Electrostimulation usually involves feeding the muscles low current electrical impulses via moistened electrode pads placed firmly on the skin. The effectiveness, comfort and depth of excitation depends on factors such as pulse shape, frequency, duration, intensity and modulation pattern. The resulting number of possible stimulation combinations immediately emphasizes how difficult it is to determine the optimum balance of variables and compare the results of different researchers.
Applications of Macrocurrent Stimulation
The Emergence of Microcurrent Stimulation
Recent research and clinical experience have revealed that electric currents as much as 1000 times smaller than that of all the traditional physical therapy modalities can be far more successful than the latter in achieving many of the benefits outlined in the previous section.
Currents as low as 10 microamps (millionths of an amp) pulsating at between 0.1 to 400Hz are too weak to cause muscle contraction, block pain signals or cause local heating, yet their effectiveness and safety is often superior in many applications to that of faradism, interferentialism and conventional TENS (Matteson & Eberhardt, 1985).
Pilla (1974) has paid particular attention to electrochemical information transfer across cell membranes. The model in this case hypothesizes that the molecular structure of the cell membrane reflects its current genetic activity. Here, the function of a cell at any instant is determined by feedback between DNA in the cell nucleus and a macromolecule inducer liberated from the membrane by means of a protein (enzyme) regulator derived from messenger RNA activity within the cell. The activity of these membrane-bound proteins is strongly modulated by changes in the concentration of divalent ions (such as calcium Ca++) absorbed on the membrane. ES may elicit these ionic changes and thereby modify cell function.
It has been shown that ES at 5Hz stimulates synthesis of DNA in chick cartilage cells and rat bone by as much as 27%, but not in chick skin fibroblasts or rat spleen lymphocytes (Rodan et al, 1978). Not only does the effect of ES appear to be tissue-specific, but the increase in DNA synthesis occurs 4-6 hours after 15 minutes of ES. The process of membrane depolarisation carried by sodium ions seems to be followed by an increase in intracellular Ca++ concentration, thereby triggering DNA synthesis in cells susceptible to the particular stimulus. Further work by Pilla (1981) has confirmed the existence of cellular 'windows' which open most effectively to certain frequencies, pulse widths and pulse amplitudes. To attune the ES signal to these parameters, monitoring of tissue impedances is preferable, a system employed by so-called 'Intelligent TENS' devices.
In addition, Cheng et al (1982) have shown that stimulation with currents from 50-1000 microamps can increase tissue ATP concentrations in rats by 300-500%, and enhances amino acid transport through the cell membrane and consequent protein synthesis by as much as 40%. Interestingly, the same study reported that increasing the current above only one milliamp was sufficient to depress tissue ATP and protein synthesis - and traditional ES most commonly applies currents exceeding 20 milliamps, at which stage this depression being nearly 50%.
An Integrated Theory of Electrostimulation
Therefore, it appears as if macrocurrent stimulation (MACS - currents exceeding one milliamp) acts as a physiological stressor, which in the short term causes the typical alarm response described by Selye (1975). This is supported by the work of Eriksson et al (1981), who found that the acute effects of traditional ES are similar to those found for intense voluntary exercise. Furthermore, Gambke et al (1985) have found in animal studies that long-term MACS causes some muscle fibres to degenerate and be replaced by newly formed fibres from satellite cell proliferation. This fibre necrosis occurs a few days after application of ES and seems to affect mainly the FT fibres. The fact that the various muscle fibres do not transform at the same time may be due to different thresholds of each fibre to the stimulus that elicits the transformation. Possibly, the earlier changes might induce subsequent ones.
Thus, if Selye's General Adaptation Syndrome model is applied to MACS-type stimulation, the body would have to draw on its superficial adaptation energy stores and adapt to the ES-imposed stress by increasing strength or endurance, or by initiating transformation of muscle fibre types. If the ES is too intense, too prolonged or inappropriately used to augment a weight training programme, adaptation might not occur or it might increase the proportion of slow twitch fibres and thereby reduce strength. This could explain some of the negative research findings discussed earlier.
Furthermore, excessively demanding MACS conceivably might cause the body to draw on its deep adaptation energy and lead to permanent tissue damage. Consequently, any athlete who may derive definite performance benefits from MACS should not assume that increased dosage will lead to further improvement. The contrary may well prove to be true.
Microcurrent stimulation (MICS - currents below one milliamp), on the other hand, would not act as a stressor. Instead, the evidence implies that it elicits biochemical changes associated with enhanced adaptation, growth and repair. Since MICS appears to operate more on the basis of resonant attunement of the stimulus to cellular and subcellular processes, the specific therapeutic effects are determined by how efficiently the stimulation parameters match the electrical characteristic of the different cells, in particular, their impedance at different frequencies. MICS may be applied in several ways to facilitate restoration:
It is generally entirely safe to apply MICS anywhere on the body, because the current and energy transmitted is too low to produce any thermal or electrolytic effects on vital tissues. Under no circumstances should MACS be applied across the brain, as it can cause serious harm. It is generally not advisable to apply any form of ES to epileptics, pregnant women, cardiac patients or persons with heart pacemakers.
The Validity of Microcurrent Application?
There has been considerable debate about the value of microcurrent (small electrical currents of less than 1 ampere) in physical therapy, with its supporters claiming consistently good results and its detractors claiming that any benefits are probably due to a placebo effect. Some therapists have stated that there is scant evidence of any research and practical evidence of the value of microcurrent, so, for their interest and that of others conducting research into microcurrent therapy, we have compiled a lengthy, but incomplete, list of English language references that relate to the theoretical foundations and clinical applications of microcurrent and can be found here.
