group of the positive stimulation electrodes  31 A (m, n) exposed on the insulating sheet  32 A and the group of the negative stimulation electrodes  31 B (m, n) exposed on the insulating sheet  32 B. 
     Subsequently, the controller  3  is switched to the muscle condition determination mode at a constant frequency in the process of the training mode. Then, control is made such that output of the training stimulation signal from the electrical stimulation output part  41  is stopped and that the muscle stimulation induction signal is generated and output. Moreover, a mode switch signal for selecting, as positive stimulation sub-electrodes  42   a,  three electrodes  31 A (m, 3) surrounded by a dashed line  42 A of  FIG. 6  and exposed in a third row on the insulating sheet  32 A, selecting, as negative stimulation sub-electrodes  42   b,  three electrodes  31 A (m, 1) surrounded by a dashed line  42 B of the figure and exposed in a first row on the insulating sheet  32 A, and selecting, as myoelectric detection electrodes, all of the nine electrodes  31 B (m, n) exposed on the insulating sheet  32 B to make transition to the muscle condition determination mode is output from the controller  3  to the switch  13 . Accordingly, each common terminal connected to a corresponding one of the electrodes  31 A (m, 3) surrounded by the dashed line  42 A is switchably connected to a first selection terminal  13   a  to which a training stimulation signal (+) is output, each common terminal connected to a corresponding one of the electrodes  31 A (m, 1) surrounded by the dashed line  42 B is switchably connected to a first selection terminal  13   a  to which a training stimulation signal (−) is output, and all of the electrodes  31 B (m, n) exposed on the insulating sheet  32 B are switchably connected respectively to second selection terminals  13   b  connected to an input of a multiplexer  15 . 
     The negative stimulation sub-electrode  42   b  and the positive stimulation sub-electrode  42   a  in a pair are exposed respectively on the right and left sides of the elongated band-shaped insulating sheet  32 A. The positive electrodes  42   a  closely contact a body surface on an outer side with respect to the myoelectric detection electrodes  31 B (m, n) exposed on the insulating sheet  32 B, and the negative electrodes  42   b  closely contact the body surface on an inner side with respect to the positive electrodes  42   a.  When the muscle stimulation induction signal flows between the group of the positive electrodes  42   a  closely contacting the body surface above the muscle  50  and the group of the negative electrodes  42   b  closely contacting the body surface above the muscle  50 , such a muscle stimulation induction signal flows, during application thereof, inward through the positive electrodes  42   a,  flows in a longitudinal direction in nerve fibers, and flows outward in the negative electrodes  42   b.  Outward current induces excitation, and therefore, it is typically considered that the induced muscle action potential is generated in the vicinity of the negative electrodes  42   b  when signal application begins. Subsequently, the induced muscle action potential is also generated in the vicinity of the positive electrodes  42   a.  However, since the muscle stimulation induction signal is a short stimulation with 0.5 msec as described above, only the negative electrodes  42   b  has a stimulation effect when signal application begins. Subsequently, the induced muscle action potential is, along the direction of muscle fibers of the muscle  50 , transmitted from the closely-contact positions of the negative electrodes  42   b  to the myoelectric detection electrodes  31 B (m, n) exposed on the insulating sheet  32 B. 
     The waveform of such an induced muscle action potential is the waveform of a combination of waves called “M-waves” and indicating electric action of many muscle fibers of the muscle  50  to be evaluated. A muscle condition determination part  12  monitors the M-wave detected at each of the myoelectric detection electrodes  31 B (m, n), and obtains, every time the controller  3  transitions to the muscle condition determination mode in the process of the training mode, a time (a latent time) after application of the muscle stimulation induction signal to the pair of stimulation sub-electrodes  42   a,    42   b  until detection of rising of the M-wave in each of the myoelectric detection electrodes  31 B (m, n). The latent time may be obtained for any of the myoelectric detection electrodes  31 B (m, n), or may be obtained from an average for the plurality of myoelectric detection electrodes  31 B (m, n). 
     The transmission velocity of the M-wave of the muscle action potential induced by an electrical stimulation decreases with an increase in fatigue of the muscle  50  due to training. On the other hand, the transmission velocity of the M-wave is in inverse proportion to the latent time because a distance from an electrical stimulation position (the stimulation sub-electrode  42   b ) to each of the electrodes  31 B (m, n) is constant. Thus, the muscle condition determination part  12  compares the latent time obtained every time the controller  3  transitions to the muscle condition determination mode, thereby evaluating the degree of fatigue of the muscle  50 . 
     The controller  3  displays, on a display  19 , an evaluation result of the degree of fatigue of the muscle  50 , the evaluation result being input from the muscle condition determination part  12  every time the controller  3  transitions to the muscle condition determination mode. Moreover, when the degree of fatigue of the muscle  50  reaches a certain value, the controller  3  stops subsequent operation in the training mode to terminate training using the muscle training device  40 . 
     According to the muscle training device  40 , the electrical stimulation is, at a constant frequency, applied from each pair of stimulation sub-electrodes  42   a,    42   b  to the muscle  50  during training so that the induced muscle action potential induced by the electrical stimulation can be, even during training, detected distinctively from an unstable muscle action potential generated by a nerve stimulation from the brain. Moreover, the muscle  50  does not need to be consciously expanded and contracted every time the state of action of the muscle  50  is determined. Further, even when the muscle  50  is other muscles than a skeletal muscle, the state of action of the muscle  50  can be determined from the induced muscle action potential shown at each of the myoelectric detection electrodes  31 B (m, n). 
     In each of the above-described embodiments, the plurality of electrodes are dispersedly exposed at the linearly-arranged positions of the bottom surface of the insulating sheet or the positions in a matrix on the bottom surface of the insulating sheet. However, the profile of the insulating sheet and the positions of the plurality of exposed electrodes can be optionally set. For example, in the case where a plurality of electrodes are dispersedly exposed on a two-dimensional plane of a bottom surface of an insulating sheet, the insulating sheet  5  may be in a discoid shape, and the plurality of electrodes  6  may be, on the bottom surface of the insulating sheet  5 , dispersedly exposed in a concentric pattern at positions on a plurality of circles concentric with the center of the discoid shape, as illustrated in  FIG. 7 . 
     According to the above-described insulating sheet  5 , the plurality of electrodes  6  are dispersedly exposed on the two-dimensional plane of the bottom surface. Thus, even when the body surface near the muscle  50  to be trained is curved, any of the electrodes  6  closely contacts the vicinity of the muscle  50 , and a relatively-high muscle action potential is detected from such an electrode  6 . As a result, a training stimulation signal can be effectively applied to the muscle  50 . 
     In the above-described embodiments, each electrode attached to the insulating sheet and the training device body  10  are connected together via the connection cable  7 . However, a communication interface configured to perform two-way wireless communication may be provided at each of the insulating sheet and the training device body  10  such that wireless connection among the plurality of electrodes and the common terminals of the switch  13  is realized. 
     Moreover, the bottom surface of the insulating sheet is, with the adhesive layer, bonded to the body surface above the muscle  50 . However, as long as the insulating sheet can be at a predetermined position of the body surface, the insulating sheet may be positioned in such a manner that the insulating sheet is, using a band etc., wound around the body surface. 
     Further, part or the entirety of the configuration of the training device body  10  is wearable with such a configuration being disposed in a device to be attached to the body with a band etc. 
     The embodiments of the present invention are suitable for a muscle training device configured to train a muscle by application of an electrical stimulation.