The effect of the subcutaneous fat on the transfer of current through skin and into muscle
Introduction
The standard model for the movement of current through tissues is that of movement in a volume conductor [1], [2], [3]. In this model, the three-dimensional flow of current is only governed by tissue resistance. Thus, if this model is correct, an alternating waveform would pass through tissue undistorted since there would be only a small effect of tissue or transmembrane capacitance and inductance [1]. Recently, Plonsey pointed to the potential effect of non-homogenous tissue on the transmission of electricity through such a volume conductor [1]. Miranda et al. [4], [5] studied the effect of changes in the conductivity of small structures buried in a volume conductor on the stimulation of neurons. They found that at the boundaries where electrical resistance changed (e.g., at glands or bone) conductivity in a volume conductor changes abruptly and alters the signal transmission characteristics of the media reflecting transmitted energy along the boundary lines of the impedance change in tissue [4], [5], [6]. Thus, in real tissue, an ideal homogenous media is not present and changes in conductance in real tissue alters signal transmission [7].
When electrical stimulation is used on the surface of the skin to elicit a contraction in muscle, a larger problem is created for the movement of current due to the subcutaneous fat layer under the skin. The fat layer forms a dielectric and, due to its high electrical resistance, a capacitance is developed by high skin conductance (low resistance) and muscle conductance (low resistance) separated by high resistance in subcutaneous fat [8], [9]. With a thick subcutaneous fat layer in people who are overweight, this subcutaneous capacitance would be lower than that seen in thin people. Since there is no evidence that membrane capacitance of the cells is altered in people who are overweight, this subcutaneous fat layer may alter signal movement from the skin and into deeper tissues [8], [10], [11]. Since subcutaneous fat thickness can increase by 400% in thin vs. overweight people, considerable capacitance differences might be seen. The effect of this capacitance should be to create a time constant for the movement of charge that would alter the energy transferred into muscle from surface electrodes [4], [5], [10]. This may be particularly important since the standard delivery mode for electrical stimulation is a square wave [12], [13]. Because a square wave can be distorted through a series resistance (the skin) and capacitance (subcutaneous fat), the waveform arriving in muscle might be different than that seen at the surface electrodes.
Transcutaneous electrical stimulation is commonly used in therapy. It has been used for wound healing [12], [14], for muscle strengthening [15], [16], to reduce muscle spasticity [17], to decrease pain [18], and reduce muscle atrophy [19]. But recent studies show that for the same current applied to the surface of the skin, square wave stimulation is much less effective in eliciting a muscle contraction than sine wave stimulation [20], [21], [22]. Further, when subcutaneous fat was measured, people with the greatest subcutaneous body fat required the greatest current to elicit a contraction in skeletal muscle [23]. Interestingly, in the same person, if skin blood flow was high, it took substantially more current to stimulate the underlying muscle [23]. These findings might be explained due to the resister capacitor (RC) time constant that would be seen by a current source moving transcutaneously from the skin and into muscle. If either R or C is changed, transmission would be altered. R can be changed by increasing or decreasing blood flow [23] while C might be changed by altering the thickness of the subcutaneous fat layer overlying muscle.
Therefore, in the present investigation these relations were investigated further. The hypothesis to be tested here is that the subcutaneous fat layer creates a capacitor and filters square wave stimulation; the thicker the layer, the longer the time constant for the rise time of a square wave transmitted into muscle, creating a low pass filter. To accomplish this, two series of experiments were conducted. First, on a limited group of subjects, transmission and waveform distortion from a sine and square wave impulse delivered through the skin and into muscle were measured. In the second, the relationship between that distortion and subcutaneous body fat thickness was measured on a larger group of subjects.
Section snippets
Subjects
There were two groups of subjects examined here. In group 1, the subjects were three males and three females in the age range of 21–38 years old. The demographics of the subjects are shown in Table 1. In the second group, 15 male and 15 female subjects were examined to see the relationship between subcutaneous body fat and waveform transmission into muscle. All subjects were informed of all experimental procedures and signed a statement of informed consent as approved by the Institutional
Sine and square wave stimulation
Sine and square wave biphasic electrical stimulation were provided by a Challenge 8000 current controlled powered muscle stimulator (MPTS Inc., Tustin, CA). The electrode separation distance was 20 cm on the skin above the quadriceps muscle. The stimulation frequencies tested was 30 Hz, electrode size was 2 cm × 4 cm at a current of 5 mA and pulse width of 100 μs. Sine and square wave stimulation consisted of a pulse (100 μs) (Fig. 1) followed by a delay. The voltage was then held at zero (isopotential)
Procedures
Two series of experiments was conducted. The purpose of series 1 was to quantify how much current passed from surface electrodes to muscle. Both sine and square wave stimuli were applied to the skin to examine any distortion in the two waves that might occur during movement of the current into muscle due to resistance and capacitance of the tissue. The results from this series showed that for sine wave stimulation, there was a phase shift in the waveform from the surface of the skin to muscle.
Dispersion of current across the skin and into muscle
The results of the first series of experiments on six subjects are shown in Fig. 3. Illustrated here is the current measured on five locations on the surface of the skin (panel A) and the current measured with the needle electrodes (panel B) for sine and square wave stimulation. As can be seen here, with an electrode separation of 20 cm and 100 μs pulse width for electrical stimulation, the current, as assessed by the five electrode pairs placed on the surface of the skin, was fairly evenly
Discussion
Electrical stimulation is a modality commonly used both in physical therapy and in sports medicine [15], [19], [24]. It has also been used in orthopedic surgery, as an aid to muscle strengthening after surgery [9], [13]. However, some people report severe pain with electrical stimulation while others do not [20], [21], [25], [26]. This pain can be so uncomfortable that many patients prefer not using this modality even though there is good therapeutic benefit [20], [21], [26].
In recent years,
Conflict of interest
No conflict of interest.
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