Method and apparatus for measuring body fat

[0002] This application claims the benefit under 35 U.S.C. § 119 of U.S. Provisional Patent Application No. 60/239,469 filed Sep. 30, 2000.

[0003] The present invention relates to an improved method and apparatus for measuring the body fat of a live subject, and more particularly, of a living human being.

[0004] Body composition, and in particular percent body fat, is a well-recognized measure of physical health. Various techniques of determining percent body fat are presently in use, including caliper measurements, underwater displacement measurement, and bioelectrical impedance measurement.

[0005] In this last technique, as described in U.S. Pat. Nos. 5,415,176 and 5,611,351, both to Sato et al., a live subject whose body fat is to be measured stands upon a scale-like device with four electrodes mounted on its upper surface. A 50 kHz, 800 microampere electric current is produced by constant current source. This current is passed first through two electrodes in contact with the subject's toes and then through two reference resistors located in series with the subject's body and with each other. The electric current flowing through the subject causes a voltage potential to develop across the subject's heels.

[0006] Using a microprocessor-controlled switch array and a voltage measuring circuit, the heel-to-heel voltage is measured via two other electrodes in contact with the subject's heels. The voltages across the reference resistors are also measured while the electric current is applied to the subject's toes. A comparison of the voltages measured across the reference resistors with the heel-to-heel voltage provides a highly accurate measure of the heel-to-heel impedance. After certain additional parameters such as age, weight, and height are entered into the microprocessor, it calculates body density using an algorithm relating body impedance and the additional parameters to body density. Once body density is obtained, the microprocessor performs a second calculation to convert body density to percent body fat.

[0007] The present inventors have identified several disadvantages of the device taught by Sato et al. First, the 800 microampere current produces a relatively strong electric field that is centered on the current-supply electrodes. This field distribution restricts the location of the current-supply electrodes with respect to the voltage-detecting electrodes: the two sets of electrodes are required to be at least 5 cm apart. This distance limitation can present a problem in the body fat measurement of small children.

[0008] Second, the electrodes in the device of Sato et al. are flat and quite large, to accommodate a range of adult feet sizes. The pressure of the subject's weight on his feet, which are in contact with the wide, flat surface of the electrodes, restricts blood flow to the tissues above the electrodes. It is believed that this constriction causes the measured body impedance to be artificially increased, creating a source of error in the body fat measurement. Furthermore, the large electrodes of the device of Sato et al. causes the equipment to appear clinical and unaesthetic, and not at all user-friendly.

[0009] Third, the reference resistors in the device of Sato et al. are placed in series with each other and with the subject's body. This configuration limits the device's durability and reliability, for if the connection between any of the resistors or the current supply electrodes is accidentally broken, the device becomes entirely unable to function. In addition, the device is only capable of measuring combinations of reference resistors which are adjacent to each other. This arrangement thus limits the resolution of the measurement.

[0010] Finally, the current-supply electrodes in the device of Sato et al. contact the subject's toes, while the voltage-measurement electrodes contact his heels. Thus, the current flows from the toes through the feet toward the heels and then up through the legs. The measured body impedance thus includes the toe-heel impedance of each foot. Because toe-heel impedance is not included as an independent variable in most equations correlating body impedance and percent body fat, however, variations in foot size and foot impedance from subject to subject introduce additional error in the body fat calculation.

[0011] Accordingly, it is an object of the invention to overcome these and other problems associated with the prior art, and to provide an accurate, robust, high-resolution body fat analyzer that may be used by a wide range of subjects.

[0012] In accordance with one embodiment of the present invention, a subject's body composition is measured by (1) supplying a 50 kHz, 300 microampere electrical current to the subject's body via a group of current-supply electrodes that contact the subject's heels, (2) measuring the voltage across a group of voltage-detecting electrodes that contact the balls (and/or toes) of the subject's feet and across a plurality of internal reference resistors connected in parallel with the subject's body, and (3) calculating from these measurements the body fat percentage as a function of body impedance.

[0013] A preferred embodiment of the invention comprises a current source connected in parallel with two or more reference resistors and with the subject's body. The resistors and the subject's body are switched in and out of the circuit, and the various voltages across the resistors and the body are detected by a differential amplifier. The output of the differential amplifier is conditioned by a rectifier and low-pass filter and input to an analog-to-digital convertor (ADC). The output from the ADC is presented to a microprocessor control unit, which calculates (1) the impedance of the individual's body based upon the various voltage measurements, and (2) the percent body fat as a function of that impedance and other variables such as height, weight, age, and sex.

[0014] Additionally, the electrodes in the present invention are preferably designed as an array of small round knobs raised slightly above the surface upon which they are mounted. The electrodes are grouped into four groups—two current-supply electrode groups that contact the right and left heels, and two voltage-detecting electrode groups that contact the balls of the feet. Because a current of only 300 microamperes is preferably used, rather than the more conventional 800 microamperes, the current-supply electrodes and the voltage-detecting electrodes may be quite close to each other. In a preferred embodiment, the electrodes in the current-supply group are separated from the electrodes in the voltage-detecting group by a distance as small as about 1 cm.

[0015] This electrode array configuration has a number of advantages over the prior art. First, the electrodes may be spaced sufficiently close together to allow even small-footed children to use the device. Second, the subject has great flexibility as to the specific location of his feet on the device. Lastly, since the subject's foot makes contact with a number of small electrodes distributed over the surface of the foot, the blood circulation in each foot is enhanced, as compared with the flat plate electrodes as described in the background above, and the measurement is rendered thereby more reproducible.

[0016] FIG. 1 is a block diagram illustrating a body fat analyzer embodying present invention.

[0017] FIG. 2A illustrates a top-view of an electrode array mounted upon the upper surface of a body fat analyzer embodying the present invention.

[0018] FIG. 2B illustrates a cross-sectional view of the device shown in FIG. 2A.

[0019] A device embodying the present invention, as shown in a preferred embodiment in FIG. 1., comprises a 50 kHz, 300 microampere constant current source 10 connected in parallel with a plurality of reference resistors Ri (R1, R2, and R3 being shown as examples in FIG. 1) and with the subject's body 60. Resistors Ri and the subject's body 60 are switched in and out of the circuit by switches Sj (S1 through S8 in the embodiment shown in FIG. 1), which are controlled by microprocessor control unit (“MCU”) 120. In the embodiment shown in FIG. 1, the various voltages across the resistors and the body are connected sequentially to voltage drop measuring device 70 via switches S9 through S12, also controlled by MCU 120.

[0020] For example, to measure the voltage developed across body 60, switches S7, S8, S11, and S12 are closed, while switches Si through S6, S9 and S10 are opened. Thus, current flows through the current-supply electrode group 20, which is in contact with one of the subject's heels, through body 60 and through the current-supply electrode group 30 in contact with the subject's other heel. Simultaneously, the voltage generated between the balls of the subject's feet is passed through voltage-detecting electrode groups 40 and 50, which are in contact with the balls of the subject's feet, to the inputs of voltage drop measuring device 70. Similarly, to measure the voltage developed across resistor R1, switches S1, S2, S9, and S10 are closed, while switches S3 through S8, S11, and S12 are opened. In this way, the voltage across any of the resistors or the subject's body may be measured.

[0021] Additionally, combinations of the references resistors Ri (R1, R2, and R3 in FIG. 1) and body 60 can be connected and the voltage across the combination measured. For example, resistors R1 and R2 can be connected by closing switches S1, S2, S3, S4, S9, and S10, and opening the other switches. Thus, a large number of resistance/voltage data points with a narrow resolution of resistance values can be obtained. Furthermore, the number of potential data points can readily be increased by increasing the number of reference resistors Ri in parallel with body 60. The resolution of the data points is set by the values of the reference resistors and the equivalent resistances of groups of resistors; the reference resistors are selected such that the full range of body impedance, which is typically 0 to 1000 ohms, is bracketed.

[0022] Since the voltages presented to voltage drop measuring device 70 are small AC voltages, voltage drop measuring device 70 comprises a differential amplifier 80 to amplify the AC voltage signal, a rectifier circuit 90, and a low pass filter (LPF) 100. Voltage drop measuring device 70 thus presents a DC voltage corresponding to the amplitude of the AC voltage signal to analog-to-digital convertor (ADC) 110, which in turn passes the digitized voltage measurement to MCU 120. MCU 120 then interpolates the body impedance by comparing the measured body voltage with the measured reference resistances. In this way, voltage measurement errors caused by contact resistances or amplifier nonlinearities are eliminated.

[0023] Next, MCU 120 uses a formula to calculate the percent body fat from the measured body impedance. In U.S. Pat. No. 5,415,176, Sato et al. teaches one such formula, a two-step calculation process in which body density is calculated first as a function of body impedance, and percent body fat is then calculated as a function of body density. The disclosed formula for the first calculation step is: $\mathrm{BD}=1.1144-\frac{0.0976*W*Z}{H^2}+0.00084*Z$

[0024] where BD is body density, W is weight in kilograms, Z is body impedance, and H is height in centimeters. The formula disclosed for the second step is: $\%\mathrm{BF}=\frac{4.57}{\mathrm{BD}}-4.142*100$

[0025] where %BF is percent body fat and BD is body density. However, any formula relating body impedance to body fat may be used. Once MCU 120 calculates the body fat of the subject, it displays the results on a display screen 130.

[0026] FIGS. 2A and 2B illustrate the electrode array configuration in a preferred embodiment of the invention. When the subject stands on the body fat analyzer, the feet 270, 280 rest on the discrete electrodes. In the specific embodiment illustrated, the electrodes are divided into four groups 230, 240, 250, and 260. Groups 230 and 240 are voltage-detecting electrodes, while groups 250 and 260 are current-supply electrodes. Compared with large flat electrodes, these discrete-geometry electrode groups stimulate the subject's feet, improving blood circulation and rendering the body impedance measurement more reproducible. Additionally, the subject is not restricted in the placement of his feet 270, 280, so long as the heel of the foot, 280 for example, contacts at least one of the electrodes in current-supply electrode group 260 and the ball of the foot 280 contacts at least one of the electrodes in voltage-detecting electrode group 230. In one embodiment, the electrodes of voltage-detecting groups 230, 240 and the electrodes of current-supply groups 250, 260 are separated by a distance of 1 cm, so that both children and adults can use the device. Also shown in FIG. 2A are the display panel 220 and a keypad 210 for entering parameters such as height, age, weight, and sex into the MCU.

[0027] FIG. 2B depicts a cross-sectional view through the body fat analyzer shown in FIG. 2A. For a given foot 280, voltage-detecting electrodes 300 contact the ball and toe of the foot, while current-supply electrodes 310 contact the heel. It may be seen from FIG. 2B that the voltage-detecting electrodes 300 are electrically connected, and that the current-supply electrodes 310 are electrically connected.

[0028] Preferably, non-conductive surface 320 is sufficiently rigid to support the weight of the subject without deforming, and may be made from any substantially rigid, nonconductive material, such as glass. Alternatively, conductive materials, such as aluminum, may also be used, as long as some gap or insulation is provided between the electrodes and surface 320.

[0029] While the invention has been described with reference to a specific embodiment, it will be appreciated by those of ordinary skill in the art that modifications can be made to the structure and form of the invention without departing from its spirit and scope. For example, it will be readily apparent to those of ordinary skill in the art that the electrode array configuration described above may be easily applied to body fat analyzers that use a two-point impedance measurement technique, rather than the four-point technique described above. So, too, it will be recognized that the arrangement of the reference resistors in parallel may also be applied to two-point measurement systems. It will further be apparent that although the electrodes in the present invention are described above as knob-shaped, they may also be configured as strips, bars, or any geometric shapes that do not substantially constrict blood flow to the surface of the foot. Accordingly, the scope of the invention is not limited to the embodiments described above, but is defined solely by the following claims.