We used an insulated acupuncture needle to deliver a 20 kHz alternative current to detect the changes on the electromyogram (EMG) and deep muscle impedance. We found that impedance increased (with a duration of 224.1 +/- 75.3 msec) and then decreased (with a duration of 293.4 +/- 97.3 msec) after motor unit action potential (MUAP) firing during isometric voluntary contraction (VC) of the tibialis anterior muscle (TAM). We divided the positive peak into pattern I (with a latency range of > or = 30 msec and < or = 75 msec) and pattern II (with a latency range of > 75 msec and < or = 140 msec). These patterns of change corresponded to the contraction times of tension caused by the electrical stimulation and VC. These impedance changes may be caused by a change in the extracellular and intracellular fractions, and may reflect the variance of tension intensity in the muscle tissue.