TENS is short for Transcutaneous Electrical Nerve Stimulation.

The correct interptation of pain

PAIN LEVEL     DESCRIPTION
10 EXCRUCIATING, often debilitating.
8 INTOLERABLE, concentration difficult, able to perform certain tasks of an undemanding nature.
6 DISTRESSING, but able to continue tasks.
4 DISCOMFORTING may be ignored at times.
2 MILD, only aware of it when attention is brought to it.
1 NO PAIN.

TENS units stimulate nerves with electrical impulses, delivered with electrodes in pairs.

1. Place electrodes (must use in pairs)

Where you place the electrodes is the most important variable for optimum results. You must always use electrodes in pairs. Peel the self-adhesive electrodes off of the protective backing and save the backing to store the electrodes after use. If the adhesive is drying out and the electrodes do not stick well, you may wet the electrodes by wetting your finger with water and lightly rubbing it over the electrode for just one second (more time will saturate them and actually shorten their life). Discard and replace electrodes when adhesive has split or worn through to the conductive backing.

Visualize where your pain is or where your doctor has determined your pain is originating from. If possible, place one electrode directly over the area of pain, and the other where an imaginary line drawn between the two electrodes would go directly through the area of pain to the opposite side of that part of the body. If it is not possible to place one directly over the painful area, or if that does not produce the best results, then still place the two electrodes in such a way that a line drawn between them would go through the area of pain. Keep in mind that the body is three-dimensional and many different lines can be drawn through your pain. One of these lines should work better than the others. Therefore the best electrode location for you is the one that works best for you. Several attempts to find the best electrode locations are usually necessary.  After treatment the pain may move around. This will require moving the electrodes to "follow" the pain.

Do not place the electrodes too close together. For example, one right next to each side of the spine is not a good placement for back pain. In this example the current will travel just under the surface of the skin to the other electrode and not penetrate to the area requiring treatment. Here it would be better to put one electrode next to the spine and the other on the opposite side of the body, on the side, near the front.  Another possible electrode placement for back pain is to place one on each side of the body (between the shoulders and the hips) and at the level of pain. Finally, back pain may respond to one pair of electrodes on the right shoulder and left hip. You will have to experiment until the best location is found. Be sure to discuss this with your doctor initially, and again from time-to-time.

Most TENS units that have ajustable settings have at lease two of the following (some have more).

Modes of operation and settings explained

BURST Mode (explanation)

The burst mode provides a burst of seven pulses. There are two bursts that are delivered per second. Positive pulse and negative pulse interact continuously at fixed 100 Hz Intensity levels 0V to 40V adjustable. Pulse width from 50-260 micro amps. Use the burst mode at low intensity setting to treat acute, superficial pain. Use at higher intensity settings resulting in strong muscle contractions to treat chronic, deep pain. This type of treatment is usually given in sessions lasting 15-30 minutes. You will feel a strong rhythmic pumping action of the affected muscle.

CONTINUOUS Mode (explanation)

The continuous mode produces a continuous train of impulses. The stimulation parameters are not automatically interrupted nor varied in any way. In this mode, the pulse rate (from 2 to 120 Hz), pulse width (from 40 to 260 micro amps), and amplitude (from 0 to 8 on the on-off-intensity dial) are fully adjustable. The continuous mode is quite versatile because it may be applied with a variety of rate and width settings.

High Rate Normal Mode Settings (most common):
1. Rate: 60-12Ohz (60 default in many units)
2. Width: 70- 100µs (70 default in many units)
3. For most general applications

Low Rate/High Width Normal Mode Settings (acupuncture-like)
1. Rate: 2hz
2. Width: 225µs
3. Treat 1 5-30 minutes
4. Great endorphin release

Brief Intense Normal Mode Settings (analgesic effect)
1. Rate: I20hz   
2. Width: 260µs
3. Restrict to less than 20 minutes (no timer in unit), depending on degree of muscle stimulation and discomfort.
4. Brings short-term relief for acute pain

Modulated Mode:
The pulse rate and width are automatically varied in a cycle to produce a pleasant, massage-like sensation. It's believed that nerves can become accustomed to, or "accommodated" to the same electrical stimulus after a period of time and thus would require increasing the intensity to further "block" the pain. The Modulation mode was produced to offer a variety of different electrical stimulation, thus preventing nerve accommodation so that less intensity is required for long and effective treatment.  In this mode, during the beginning of 0.5-sec. period, the WIDTH decreased to 50% of its original setting and then during the next 0.5-sec. period, the RATE is decreased to 50% of its original setting. Therefore, the total cycle time is 1 second.

Using high intensity and/or wide pulse widths with this mode is recommended for 30min. to one hour. The analgesic effect may last an hour or more after the unit is turned off.
If narrow pulse widths, under 100µs. and/or low intensities are used, the treatment may be prolonged.
Intensity and pulse width may be set high for shorter treatment time. Conversely, the pulse width may be set narrow and the stimulation continued all day.

Caution!
Electronic devices must not be used above the waist, or more accurately, above the navel. Some people maintain that as long as both terminals are on the same arm this is safe, too.
If you use a pacemaker or other electronic medical device do not use a TENS unit without clearing it with your physician first!