The effect of the subcutaneous fat on the transfer of current through skin and into muscle

The present investigation was conducted to see the effect of subcutaneous fat on the transmission characteristics of an electrical stimulus applied to the skin and conducted to skeletal muscle. Two groups of subjects participated. In one, the subjects were three males and three females whose average age was 24.6 ± 1.5 years, average weight was 74.8 ± 18.2 kg, and average height was 176.4 ± 10.3 cm. The other was a group of 30 subjects who average age was 26.2 ± 1.9 years, average height 177.3 ± 11.5 cm, average weight 92.4 ± 19.8 kg. Electrical stimulation was applied above the quadriceps muscle at a current of 5 mA and with sine and square wave stimulation at a frequency of 30 Hz and a pulse width of 250 μs. Current movement was measured on the skin and into muscle with surface and needle electrodes. The results showed that the thickness of the subcutaneous fat layer was directly related to signal loss from the skin (correlation between subcutaneous fat thickness and RC time constant was 0.96, p < 0.001). Because of the subcutaneous fat layer and the resulting capacitance, an RC low pass filter is created such that square wave stimuli are not transmitted well into muscle whereas sine wave stimuli pass easily. Thus, when considering surface stimulation of nerve or muscle, any volume conductor model must take subcutaneous fat into consideration since the RC low pass filter created by fat will filter surface signals or, conversely, signals such as EMG which are generated in muscle but measured on the surface of the skin.