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OBJECTIVE:To better understand the working mechanism of acupuncture,we investigated the skin electrical impedance distribution around acupoints,and the impedance changes at 12 original acupoints bilaterally after bending the limbs.METHODS:We measured the skin electrical impedance in three study subjects in the frequency range of 40 to 10 kHz using the four-electrode method with a sharp probe and a large reference electrode.A measurement matrix of 7 mm x 7 mm with spacing of 2.0 (or 3.0) mm was measured to obtain 2D impedance mapping of acupoints.The impedance spectra of 12 original acupoints were measured at the 0° position and the 90° position.RESULTS:The electrical impedance of some acupoints,such as Yangchi (TE 4),was 16 times lower than that of the surrounding area,showing a recognizable small central area of low impedance with a diameter of less than 4 mm.In contrast,other acupoints,such as Laogong (PC 8),had an electrical impedance that was not significantly different from that of the surrounding area.When the limb was bent from a straight position (0°) to a vertical position (90°),the electrical impedance of the 12 original acupoints showed varied trends,either increasing or decreasing by a factor of up to ten times,or remaining at the same level.CONCLUSION:Not all acupoints tested show the property of low impedance,which might be related to the varied depth of the openings of superficial collaterals.The unexpected dependence of acupoint impedance on limb angle is a novel discovery,which implies that the channel paths are located in interstitial structures in the limbs.It might be possible to determine an optimized limb position for each particular acupuncture treatment in clinical practice.