According to Michael Crary who wrote the book Dysphagia: Clinical Management in Adults and Children, the electrodes may be placed on the skin, on the muscles or by implantation.2
But for most neck or back injuries treated in outpatient physical therapy clinics, the electrodes will be placed on your skin.
The exact placement of the electrodes depends on the reason for which the treatment is given, as well as how deep or superficial your therapist intends for the current to go. The way the muscle changes shape once it’s exposed to the current is another factor in determining placement of the electrodes.
Your therapist will adjust the controls of the electric stimulation machine up to the point where you feel a prickly sensation. This sensation should be noticeable, but not overly intense. An electric stimulation treatment lasts about 10 or 15 minutes.
