Because ions dissociate by electrolysis in the presence of electrical current, living tissue can become polarized in a direct current field. This can be disastrous in neuronal tissue. Therefore, modern stimulators usually provide alternating or biphasic current. That is, current that reverses polarity each half cycle so that the electrolytes factor sum to zero. If the current continued to flow in the same direction, polarity stress could result in irreversible tissue damage.

As an analogy, picture a group of soldiers marching across a bridge. Before they get to one side, an about face order is given and they return. Before they reach the opposite side, another about face order is given, and so on, so that they never actually reach a side. By going back and forth, biphasically, there is no net electron flow across the bridge and no soldiers are added or subtracted. They never get across the bridge to cause an irreversible balance in the status quo.

Since the first edition of this book a few engineers have commented that biphasic is incorrect and should be replaced with bipolar. Technically, this is true but the designation of biphasic has already become the usual and customary term in electromedicine, so we have maintained it.

Electricity travels in a circuit. The number of electrons moving per unit of time is called amperage. This is a measure of the amount of current. Voltage is a measure of the pressure in the circuit. Resistance to the electron flow in the circuit is measured in Ohms. A classic analogy of this is a garden hose setup. The amount of water in the hose would correspond to the amperage. The water pressure would correspond to the voltage. The hose could only take so much water pressure at a given time. Any more pressure or water would be met by more resistance from the hose. This concept has been mathematically related by Ohm's law of E = IR, where E is the voltage, I is the current and R is the resistance. One can increase the current and decrease the voltage by decreasing resistance, just as more water could pass with a lower pressure through a fire hose instead of a garden hose. Similarly, more current can pass through a larger diameter wire or through a highly conductive metal such as copper. In both cases, the thicker wire and more conductive metal have lower resistance.

In the case of a human body, resistance is determined by factors such as fluid content, general health, skin thickness, amount of oil on the skin, humidity in the air, etc. If a person has a higher resistance, less current will flow through. The voltage can be raised to maintain the desired level of current. The better electromedical devices deliver a constant current by self-adjusting the voltage as the skin resistance changes.

Our understanding of pain mechanisms and how pain is perceived has undergone significant changes. Initially, pain was viewed as an emotion rather than a sensation. It was said to be the opposite of pleasure. This theory dated back to the time of Aristotle. The traditional theory, described as the specificity theory by Descartes in 1664, conceived of a pain system as a direct channel from the skin to the brain (Melzack & Wall, 1982). This concept was supported and expanded on by Von Frey, who argued that there were specific pain receptors (Sinclair, 1953). As scientific experimentation grew towards the later part of the last century, Goldscheider developed the pattern theory, which stated that sensation was based on spatial and temporal patterns of a number of impulses (Nafe, 1929; Geldard, 1953).

Zotterman (1936) discerned A-Delta and C afferent nerve fibers. Later research stated that A-delta fibers carried fast, but short-lived, well-defined pain impulses while C fibers carried slow, diffuse, long-duration pain.

The modern basis for the widespread use of transcutaneous electrical nerve stimulation (TENS) for pain control is the gate control theory proposed by Dr. Ronald Melzack and Dr. Patrick Wall (1965). This theory suggests that hypothetical "gates" in lamina V of the substantia gelatinosa could be "closed" to block pain stimuli traveling from A-delta and C fibers in the peripheral nervous system where they synapse into the central nervous system. Although this theory was later modified, it created much notoriety at the time and revolutionized the way most practitioners viewed and treated chronic pain.

In 1967, C. Norman Shealy, M.D., a neurosurgeon who had been implanting dorsal column stimulators (DCS), discovered that devices that transmitted electricity transcutaneously were just as effective without the risks associated with surgery (Shealy, Mortimer, & Reswich, 1967). With the Melzack-Wall theory behind him, Shealy's work renewed interest in electromedicine and resulted in the beginning of the modern day TENS devices.

Transcutaneous electrical nerve stimulation is an established modality with over 250,000 TENS units prescribed annually in the United States alone. Accordingly, much information is available on standard TENS applications, precautions, contraindications, side effects and results (Mannheimer & Lampe, 1984; Tapio & Hymes, 1987).

Mixed results have occurred due to several factors, such as waveform, frequency, pulse repetition rate, intensity, alternating vs. direct current, electrode placement and treatment time. Perhaps the single largest variable is the education, or lack thereof, provided to health care practitioners recommending these devices and instructing patients in their use. Many professionals simply give the unit to a patient with the factory settings and never alter them, leaving each device as a hit or miss item. TENS devices must be tried at a variety of settings to achieve optimal pain relief for a given patient.

In principle, TENS applies an electrical force that stimulates pain-suppressing A-beta afferent nerve fibers which compete against pain-carrying afferent nerve fibers. The same stimulation would occur if one lightly tapped the painful body part repeatedly with a pen, spoon, or other blunt object. Ice, massage, heat, manipulation and other long-standing therapy techniques have relieved pain this way for centuries.

Janet Travell, M.D. started researching trigger points in 1957 (Travell & Simons, 1983). Trigger points are named such because they trigger pain when pressed. They are areas of localized tenderness that usually form with a persistent muscle spasm. For the purpose of this discussion, they can be measured as having higher conductivity than normal tissue. In most cases, electrical treatment directly on the point is indicated. Sometimes, this will increase the pain (as explained by the pattern theory of pain). If this occurs, treating across the area with microcurrent stimulation at 0.5 Hz will almost always alleviate it. With the parameters discussed, it is apparent that several combinations of PRR, pulse width, intensity, electrode placement, time and frequency of stimulation may have to be attempted before desirable results are obtained. Perhaps this is the reason why most major medical insurance companies prefer to rent a TENS device to a patient for the first month before approving it for purchase.

TENS is now a widely used modality for the treatment of general acute and chronic pain syndromes. When effective, the ease of use, general safety and portability make it a preferred