The relationship between fat thickness t(fat) and current density magnitude |i(phi)| has been clarified by coupling of electromagnetic simulation and electrical impedance tomography (ES-EIT) under electrical muscle stimulation (EMS) in three responsive calf muscle compartments which are called M-1 compartment of gastrocnemius muscle, M-2 compartment of tibialis anterior, extensor digitorum longus, and peroneus longus muscles, and M-3 compartment of soleus muscle. ES-EIT images the current density magnitude |iu| from the conductivity distribution r reconstructed by EIT for the decision of optimum voltage intensity ui OVI of EMS. As a result, the highest sensitivity of |i(phi)| to fat thickness tfat is M-1, the physiological-induced conductive response in M-2 depends not only on the |i(phi)|, but also on the muscle mechanical motion. The Pearson correlation coefficient (PCC) r between spatial-mean current density magnitude<|i(phi)|> and spatialmean conductivity difference Delta <sigma(phi-pre)> in M-1, M-2 and M-3 shows high correlation under EMS. The tfat and |i(phi)| are negatively correlated in muscle compartments of each subject. The individual tfat and |i(phi)| are negatively correlated in muscle compartments of each subject because the fat layer becomes an obstacle against |i(phi)| from the skin to the subcutaneous tissue. As a discussion, the optimum voltage intensity phi(i)(OVI) and phi(sigma)(OVI) of M-1 are defined as the highest point of second-order difference of spatial-mean current density magnitude <partial derivative(2) vertical bar I-phi(M1) vertical bar/partial derivative phi(2) vertical bar > and spatial-mean conductivity <partial derivative(2) sigma(M1)(phi)/partial derivative phi(2) vertical bar(phi)> under EMS, which are increased with the increase in tfat. The absolute difference |DuOVI| between ui OVI and ur OVI is decreased with increasing tfat, which is due to the fact that the effect of EMS loading voltage on muscle diminishes with increasing tfat, thus reducing the effect of muscle quality on the results of optimum voltage intensity calculations.
