TENS works on voltage not current. By stimulating the smaller sensory nerves, it is shown to inhibit the flow of pain signals along the larger nerve trunks. The reason that alkaline cells work better than rechargeable is that the AA/AAA/C/D type of cells deliver a nominal 1.5 volts, whereas rechargeable of similar physical type deliver only 1.2 volts. The PP3 type of battery is a series of 6 cells, hence the alkaline type deliver nominal 9 volts and the rechargeable equivalent only 7.2 volts, hence the voltage on a lot of drills, screwdrivers etc. Commercially, TENS has become a very competitive field. The cheaper machines use very simple circuitry and the effect is dependent on the supply voltage. The more sophisticated machines use chips which will deliver a pre-determined peak voltage and wave form. Regrettably, without a knowledge of electronics, obtaining copies of CCT diagrams and identifying specific component types that latter cannot be determined. I am frequently appalled at the built quality of the plastic cases of such machines as they are used for hours at a time in normal waking and sleeping life and hence are vulnerable, also the connections from plugs to unit/electrodes, and wires to plugs. I don’t think that a limit exists to the length of time which it can be used in a session. The limiting factor is usually bi-fold:
A. sensitivity to the coupling electrolyte gel/solution.
B. ‘Accommodation’ to the waveform.
You will find many publications on electrode positioning. In the event trial and error seems to be the best known for the ley person. IT IS INADVISABLE TO CAUSE MUSCLE STIMULATION WITH TENS MACHINES. Yes, medical electrical stimulation has been used for over a century in the form of faradism named after Michael Faraday (1791-1867) who did some very early work on electrolysis (and a lot more). Significant research has been carried out in this field and it has been shown at Liverpool University that the wave forms produced by the devices such as your cousin used, can damage muscle fibers and nerve tissue. I am sorry about the length of this screed but felt it important to clarify some points and issue the warnings for the benefit of other list users.
Comment 1: My mistake ohms amps and volts was a long time ago. So am I, I used to import 50p digital watches that were better built than some! “It is inadvisable to cause muscle stimulation with TENS Machines”. This is a bit of info that I will accept, when I pointed this effect out to the supplier he said that I could use it for that function (things have either moved on or another salesman at it!), I have no doubt that you are right. As this is the case why is it possible and why inadvisable?
Given a choice of standing on my hind legs and walking with damage or being safe and immobile I would chose the former. It did cause a lot of confusion that my damaged leg was 1-2 inches larger around the thigh than my ‘normal one’, this being a combination of normal and electric exercise. The pro machines were used by a highly qualified AND experienced physio. Does this apply to the commercially available electronic exercisers as well? The instructions are basic. Thank you it’s a pity that the instructions are of the same standard as the build.
Comment 2: I found it very interesting, and I agree it is certainly important. I had no idea that some of the TENS machines on sale might be poor quality. I can recommend a couple of books to anyone who wants to read more about exactly how TENS machines affects pain. It’s an absolutely fascinating account of the brain’s influence on the intensity of pain. The books are “The Challenge of Pain”, by Ronald Melzack and Patrick Wall, Penguin, 1988; and “Pain: the Science of Suffering”, by Patrick Wall, Weidenfeld & Nicolson, 1999, ISBN 9-780297-842442. Patrick Wall was co-inventor of TENS so the explanations in these books come straight from the horse’s mouth. Some of what the horse says is pretty complicated but you can get the picture without having to absorb all the details.
Comment 3: I guess I should have been a bit more specific. I should have said that I think that the use of TENS for muscle stimulation in RSI is inadvisable. Sure, it can be used for that purpose if so desired. I was not casting any opinion on another professional’s use of muscle stimulation equipment, just reporting findings over the last few years. I have used both faradism and galvanism (IDC) myself. Its use was being taught till the mid eighties to my knowledge, may be later. Incidentally, the 4 electrode T E N S shouldn’t be confused with interferential. With T E N S we are talking about 2 * 2 channels interferential is another modality altogether.