As you read, both TENS and EMS employ electrotherapy to treat acute and chronic pain, but the methodology differs. (Note that some of the references are to NMES, which stands for Neuromuscular Electrical Stimulation, and is considered synonymous with EMS.)
Transcutaneous Electrical Nerve Stimulators (TENS) use electrotherapy to stimulate the nerves and active therapeutic healing. Electronic Muscle Stimulators (EMS), on the other hand, sends electric impulses that cause muscle contraction. EMS units are used to prevent atrophied muscles, for retraining muscles (e.g. in partial paralysis), to increase range of motion, and other uses.
Some of the technology is described here.
A TENS unit uses microcurrent to increase the production of ATP, which is adenosine triphosphate and the substance that supplies energy to the body on a cellular level. Although lasting pain reduction may require regular microcurrent sessions, in some individuals the pain reduction begins to last longer and the time between sessions lengthens. This may be due to the increased circulatory action that enables the lymph system to begin clearing the body of accumulated toxins. The sensations of both the microcurrent and conventional TENS treatments are more like tingling sensations than the unpleasant feeling of a small electrical shock.
EMS uses a cycle of stimulation, contraction and then relaxation, to increase blood flow to the area, which decreases inflammation and promotes healing and muscle growth. By stimulating the muscles at their basic structure, an EMS unit causes muscle contractions similar to those obtained by exercise. The EMS units are specifically used to prevent atrophied muscles or for retraining muscles, such as in an individual who has been paralyzed or partially paralyzed. In addition, EMS is used for many other applications such as range of motion improvement, increased circulatory action and the prevention of blood clots.
Both types of therapies are convenient, in that they're used at home, and the patient controls the dose, generally by way of either a knob or a switch. Electrodes are user-replaceable, though placement instructions are given and monitored by a physician or physical therapist. TENS units also have a "burst" feature, enabling the patient intermittent increases, which is helpful after strenuous activity or if you're just having a bad day. I personally found that feature very helpful!
A Yale University study entitled Neuromuscular Electrical Stimulation for Skeletal Muscle Function(Yale J Biol Med. 2012) discussing electrical stimulation (ES), neuromuscular electrical stimulation (NMES), transcutaneous electrical nerve stimulation (TENS), and functional electrical stimulation (FES) states that the potential that electrical stimulation holds for rehabilitation recovery is immeasurable. It is used to improve muscle strength, increase range of motion, reduce edema, decrease atrophy, heal tissue, and decrease pain.
Neuromuscular electrical stimulation (NMES), used interchangeably with electrical stimulation (ES), is typically provided at higher frequencies (20-50 Hz) expressly to produce muscle tetany and contraction that can be used for “functional” purposes and can be found in literature as early as 1964. TENS is an alternate form of electrical stimulation that historically used high frequencies for pain relief but is now also administered at very low frequencies (sensory level TENS, 2-10 Hz). TENS propagates along smaller afferent sensory fibers specifically to override pain impulses. When very low frequencies are used, TENS specifically targets sensory nerve fibers and does not activate motor fibers; therefore, no discernible muscle contraction is produced.
This source also has chapters on other subjects pertinent to your question, including, among others:
- Parameters of electrical stimulation
- Pulse width/duration
- Stimulation pulse patterns
- Stimulation intensity
- Dosing of stimulation
- Limitations of electrical stimulation.
In addition, you may also be interested in the following charts:
For EMS, this describes in detail which muscles are targeted by names, and at which frequencies.
shows placement of electrodes of TENS unit, based on the location of the pain.
For further information, including uses, cautions and case studies, see:
Transcutaneous Electrical Nerve Stimulation
Effects of Three Recovery Protocols on Range of Motion, Heart Rate, Rating of Perceived Exertion, and Blood Lactate in Baseball Pitchers During a Simulate Game.
Serratus muscle stimulation effectively treats notalgia paresthetica caused by long thoracic nerve dysfunction: a case series
Understanding the Differences Between TENS and EMS Units
How Does A Tens Unit Work?