Hi all. Sorry I have come in so late on this one. (Yes, one year is VERY late!)
Others have noted that there is no evidence and as far as I can tell, this is the case although recently some have started to look at penetration relative to laxity or stretch of tendons subjected to light.
But let's look at the physical characteristics of tissue and combine that with the theoretical and mathematical modelling of photon penetration and absorption. Theoretically:
- red light and similar wavelengths in the visible range of the electromagnetic spectrum are absorbed well superficially (i.e., do not penetrate far - perhaps a few millimetres at most)
- invisible (infrared) wavelengths penetrate better (small percentage up to a few centimetres according to MonteCarlo modelling)
- photons / wavelengths of light energy are subject to the principles of physics and so penetration is 'inverse' to absorption
- effect of photons on cells is subject not only to the direct action of the photons that reach target cells but also to the effect of stimulated cells communicating with other cells nearby or remotely (i.e., in theory, you do not have to stimulate every cell to achieve a response - a response can occur indirectly by the activation of only some cells in the target area that then transmit the effect from cell to cell) - this is the abscopal effect
So, in theory we needn't apply photobiomodulation (PBM) therapy with pressure at all except to ensure that we reduce reflection and scatter - so keep the laser probe in contact where possible and ensure that the entire diode face is in contact and preferably at 90 degrees to your target tissue.
However, any EPA is going to have its best effect if you try to optimise all facets of the treatment. If you truly wish to optimise the amount of photons reaching a deeper target lesion or anatomical site, compression of overlying tissue (assuming it's not painful to the patient) may help reduce the depth of overlying tissue and to situate photons closer to your target tissue. This then is the applied theory of what was originally called the 'woodpecker' technique suggested in some of the earliest clinical papers. It's a technique I still use - why not.