Sometimes clients come in with a very specific pain issue that they comment that their TENS machine did not work. Also, some clients visit a Dr. or chiropractor where this machine was hooked up to them and again, come back to me as there was no change in their discomfort. The problem with owning your own personal TENS machine, is that most people do not educate themselves as to when to actually use it. So in many cases, it results in worse spasms and more intense pain.
The usage of a TENS machine is based on the Gate Theory. http://en.wikipedia.org/wiki/Gate_control_theory_of_pain
In layman terms, the appliance works by "tricking" your brain into not feeling the pain, just the pulsing sensation of the electrode. OK, this makes sense, I rub my toe when I stub it on the coffee table... So why would it not work with a muscle spasm? With most muscle spasms, their occurrence stems from being over-worked or used incorrectly. When you electrically stimulate your muscle, you are actually making it worse by perpetuating the cause of the pain! There may be better applications for the TENS, but I am from the school of thought where you solve a problem by eradicating the cause of it. Not trying to mask it for a period of time. So what are you supposed to do instead? Of course, come see me and get a massage (that's a Captain Obvious) but you can also try stretching it out, work with heat and ice application or rolling a lacrosse ball or foam roller on the area.