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+/-18.2kg, and average height was 176.4+/-10.3cm. The other was a group of 30 subjects who average age was 26.2+/-1.9 years, average height 177.3+/-11.5cm, average weight 92.4+/-19.8kg. Electrical stimulation was applied above the quadriceps muscle at a current of 5mA and with sine and square wave stimulation at a frequency of 30Hz and a pulse width of 250micros. 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.     

Interrelationships between body fat and skin blood flow and the current required for electrical stimulation of human muscle

There is variability between individuals in the current needed to elicit a contraction in human muscle with surface electrodes. To understand what might be causing some of this variability, 25 subjects whose average age was 24.4+/-2.3 years, whose height was 165.5+/-9.5 cm, and whose average weight was 70.3+/-21 kg were examined. Electrical stimulation was applied above the motor point of the quadriceps, biceps, and lateral gastrocnemius muscles. To assess body fat, 2D ultrasound was used with a 1cm stand off. Electrical stimulation was applied with sine wave stimulation at 100 micros pulse width and at a frequency of 30 Hz. To alter skin blood flow, aside from the natural difference in skin blood flow at rest, hot packs and cold packs were used for 5 min. The average fat thickness below the quadriceps and gastrocnemius muscles was 0.75+/-0.13 cm and under the biceps was 0.48+/-0.16 cm. Without the use of hot or cold packs, the currents for the quadriceps and gastrocnemius muscles were significantly higher than that of the biceps (p<0.01). While there was some relationship between stimulation current and blood flow without the application of hot or cold packs, when hot packs were applied, skin blood flow increased as did the current required to stimulate muscle to threshold. When cold packs were applied, there was a decrease in the current required to stimulate these muscles. In conclusion, there is a causal relationship between skin blood flow, the thickness of the fat layer below the skin, and the current required to stimulate the muscle.     

A multi-channel stimulator and electrode array providing a rotating current whirlpool for electrical stimulation of wounds

When electrical stimulation is used on wounds, the electrical current has difficulty penetrating areas where there is necrotic tissue. Further, for an irregularly shaped wound, current distribution is poor in some areas of the wound since conventional two-electrode delivery systems provide the greatest current in a line directly between the electrodes. A new stimulator and electrode system is described which uses three electrodes spaced around a wound to disperse current more evenly. The stimulator senses tissue impedance and then redirects current by altering its Thevenin's output impedance for each electrode; each of the three electrodes becomes the active one in sequence while the remaining are the sink electrodes. Eight subjects were examined to test the stimulator. Electrical stimulation was applied to the skin above the quadriceps muscle at currents of 15 mA in six subjects without wounds and in two subjects with wounds. The relationship between electrode position and current dispersion on the skin was examined with a two-electrode vs. a three-electrode system to set stimulation parameters for the computer. The results showed that the three-electrode system could (1) detect areas of the skin with high impedance; (2) compensate by altering the Thevenin's output impedance at each of the three electrodes to shift current to high impedance areas; (3) provide uniform current across the skin as assessed by skin current and blood flow measurements with a laser Doppler flow imager.     

A multi-channel stimulator and electrode array providing a rotating current whirlpool for electrical stimulation of wounds

When electrical stimulation is used on wounds, the electrical current has difficulty penetrating areas where there is necrotic tissue. Further, for an irregularly shaped wound, current distribution is poor in some areas of the wound since conventional two-electrode delivery systems provide the greatest current in a line directly between the electrodes. A new stimulator and electrode system is described which uses three electrodes spaced around a wound to disperse current more evenly. The stimulator senses tissue impedance and then redirects current by altering its Thevenin's output impedance for each electrode; each of the three electrodes becomes the active one in sequence while the remaining are the sink electrodes. Eight subjects were examined to test the stimulator. Electrical stimulation was applied to the skin above the quadriceps muscle at currents of 15 mA in six subjects without wounds and in two subjects with wounds. The relationship between electrode position and current dispersion on the skin was examined with a two-electrode vs. a three-electrode system to set stimulation parameters for the computer. The results showed that the three-electrode system could (1) detect areas of the skin with high impedance; (2) compensate by altering the Thevenin's output impedance at each of the three electrodes to shift current to high impedance areas; (3) provide uniform current across the skin as assessed by skin current and blood flow measurements with a laser Doppler flow imager.     

The transfer of current through skin and muscle during electrical stimulation with sine,square, Russian and interferential waveforms

Electrical stimulation is a commonly used modality for both athletic training and physical therapy. However, there are limited objective data available to determine the waveform which provides the maximum muscle strength as well as minimizing pain. In the present investigation, two groups of subjects were examined. Group 1 was composed of six males and four females and group 2 was composed of three male and three female subjects. The first series of experiments investigated muscle strength with stimulation at currents of 20, 40 and 60 milliamps using sine, square, Russian and interferential waveforms evaluating strength production and pain as outcomes. The second phase of experiments compared the effect of the different waveforms on current dispersion in surface versus deep muscle electrodes with these same waveforms. The results of the experiments showed that sine wave stimulation produced significantly greater muscle strength and significantly less pain than square wave, Russian or interferential stimulation at that same current. The most painful stimulation was square wave. Strength production was greatest with sine wave and least with Russian and interferential. An explanation of these findings may be the filtering effect of the fat layer separating skin from muscle. The highly conductive muscle and skin dermal layers would form the plates of a capacitor separated by the subcutaneous fat layer providing an RC filter. This filtering effect, while allowing sine wave stimulation to pass to the muscle, reduced power transfer in square wave, Russian and interferential stimulation is observed.     

Atlas of the muscle motor points for the lower limb: implications for electrical stimulation procedures and electrode positioning

The aim of the study was to investigate the uniformity of the muscle motor point location for lower limb muscles in healthy subjects. Fifty-three subjects of both genders (age range: 18–50 years) were recruited. The muscle motor points were identified for the following ten muscles of the lower limb (dominant side): vastus medialis, rectus femoris, and vastus lateralis of the quadriceps femoris, biceps femoris, semitendinosus, and semimembranosus of the hamstring muscles, tibialis anterior, peroneus longus, lateral and medial gastrocnemius. The muscle motor point was identified by scanning the skin surface with a stimulation pen electrode and corresponded to the location of the skin area above the muscle in which an electrical pulse evoked a muscle twitch with the least injected current. For each investigated muscle, 0.15 ms square pulses were delivered through the pen electrode at low current amplitude (<10 mA) and frequency (2 Hz). 16 motor points were identified in the 10 investigated muscles of almost all subjects: 3 motor points for the vastus lateralis, 2 motor points for rectus femoris, vastus medialis, biceps femoris, and tibialis anterior, 1 motor point for the remaining muscles. An important inter-individual variability was observed for the position of the following 4 out of 16 motor points: vastus lateralis (proximal), biceps femoris (short head), semimembranosus, and medial gastrocnemius. Possible implications for electrical stimulation procedures and electrode positioning different from those commonly applied for thigh and leg muscles are discussed.     

Toward the optimal waveform for electrical stimulation of human muscle

Electrical stimulation of the quadriceps muscle was used to elicit 4-min isometric contractions at 10% of the maximal voluntary contraction (MVC) in four male and three female subjects. The effect of four waveforms, including Russian, interferential, sine, and square, on the mean stimulation current required to achieve the desired contraction force, subjective comfort, and physiological responses was studied. Interferential stimulation, even at full power, could not elicit a sustained contraction at 10% MVC. The contractions elicited by electrical stimulation utilizing the sine waveform required significantly less mean stimulation current to maintain the desired force of contraction with consistently lower verbal rating scale scores and greater increases in oxygen consumption than either the Russian or square waveform stimulations. Russian waveform stimulation produced a significantly greater rise in galvanic skin resistance than the sine or square waveform while the changes in respiratory quotient were similar between waveforms. The data support sine wave stimulation as working the best by producing the desired muscle tension with the least mean stimulation current and therefore, the least tissue trauma while providing the most subjective comfort. Electronic Publication     

Effect of skin temperature on vibrotactile sensitivity

The vibration problems relating to living bodies have so far been studied from the perspectives of engineering physiology and psychology. This study shows the relationship between vibratory sensibility and temperature in the living body. Psychological experiments were carried out by using the vibrometer of an acoustic calibration apparatus in sine, triangular and square waves. The sensibility-threshold measurements were made using 30–700 Hz sine waves, 30–300 Hz triangular and sawtooth waves, or 30–250 Hz square waves. Each of ten subjects was kept seated. The average value of the vibratory levels, varied by ascending and descending steps, was taken as that of the threshold. As the vibrometer in the apparatus used makes a noise at frequencies greater than 250 Hz it was masked from the subject by presenting him with a different noise. The threshold curve for square waves is lower by 12·3 dB than that for sine waves at about 30Hz. The threshold curve of the 26°C sine wave was lower by 10 dB than that of the 58°C sine wave vibration near 200 Hz. For example with a sine wave, at 58°C the amplitude threshold was lowest at about 270 Hz, but at −11°C at about 200 Hz. At frequency stimulation higher than 120 Hz, as the temperature of the contact point was lowered, the amplitude threshold increased and the frequency at which the threshold curve was at a minimum shifted to a lower frequency.     

Rebound responses to prolonged flexor reflex stimuli in human spinal cord injury

The purpose of this study was to examine the reflex effects of electrical stimulation applied to the thigh using skin electrodes, targeting the sensory fibers of the rectus femoris and sartorius, in people with spinal cord injury (SCI). Thirteen individuals with SCI were recruited to participate in experiments using prolonged electrical stimuli on the right medial thigh over the regions of the sartorius and rectus femoris muscles. Three stimuli, spaced 20 s apart, were applied at 30 Hz for 1 s at four different intensities (15–60 mA) while subjects rested in a seated position. Isometric joint torques of the hip, knee and ankle, and electromyograms (EMGs) from six muscles of the leg were recorded during the stimulation. Early in the stimulation, a flexion response was observed at the hip and ankle, analogous to a flexor reflex; however, this response was usually followed by a “rebound” response consisting of hip extension, knee flexion and ankle plantarflexion, occurring in 10/13 subjects. Stimuli applied in a more lateral (mid thigh) electrode position (i.e. over the rectus femoris) were less effective in producing the response than medial placement, despite vigorous quadriceps activation. This complex reflex response is consistent with activation of a coordinating spinal circuit that could play a role in motor function. The reversal of the reflex pattern emphasizes the potential connection between skin/muscle afferents of the thigh, possibly including sartorius muscle afferents and locomotor reflex centers. This knowledge may be helpful in identifying rehabilitation strategies for enhancing gait training in human SCI.     

Development of a platinized platinum/iridium electrode for use in vitro

Silver/silver chloride (Ag/AgCl) electrodes possess excellent electrical properties for measuring the electrical activity of gastrointestinal smooth muscle but exert toxic effects on this tissue in vitro. We thus developed a platinum electrode for use in vitro, the construction of these electrodes relying upon the formation of a glass-platinum/iridium seal. The platinum/iridium (Pt/Ir) electrodes were platinized using a current density of 0.45 mA mm⁻². The electrode impedance at 0.01 Hz showed a minimum with platinization current-time products greater than 500 mA s mm⁻². However, deposits in excess of 600 mA s mm⁻² were readily removed by mechanical abrasion and proved unsatisfactory. Optimal platinization was obtained with a deposit of platinum-black corresponding to a current-time product of 550 mA s mm⁻². Optimally-platinized electrodes (geometric surface area 0.11 mm²) had a stable and reproducible potential with a drift of less than 1 μV min⁻¹ and a lower impedance than optimally chlorided silver electrodes (geometric surface area 0.46 mm²) at frequencies higher than 0.25 Hz. The platinized Pt/Ir electrodes were used to record the electrical activity of gastrointestinal smooth muscle in vitro.     

Design of electrodes and current limits for low frequency electrical impedance tomography of the brain

For the novel application of recording of resistivity changes related to neuronal depolarization in the brain with electrical impedance tomography, optimal recording is with applied currents below 100 Hz, which might cause neural stimulation of skin or underlying brain. The purpose of this work was to develop a method for application of low frequency currents to the scalp, which delivered the maximum current without significant stimulation of skin or underlying brain. We propose a recessed electrode design which enabled current injection with an acceptable skin sensation to be increased from 100 μA using EEG electrodes, to 1 mA in 16 normal volunteers. The effect of current delivered to the brain was assessed with an anatomically realistic finite element model of the adult head. The modelled peak cerebral current density was 0.3 A/m², which was 5 to 25-fold less than the threshold for stimulation of the brain estimated from literature review.     

Effects of a 2-, 3- and 4-electrode stimulator design on current dispersion on the surface and into the limb during electrical stimulation in controls and patients with wounds

Electrical stimulation is a widely used modality in the field of physical therapy and exercise physiology. The most common method for the application of electrical stimulation is a two-electrode system where one electrode is the source and the other is a reference. However, recent studies report that a more effective delivery system can be achieved if more than two electrodes are used. In the present investigation, the circuitry to deliver electrical stimulation through a 2-, 3- or 4-electrode delivery system was designed. The system was evaluated by its ability to deliver current on the surface of the skin as well as deep into the quadriceps muscle in six control subjects and in and around wounds in six other subjects. The results of the experiments showed that much better depth of penetration was achieved in a 4-electrode system (one electrode was on the opposite side of the limb and three electrodes were on top of the limb) than in either a 2- or a 3-electrode delivery system. In non-wounded skin, given the same current from the stimulator, the current in the quadriceps muscle was found to be double with a 4-electrode versus a 2-electrode system. In wounds, this same finding was seen. Here, blood flow, an indicator of the effectiveness of electrical stimulation in wounds, was three times higher if a multi-channel stimulator was used versus a 2-channel stimulator. Thus a multi-channel electrical stimulation system is more effective than a 2-electrode system.     

Effects of stimulation parameters on modification of spinal spasticity

Various electrical stimuli with frequencies from 10 Hz to 1000 Hz and pulse widths from 10 μs to 1000 μs were applied to seven spinal-cord injured patients with spasticity of the knee muscle. Spasticity was assessed with the pendulum test and EMG activity in the quadriceps and hamstrings. No universal optimum combination of stimulation parameters could be established but stimuli of 100 Hz and 100 μs pulse width were more effective than other combinations. Subjective remarks of patients regarding the effects over 24 h did not always correlate with measured data obtained within one hour after stimulation.     

Safety of multi-channel stimulation implants: a single blocking capacitor per channel is not sufficient after single-fault failure

One reason given for placing capacitors in series with stimulation electrodes is that they prevent direct current flow and therefore tissue damage under fault conditions. We show that this is not true for multiplexed multi-channel stimulators with one capacitor per channel. A test bench of two stimulation channels, two stimulation tripoles and a saline bath was used to measure the direct current flowing through the electrodes under two different single fault conditions. The electrodes were passively discharged between stimulation pulses. For the particular condition used (16 mA, 1 ms stimulation pulse at 20 Hz with electrodes placed 5 cm apart), the current ranged from 38 to 326 μA depending on the type of fault. The variation of the fault current with time, stimulation amplitude, stimulation frequency and distance between the electrodes is given. Possible additional methods to improve safety are discussed.     

Histologic and physiologic evaluation of electrically stimulated peripheral nerve: Considerations for the selection of parameters

Helical electrodes were implanted around the left and right common peroneal nerves of cats. Three weeks after implantation one nerve was stimulated for 4–16 hours using charge-balanced, biphasic, constant current pulses. Compound action potentials (CAP) evoked by the stimulus were recorded from over the cauda equina before, during and after the stimulation. Light and electron microscopy evaluations were conducted at various times following the stimulation. The mere presence of the electrode invariably resulted in thickened epineurium and in some cases increased peripheral endoneurial connective tissue beneath the electrodes. Physiologic changes during stimulation included elevation of the electrical threshold of the large axons in the nerve. This was reversed within one week after stimulation at a frequency of 20 Hz, but often was not reversed following stimulation at 50–100 Hz. Continuous stimulation at 50 Hz for 8–16 hours at 400 μA or more resulted in neural damage characterized by endoneurial edema beginning within 48 hours after stimulation, and early axonal degeneration (EAD) of the large myelinated fibers, beginning by 1 week after stimulation. Neural damage due to electrical stimulation was decreased or abolished by reduction of the duration of stimulation, by stimulating at 20 Hz (vs. 50 Hz) or by use of an intermittent duty cycle. These results demonstrate that axons in peripheral nerves can be irreversely damaged by 8–16 hours of continuous stimulation at 50 Hz. However, the extent to which these axons may subsequently regenerate is uncertain. Therefore, protocols for functional electrical stimulation in human patients probably should be evaluated individually in animal studies.     

Modelling the response of scalp sensory receptors to transcranial electrical stimulation

Transcranial electrical stimulation of the brain cause considerable discomfort to the patient. The purpose of the study was to find out whether this could be affected by the choice of stimulation parameters. A spherical volume conductor model of the head and active compartmental models of a pyramidal motor nerve and scalp nociceptor were used in combination to simulate the scalp nociception to transcranial electrical stimulation. Scalp nociceptors were excited at distances of several centimetres from the electrodes. The size of the excited scalp area correlated with the length of the stimulation pulse. The area was 12.3, 20.4 and 26.0 cm², for a 10μs, 100μs and 1 ms constant current pulse, respectively. With a 100 μs constant current pulse, the threshold for motor excitation was 205 mA and, for nociception, it was 51 mA. There was no significant difference between constant current and capacitor discharge pulses or between electrodes of different sizes. The results imply that the use of very short stimulation pulses can reduce the pain. If a topical anaesthesia is used to reduce the pain, it has to be applied on a large area around the electrodes.     

Frequency dependence of the cardiac threshold to alternating current between 10 Hz and 160 Hz

It is still unclear what fundamental criteria influence the ability of alternating current (AC) to induce ventricular fibrillation (VF) in vivo. As the VF threshold has a bowl-shaped relationship with frequency (showing a minimum threshold at some frequency), similar to the nervous system, one proposed model has assumed that the mechanisms underlying AC stimulation of nerves are at work for VF induction. More recent work has suggested a second approach, whereby a simple RC-like model is sufficient to understand the cardiac AC stimulation threshold's frequency dependence, suggesting that some unarticulated mechanism is at work for VF. The paper directly tests these two models. In 12 intact dogs and 20 intact guinea pigs, DC pulses were used to stimulate AC square and AC sine waves at 10, 20, 40, 80 and 160 Hz. All electrodes were endocardial, with the return electrode being on a paw or thorax. It was found that, for square and sine wave stimulation in both dogs and guinea pigs, the stimulation threshold increased monotonically with frequency from 10 Hz up to 160 Hz (p<0.01 for dogs and guinea pigs). Between 80 and 160 Hz, the AC stimulation threshold doubled, exactly as predicted by an RC model. It was concluded that the AC stimulation threshold is not bowl-shaped and is best understood with an RC model. As the VF threshold does exhibit a bowl-shape with frequency, as opposed to the stimulation threshold which does not, the VF induction frequency dependence must have different origins.     

Short and long duration transcranial direct current stimulation (tDCS) over the human hand motor area

The aim of the present paper is to study effects of short and long duration transcranial direct current stimulation (tDCS) on the human motor cortex. In eight normal volunteers, motor evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS) were recorded from the right first dorsal interosseous muscle, and tDCS was given with electrodes over the left primary motor cortex (M1) and the contralateral orbit. We performed two experiments: one for short duration tDCS (100 ms, 1, 3 or 5 mA) and the other for long duration tDCS (10 min, 1 mA). The stimulus onset asynchrony (SOA) between the onset of tDCS and TMS were 1–7 and 10–120 ms for the former experiment. In the latter experiment, TMS was given 0–20 min after the end of 10 min tDCS. We evaluated the effect of tDCS on the motor cortex by comparing MEPs conditioned by tDCS with control MEPs. Cathodal short duration tDCS significantly reduced the size of responses to motor cortical stimulation at SOAs of 1–7 ms when the intensity was equal to or greater than 3 mA. Anodal short duration tDCS significantly increased MEPs when the intensity was 3 mA, but the enhancement did not occur when using 5 mA conditioning stimulus. Moreover, both anodal and cathodal short duration tDCS decreased responses to TMS significantly at SOAs of 20–50 ms and enhanced them at an SOA of 90 ms. Long duration cathodal tDCS decreased MEPs at 0 and 5 min after the offset of tDCS and anodal long duration tDCS increased them at 1 and 15 min. We conclude that the effect at SOAs less than 10 ms is mainly caused by acute changes in resting membrane potential induced by tDCS. The effect at SOAs of 20–100 ms is considered to be a nonspecific effect of a startle-like response produced by activation of skin sensation at the scalp. The effect provoked by long duration tDCS may be short-term potentiation or depression like effects.     

Effects of electrical stimulation of the dorsal skin on systemic and mesenteric microvascular hemodynamics in anesthetized rats

The effects of electrical stimulation of the dorsal skin area on the mesenteric arterioles were investigated in anesthetized rats by the use of an intravital microscope-television system. Changes in the diameter of the mesenteric precapillary arterioles (10-40 microm in diameter) were measured with an image processor. Blood flow velocity in the mesenteric precapillary arterioles was monitored by the dual sensor method developed by the authors. Electrical stimulation was performed through two platinum electrodes placed at the right dorsal Th5-12 level skin area by the use of an electrical stimulator (0.2 ms, 20 Hz). Continuous stimulation lasting for 30 s (1-10 mA) and intermittent stimulation lasting for 10 min (3 mA) were applied. The pressor response following the depressor response was induced by a stimulus current above 8 mA. The decrease in mesenteric blood flow velocity was induced by stimulus current above 10 mA. These responses were abolished by lidocaine injection into the subcutaneous area where the electrodes were attached. No significant change in arteriolar diameter or heart rate were induced by the stimulation for 30 s. Electrical stimulation of the skin for 10 min evoked a decrease in the diameter of arterioles (-3.4 +/- 2%, p < 0.01, n = 12). In the adrenalectomized group, electrical stimulation of the skin for 10 min elicited a slight increase in the diameter (1.1 +/- 0.5%, n = 6). It is therefore suggested that electrical stimulation of the skin for 30 s reflexly evoked decreases in MAP and in blood flow velocity, and that the constriction of the mesenteric precapillary arterioles induced by the stimulation for 10 min was mediated by humoral adrenaline and noradrenaline released by somato-adrenal medullary reflex.     

Effects of neuromuscular electrical stimulation parameters on specific tension

This study examined the effects of altering surface neuromuscular electrical stimulation (SNMES) parameters on the specific tension of the quadriceps femoris muscle. Seven able-bodied subjects had magnetic resonance images taken of both thighs prior to and immediately after four SNMES protocols to determine the activated muscle cross-sectional area (CSA). The four protocols were: (1) research (RES, 100 Hz, 450 μs, and amplitude set to evoke 75% of maximal voluntary isometric torque, MVIT); (2) pulse duration (PD, 100 Hz, 150 μs, same current as in RES); (3) frequency (FREQ, 25 Hz, 450 μs, and same current as in RES); (4) amplitude (AMP, 100 Hz, 450 μs, and current set to evoke the average of the initial torques of PD and FREQ, 45 ± 9% of MVIT). Reducing the amplitude of the current from 75 to 45% of MVIT did not alter specific tension, 25 ± 8 N/cm², suggesting that the amplitude probably affects torque and the area of activated muscle proportionally. Shortening the pulse duration from 450 to 150 μs caused specific tension to drop from 25 ± 6 to 20 ± 6 N/cm² (P < 0.05), indicating that pulse duration increased torque and the activated CSA disproportionally. Alternatively, reducing the frequency from 100 to 25 Hz decreased specific tension from 25 ± 6 to 17 ± 4 N/cm² (P < 0.05), suggesting that the frequency increased torque without affecting the activated CSA. Clinicians who administer SNMES should be aware of the magnitude of adaptations to a given amplitude, pulse duration, and frequency.