Where you put the electrodes decides where the current goes. Get this right and most of the serious risks of e-stim never arise.
Think about the path#
Each channel has two electrodes. Current flows from one to the other through the body, taking the easiest routes between them. It doesn't travel in a neat line, but most of it stays in the tissue between and just around the two contacts.
So for any placement, ask one question: what lies between these two electrodes?
- Two pads on the same thigh: thigh muscle. Fine.
- One pad on each thigh: the current crosses the pelvis.
- One pad on each arm: the current crosses the chest and the heart. Never do this.
The below-the-waist rule#
The simplest way to keep current away from the heart is to keep every electrode below the waist. With nothing above the waist, there's no path through the chest.
Medical TENS is sometimes used on the back, shoulders and arms under a professional's direction, with both pads close together on the same side. This site sticks to the stricter rule because it's easy to remember and leaves a wide margin for error.
Places to avoid#
| Where | Why |
|---|---|
| Chest, or any path across it | Current can disturb the heart's rhythm |
| Head, face and eyes | Current through the brain or eyes |
| Front and sides of the neck | Can affect blood pressure and heart rate, and close the airway by making throat muscles contract |
| Broken, irritated or sunburnt skin | Lower resistance concentrates current and it stings and burns |
| Numb skin or areas with poor sensation | You can't feel a burn starting |
| Over piercings and jewellery | Metal concentrates current into a small area |
| Over metal implants or implanted devices | Take medical advice first |
Keep pairs close together#
The further apart two electrodes are, the more of the body the current passes through, and the less sure you can be of its route. Start with both electrodes of a channel on the same limb or the same small area.
Closer together gives a shallower, more local sensation. Further apart goes deeper and brings in more muscle.
Two channels at once#
With two channels you have four electrodes. On some units a little current can flow between electrodes belonging to different channels (see How an E-Stim Unit Works). Treat every electrode as if it could pair up with any other, and keep all of them below the waist.
Electrodes that touch each other#
If the two electrodes of a channel touch, the current takes the shortcut and you feel little or nothing. On a decent unit this won't harm it, but turn down before separating them, because the sensation returns the moment they part.
Electrodes of different sizes#
When one electrode is much smaller than the other, the smaller one feels stronger, because the same current is squeezed through less skin. That's also where a burn would happen first. Contact Area and Current Density explains why.
Internal electrodes#
Tissue inside the body is moist and conducts far better than skin, so much less power is needed and levels must come down accordingly. Use only electrodes made for internal use by a reputable maker, follow their instructions and use the right gel. Never use sticky pads, bare wires or improvised objects internally.
Before you switch on: a quick check#
- Is every electrode below the waist?
- What lies between each pair? Is anything on the avoid list in the way?
- Is the skin unbroken and able to feel normally?
- Are all the electrodes in full contact, with no lifted edges?
- Is the level at zero?
If every answer is right, bring it up slowly.