Years ago I saw a "Real Sports Physiotherapist". They dealt with olympic calibre clients mostly (don't know how I got in). TENS produces approx. 1000 times the impulse that the body uses to stimulate the nerve, to get through the fat/muscle tissue. In the process of doing this it generates heat which actually can partly cook the muscle by damaging the proteins. It is used by physiotherapy clinics so that they can plug you in and move on to the next client.
Don't use one.