Muscle fatigue level measuring device

A muscle fatigue level measuring device is provided which estimates muscle fatigue without being influenced by distances between electrodes. The muscle fatigue level measuring device measures a resistance component and a reactance component in a body part as impedance in the body part by impedance component measuring means 21, measures a muscular tissue effective length in the body part by muscular tissue effective length measuring means 22, computes biologically equivalent model parameters including extracellular fluid resistivity and distribution membrane capacitance based on these resistance component, reactance component and muscular tissue effective length by biologically equivalent model parameter computation means 23, and determines a muscle fatigue level based on the ratio of the extracellular fluid resistivity to the distribution membrane capacitance by muscle fatigue level determination means 24.

BACKGROUND OF THE INVENTION

(i) Field of the Invention

The present invention relates to a muscle fatigue level measuring device which estimates the level of fatigue of muscles present in a body part (i.e., muscle fatigue level) based on impedance in the body part.

(ii) Description of the Related Art

Conventional muscle fatigue evaluation devices (muscular fatigue determination devices) place electrodes on a body part so as to measure a change in potential (myoelectric potential) caused by contraction of muscles present in the body part and evaluate (determine) muscle fatigue (muscular fatigue) (for example, refer to Patent Publications 1 and 2).

Patent Publication 1

Patent Publication 2

However, the conventional muscle fatigue evaluation devices (muscular fatigue determination devices) have a problem that the electrodes must be placed at the same distance from each other every time the measurement is made because the magnitude of the measured myoelectric potential varies if the distance between the electrodes placed on the body part varies.

Consequently, an object of the present invention is to solve the above problem of the prior art and provide a muscle fatigue level measuring device which estimates a muscle fatigue level without being influenced by a distance between electrodes.

SUMMARY OF THE INVENTION

To achieve the above object, a muscle fatigue level measuring device of the present invention comprises:impedance component measuring means,muscular tissue effective length measuring means,biologically equivalent model parameter computation means, andmuscle fatigue level determination means,
whereinthe impedance component measuring means measures a resistance component and a reactance component in a body part as impedance in the body part,the muscular tissue effective length measuring means measures a muscular tissue effective length in the body part,the biologically equivalent model parameter computation means computes biologically equivalent model parameters including extracellular fluid resistivity and distribution membrane capacitance based on the resistance component and reactance component measured by the impedance component measuring means and the muscular tissue effective length measured by the muscular tissue effective length measuring means, andthe muscle fatigue level determination means determines a muscle fatigue level based on the ratio between the extracellular fluid resistivity and distribution membrane capacitance computed by the biologically equivalent model parameter computation means.

Further, the impedance component measuring means comprises:current supply means,voltage measuring means, andimpedance component computation means,
whereinthe current supply means supplies alternating currents of multiple frequencies to a body part,the voltage measuring means measures voltages generated in the body part by supplying the alternating currents of multiple frequencies by the current supply means, andthe impedance component computation means computes resistance components and reactance components in the body part by dividing the voltages measured by the voltage measuring means by the currents supplied from the current supply means.

Further, the alternating currents of multiple frequencies are an alternating current with a frequency of 50 kHz and an alternating current with a frequency of 6.25 kHz.

Further, the muscular tissue effective length measuring means comprises:part length measuring means,part width measuring means, andmuscular tissue effective length computation means,
whereinthe part length measuring means measures a part length in the body part,the part width measuring means measures a part width in the body part, andthe muscular tissue effective length computation means computes the muscular tissue effective length in the body part based on the part length measured by the part length measuring means and the part width measured by the part width measuring means.

Further, the muscular tissue effective length computation means computes the muscular tissue effective length by use of the following expression:
Meff=k√{square root over (Ml2×Mw2)}
wherein Meff represents the muscular tissue effective length, Ml represents the part length, Mw represents the part width, and k represents a correction coefficient.

Further, the biologically equivalent model parameter computation means computes extracellular fluid resistivity, intracellular fluid resistivity and distribution membrane capacitance as biologically equivalent model parameters by use of the following expressions:
(R+jX)/Meff=ρr+jρx
wherein R represents the resistance component, jX represents the reactance component, Meff represents the muscular tissue effective length, and ρr and jρx represent a real part and imaginary part of complex resistivity, respectively,
1/(ρr+jρx)=1/Re+1/(Ri+j×2π×f×Cm)
wherein Re represents the extracellular fluid resistivity, Ri represents the intracellular fluid resistivity, Cm represents the distribution membrane capacitance, f represents a measuring frequency, j represents an imaginary number, and π represents a pi.

Further, the muscle fatigue level determination means computes the muscle fatigue level by dividing the extracellular fluid resistivity by the distribution membrane capacitance.

Further, the muscle fatigue level determination means further computes a more accurate muscle fatigue level by considering at least one of personal data including a body weight, a body height, age and sex in addition to the computed muscle fatigue level.

Further, the muscle fatigue level measuring device of the present invention comprises:a main body,first ranging portions,second ranging portions, andelectrode sets,
whereinthe main body serves as a base,the first ranging portions are disposed on the main body such that they can slide freely in a part width direction in a body part so as to measure a part width,the second ranging portions are disposed on the first ranging portions such that they can slide freely in a part length direction in the body part so as to measure a part length, andthe electrode sets comprise current-carrying electrodes and measuring electrodes which are disposed at positions on the second ranging portions which correspond to the part length,the impedance component measuring means includes the electrode sets and measures a resistance component and a reactance component in a body part which is in direct contact with the electrode sets as impedance in the body part, andthe muscular tissue effective length measuring means computes a muscular tissue effective length in the body part based on the part width measured by the first ranging portions and the part length measured by the second ranging portions.

Further, the electrode sets are disposed at the positions on the second ranging portions which correspond to the part length via flexible, elastic members.

Reference numeral1denotes a main body;2and3denote first ranging portions;4(4a,4b) and5(5a,5b) denote second ranging portions;13b,14b,15b, and16bdenote current-carrying electrodes;13c,14c,15c, and16cdenote measuring electrodes;21denotes impedance component measuring means;22denotes muscular tissue effective length measuring means;23denotes biologically equivalent model parameter computation means;24denotes muscle fatigue level determination means; and100denotes a body.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described by use of the drawings.

Firstly, the constitution of a muscle fatigue level measuring device according to the present invention will be described in detail with reference to a functional block diagram shown inFIG. 1, a front view of the device at the time of measurement which is shown inFIG. 2, a plan view thereof which is shown inFIG. 3, a bottom view thereof which is shown inFIG. 4, a right side view thereof which is shown inFIG. 5, a structural block diagram shown inFIG. 6and a biologically equivalent model shown inFIG. 10. The outer shape of the muscle fatigue level measuring device of the present invention is roughly formed by a main body1, first ranging portions2and3, and second ranging portions4and5(4aand4b, and5aand5b).

The main body1constitutes the base of the present device and has a cylindrical shape. It incorporates an electronic circuit board on which an electric power source31, an oscillator32, a V/I converter33, a current-carrying electrode selector34, a measuring electrode selector35, an amplifier36, a filter37, an A/D converter38, a storage unit39, a display unit40and a CPU41are mounted.

The first ranging portions2and3serve as jaws for measuring a part width in a body part and each have a prismatic shape. The portions2and3are attached to the main body1such that they can slide freely along the direction of the cylinder axis of the main body1(or the direction of the part width in the body part) and extend in a direction perpendicular to the cylinder axis direction of the main body1(or the direction of the part width in the body part).

Further, first electrodes21aand22aare provided on the guide surface of the main body1, and second electrodes21band22bare provided on the guide surfaces of the first ranging portions2and3. These first electrodes21aand22aand second electrodes21band22bform first encoders21and22. Just like known electronic slide calipers, they detect relative displacement magnitudes between the main body1and the first ranging portions2and3and output the data to the CPU41.

The second ranging portions4and5serve as jaws for measuring a part length in a body part and comprise a pair of prismatic members4aand4band a pair of prismatic members5aand5b, respectively. The members4aand5aare attached to the members4band5bsuch that the members4aand5acan slide freely inside the members4band5b, respectively. The second ranging portions4and5are disposed at the front ends of the first ranging portions2and3such that the prismatic members4aand5aslide in a direction perpendicular to the cylinder axis direction of the main body1and the prism axis directions of the first ranging portions2and3(or the direction of the part length in the body part).

Further, third electrodes23aand24aare provided on the guide surfaces of the prismatic members4aand5a, and fourth electrodes23b(not shown) and24bare provided on the guide surfaces of the prismatic members4band5b. These third electrodes23aand24aand fourth electrodes23band24bform second encoders23and24. Just like known electronic slide calipers, they detect relative displacement magnitudes between the prismatic members4aand5aand the prismatic members4band5band output the data to the CPU41.

In addition, the second ranging portion4has an adjustment screw6, and the second ranging portion5has an adjustment screw7and various switches8. The adjustment screws6and7fit screw grooves formed in the prismatic members4band5band press the prismatic members4aand5athrough the prismatic members4band5bso as to fix the positions of the prismatic members4aand5aand the prismatic members4band5bwhen the prismatic members4aand5aslide within the prismatic members4band5b. The switches8comprise an ON/OFF switch8awhich switches between supplying electric power to units in the electrical system and stopping supplying the electric power to the units, an UP switch8bwhich increases a numerical value displayed on the display unit40at the time of setting of various data such as personal data (e.g., a body weight, a body height, age and sex), a DOWN switch8cwhich decreases a numerical value displayed on the display unit40at the time of setting of various data such as personal data (e.g., a body weight, a body height, age and sex), and a SET switch8dwhich sets a numerical value selected by use of the UP switch8bor the DOWN switch8c.

Further, at the front ends of the prismatic members4a,5a,4band5bof the second ranging portions4and5, electrode sets are disposed via springs9,10,11and12such that they face inward in the cylinder axis direction of the main body1. The electrode sets comprise plates13a,14a,15aand16a, current-carrying electrodes13b,14b,15band16b, and measuring electrodes13c,14c,15cand16c. These electrodes are placed on the corresponding plates.

Further, the oscillator32generates constant voltages of different frequencies (50 kHz and 6.25 kHz). The V/I converter33receives the constant voltages generated from the oscillator32, converts the constant voltages to constant currents and outputs the constant currents. The current-carrying electrode selector34receives the constant currents output from the V/I converter33and outputs the constant currents to the given current-carrying electrodes13band16bor14band15b. Further, the current-carrying electrodes13b,14b,15band16bserve as communication ports for passing the constant currents supplied from the current-carrying electrode selector34through a body part. The oscillator32, the V/I converter33, the current-carrying electrode selector34and the current-carrying electrodes13b,14b,15band16bconstitute current supply means for supplying alternating currents of multiple frequencies (50 kHz and 6.25 kHz) through a body part.

Further, the measuring electrodes13c,14c,15cand16cserve as communication ports for detecting voltages caused by impedance in the body part when the currents are passed through the body part from the current-carrying electrodes13b,14b,15band16b. The measuring electrode selector35receives the voltages from the given measuring electrodes14cand15c,13cand16cand outputs the voltages. The amplifier36receives the voltages caused by the impedance in the body part from the measuring electrode selector35and amplifies and outputs the voltages. The filter37receives the voltages from the amplifier36and allows only specific frequency components (50 kHz and 6.25 kHz) to pass therethrough. The A/D converter38converts the voltages having passed through the filter37from analog signals to digital signals and outputs the digital signals to the CPU41. These measuring electrodes13c,14c,15cand16c, the measuring electrode selector35, the amplifier36, the filter37and the A/D converter38constitute voltage measuring means for measuring voltages of alternating currents of multiple frequencies (50 kHz and 6.25 kHz) occurring in a body part.

Further, the storage unit39stores various setting data set by means of the SET switch8d, and the display unit40displays various setting data and various measurement data.

Further, the CPU41serves as impedance component computation means, part width computation means, part length computation means, muscular tissue effective length computation means, biologically equivalent model parameter computation means23, and muscle fatigue level determination means24. The CPU41also controls units in the electrical system and performs computations in a known manner.

The impedance component computation means divides the voltages of alternating currents of multiple frequencies (50 kHz and 6.25 kHz) received from the A/D converter38by the currents passing through the body part from the current supply means so as to compute a resistance component and a reactance component in the body part for each frequency (i.e., 50 kHz and 6.25 kHz). The above current supply means, voltage measuring means and impedance component computation means constitute impedance component measuring means21for measuring a resistance component and a reactance component in a body part as impedance in the body part.

The part width computation means computes a part width Mw based on outputs received from the first encoders21and22, and the part length computation means computes a part length Ml based on outputs received from the second encoders23and24. The muscular tissue effective length computation means substitutes the part width Mw computed by the part width computation means and the part length Ml computed by the part length computation means into an expression 7 so as to compute a muscular tissue effective length Meff in the body part.
Meff=k √{square root over (Ml2×Mw2)}
wherein k is a correction coefficient (e.g., ½1/2which is experimentally predetermined).

The above main body1, first ranging portions2and3and part width computation means constitute part width measuring means for measuring a part width Mw in a body part. Further, the above second ranging portions4and5and part length computation means constitute part length measuring means for measuring a part length Ml in a body part. Further, the above part width measuring means, part length measuring means and muscular tissue effective length computation means constitute muscular tissue effective length measuring means22for measuring a muscular tissue effective length in a body part.

The biologically equivalent model parameter computation means23substitutes the resistance component R and the reactance component jX which have been computed for each frequency (50 kHz and 6.25 kHz) by the impedance component computation means and the muscular tissue effective length Meff which has been computed by the muscular tissue effective length computation means into an expression 8 for each frequency (50 kHz and 6.25 kHz) so as to compute a real part ρr and imaginary part jρx of complex resistivity for each frequency (50 kHz and 6.25 kHz).
(R+jX)/Meff=ρr+jρx

Further, the biologically equivalent model parameter computation means23substitutes the real part ρr and imaginary part jρx of complex resistivity which have been computed for each frequency (50 kHz and 6.25 kHz), a measuring frequency f, an imaginary number j and a pi π into an expression 9 for each frequency, and three equations are established based on the expression for each frequency (50 kHz and 6.25 kHz) so as to compute extracellular fluid resistivity Re, intracellular fluid resistivity Ri and distribution membrane capacitance Cm. The extracellular fluid resistivity Re, intracellular fluid resistivity Ri or distribution membrane capacitance Cm is called a biologically equivalent model parameter.
1/(ρr+jρx)=1/Re+1/(Ri+j×2π×f×Cm)

The muscle fatigue level determination means24divides the extracellular fluid resistivity Re which has been computed by the biologically equivalent model parameter computation means23by the distribution membrane capacitance Cm so as to compute (determine) a muscle fatigue level K. Then, the muscle fatigue level determination means substitutes the computed muscle fatigue level K and personal data (body weight W, body height H, age Ag and sex S) set by the SET switch8dinto an expression 10 so as to compute (determine) a more accurate muscle fatigue level Kh.
Kh=K×f1(W)×f2(H)×f3(Ag)×f4(S)
wherein functions f1, f2, f3and f4are functions derived in advance from data collected from a number of subjects.

Next, the procedure for using the muscle fatigue level measuring device according to the present invention and the operation of the device will be specifically described with reference to a main flowchart shown inFIG. 7, a subroutine flowchart of measurement of impedance in a body part which is shown inFIG. 8, and a subroutine flowchart of determination of a muscle fatigue level in a body part which is shown inFIG. 9.

Firstly, at the press of the ON/OFF switch8a, electric power is supplied to units in the electrical system from the electric power source31(STEP S1).

Then, numeric values corresponding to personal data (body weight, body height, age and sex) are selected by the UP switch8bor the DOWN switch8c. Once the numeric values are set by the SET switch8d, the set personal data (body weight, body height, age and sex) are stored in the storage unit39(STEP S2).

Then, after the relative positions of the prismatic members4aand5aand prismatic members4band5bof the second ranging portions are determined and fixed by the adjustment screws6and7, the second encoders23and24detect relative displacement magnitudes between the prismatic members4aand5aand the prismatic members4band5b. The CPU (part length computation means)41computes an average (part length Ml) of a distance between the electrode sets13and14and a distance between the electrode sets15and16of the second ranging portions based on the detected relative displacement magnitudes (STEP S3).

Then, the electrodes13b,13c,14b,14c,15b,15c,16band16care brought into direct contact with a body part whose muscle fatigue level is desired to be determined by adjusting a distance between the first ranging portions2and3. Then, the first encoders21and22detect relative displacement magnitudes between the main body1and the first ranging portions2and3. The CPU (part width computation means)41computes a distance (part width Mw) between the electrode sets13(14) and15(16) based on the detected relative displacement magnitudes (STEP S4).

Then, impedance in the body part is measured (STEP S5). To state more specifically, the oscillator32generates a constant voltage of 50 kHz under the control of the CPU41. Receiving the constant voltage, the V/I converter33changes the constant voltage to a constant current and outputs the constant current (STEP S21). Then, S=1 is registered in a register (STEP S22).

Then, under the control of the CPU41, the current-carrying electrode selector34selects the current-carrying electrodes13band16b, and the measuring electrode selector35selects the measuring electrodes14cand15c(STEP S23). Then, the constant current output from the V/I converter33passes through the body part situated between the current-carrying electrodes13band16, and at that time, the measuring electrodes14cand15cdetect a voltage produced in the body part (STEP S24).

Receiving the detected voltage, the amplifier36amplifies and outputs the detected voltage. Receiving the amplified voltage, the filter37allows only a frequency component corresponding to 50 kHz to pass therethrough. Then, on the receipt of the voltage of 50 kHz, the A/D converter38converts the analog signal to a digital signal and outputs the digital signal. Receiving the digital signal, the CPU (impedance component computation means)41computes a resistance component and a reactance component in the body part by dividing the voltage of 50 kHz by the current passing through the body part from the current supply means (STEP S25).

Then, it is determined based on the variations of the resistance and reactance components in the body part whether the condition of contact between the electrodes and the body part are good (STEP S26). When the variations are higher than given values, it is determined that the contact condition is not good, and STEPS S24and S25are carried out again (NO in STEP S26). Meanwhile, when the variations are lower than or equal to the given values, it is determined that the contact condition is good, and the subsequent step is carried out (YES in STEP S26).

Then, it is determined whether the measurement is stable or not, based on continuation of the variations of the resistance and reactance components in the body part which are lower than or equal to the given values (STEP S27). When the variations lower than or equal to the given values are not continued over a given time, it is determined that the measurement is not stable, and STEPS S24to S26are carried out again (NO in STEP S27). Meanwhile, when the variations lower than or equal to the given values are continued over the given time, it is determined that the measurement is stable, and the subsequent step is carried out (YES in STEP S27).

Then, values displayed on the screen of the display unit40are updated with values of the resistance and reactance values in the body part when the measurement is determined to be stable (STEP S28), and the values of the resistance and reactance values in the body part when the measurement is determined to be stable are stored in the storage unit39(STEP S29).

Then, the register increases the currently registered value S by 1 and registers the updated value S (STEP S30) and determines whether the updated S is larger than 2 or not (STEP S31). When S is not larger than 2, STEPS S23to S30are carried out again (NO in STEP S31). In STEP S23in this case, the current-carrying electrode selector34selects the current-carrying electrodes14band15b, and the measuring electrode selector35selects the measuring electrodes13cand16c, under the control of the CPU41. Meanwhile, when S is larger than 2, the subsequent step is carried out (YES in STEP S31).

Then, it is determined whether a constant current passing through the body part is 6.25 kHz (i.e., whether a constant voltage generated by the oscillator is 6.25 kHz) (STEP S32). When it is not 6.25 kHz (NO in STEP S32), generation of constant voltage of 50 kHz by the oscillator32under the control of the CPU41is stopped (STEP S33), a constant voltage of 6.25 kHz is generated (i.e., a constant current of 6.25 kHz is passed through the body part), and STEP S22and subsequent steps thereof are carried out again (STEP S34). Meanwhile, when it is 6.25 kHz (YES in STEP S32), generation of the constant voltage of 6.25 kHz by the oscillator32under the control of the CPU41is stopped (STEP S33), and the measurement of the impedance in the body part is terminated (STEP S35).

Then, a muscle fatigue level is computed (STEP S6). To be more specific, the CPU (muscular tissue effective length computation means)41substitutes the part length Ml computed in STEP S3and the part width Mw computed in STEP S4into the expression7so as to compute a muscular tissue effective length Meff in the body part (STEP S51).

Then, the CPU (biologically equivalent model parameter computation means23)41substitutes the muscular tissue effective length Meff, the resistance component R and reactance component jX at the time of measurement at 50 kHz stored in the storage unit39, and a resistance component R and reactance component jX at the time of measurement at 6.25 kHz into the expression 8 so as to compute a real part ρr and imaginary part jρx of complex resistivity at the time of measurement at 50 kHz and a real part ρr and imaginary part jρx of complex resistivity at the time of measurement at 6.25 kHz (STEP S52).

Then, based on a first expression resulting from substitution of the real part ρr and imaginary part jρx of the complex resistivity at the time of measurement at 50 kHz, an imaginary number j and a pi π into the expression 9 and a second expression resulting from substitution of the real part ρr and imaginary part jρx of the complex resistivity at the time of measurement at 6.25 kHz, an imaginary number j and a pi π into the expression 9, the CPU (biologically equivalent model parameter computation means23)41establishes three equations excluding the imaginary part jρx of the complex resistivity in the second expression so as to compute extracellular fluid resistivity Re, intracellular fluid resistivity Ri and distribution membrane capacitance Cm as biologically equivalent model parameters (STEP S53).

Then, out of these biologically equivalent model parameters, the CPU (muscle fatigue level determination means24)41divides the extracellular fluid resistivity Re by the distribution membrane capacitance Cm so as to compute (determine) a muscle fatigue level K (STEP S54). Then, the CPU (muscle fatigue level determination means24)41substitutes the muscle fatigue level K and the personal data (body weight, body height, age and sex) set in STEP S2into the expression 10 so as to compute (determine) a more accurate muscle fatigue level Kh, whereby the computation of the muscle fatigue level is completed (STEP S55).

Then, the display unit40displays the muscle fatigue level K or more accurate muscle fatigue level Kh on the screen (STEP S7). At the press of the ON/OFF switch8a, the power source31stops supplying electric power to the units in the electrical system, whereby a series of procedural steps of the present device are completed (STEP S8).

As described above, the muscle fatigue level measuring device of the present invention measures a resistance component and a reactance component in a body part as impedance in the body part by the impedance component measuring means21, measures a muscular tissue effective length in the body part by the muscular tissue effective length measuring means22, computes biologically equivalent model parameters including extracellular fluid resistivity and distribution membrane capacitance based on these resistance component, reactance component and muscular tissue effective length by biologically equivalent model parameter computation means23, and determines a muscle fatigue level based on the ratio of the extracellular fluid resistivity to the distribution membrane capacitance by the muscle fatigue level determination means24. Thus, the muscle fatigue level measuring device of the present invention can obtain a muscle fatigue level with high accuracy since it obtains the muscle fatigue level in consideration of the muscular tissue effective length in the body part which is a distance between the electrodes.

Further, data serving as a basis for computing the muscular tissue effective length in the body part can be obtained by the first ranging portions2and3for measuring a part width which are disposed on the main body1such that they can slide freely in a part width direction in a body part and the second ranging portions4and5for measuring a part length which are disposed on the first ranging portions2and3such that they can slide freely in a path length direction in the body part. Thereby, the measurement can be made more easily and more accurately.

In the above description, springs were used between the electrode sets and the second ranging portions so as to have the electrodes contact with the body part in good condition. However, the present invention can also be practiced by using rubber or other flexible, elastic members in place of the springs.

As described above, the muscle fatigue level measuring device of the present invention has the muscular tissue effective length measuring means by which the device can obtain a muscle fatigue level in consideration of muscular tissue effective length in a body part. Thus, the obtained muscle fatigue level is highly accurate.

Further, the muscle fatigue level measuring device of the present invention measures a part width and a part length by the main body, first ranging portions and second ranging portions constituted such that distances in a body part can be changed so as to obtain a muscular tissue effective length in the body part. Thus, the muscle fatigue level can be obtained more easily and more accurately.

Further, it can also be easily achieved to increase accuracy by use of conventionally used electromyography in combination with the present invention.