Rule of thumb: penetration is not what you think it is.
We recently ran a study on hair growth. The most surprising result was that the PEMF hat insert was designed to generate a PEMF field only as deep as the scalp, only about 5 or 6 mm, to avoid having too much of an effect on the brain when the intended effect is to enhance hair health.
But the biological effects were remarkable, for hair health but also for very deep tissues. There were positive effects on things like migraine, TMJ, cervical spine pain, etc.
Supposedly the magnetic fields did not penetrate that deeply. But the effects were there.
So, rule of thumb I would say is depth of penetration does not seem to have much meaning. Tissues that are 5 or 6 or 8 inches deep respond when ICES-PEMF is applied to the surface.
My hypothesis: we are not attempting to focus energy to a single point deep in tissue. To get to the deep tissue, the PEMF first must travel through the less-deep tissues. As it does so, the PEMF promotes a low-inflammation tissue state that has a beneficial effect on all surrounding tissues, including deeper tissues.
The body is a highly interconnected system with many feedback loops, so this makes sense to me.