Laser therapy is added energy to the cells. The biologic effect related to the amount of added energy is described by the so-called Arndt-Shulz law, shown in the picture.

arndt schultz law
The curve shows that adding a small amount of energy yields a small amount of power, a broad-range medium amount of energy results in a positive, biologic effect, while adding a very large amount of energy provides a biologic inhibition.

This has been well-known for years, but in practice it has been almost impossible to reach the inhibitory part of the curve, simply because the laser devices available were not powerful enough.

For a long time, the general consensus has therefore been “the more, the better”. However, with more powerful lasers now being available, there is a much larger risk of applying too large a dose, i.e. an inhibitory dose. As mentioned, laser therapy is added energy to the cells, and the amount of energy that is needed is directly proportional with the number of cells that need stimulated, or in other words: the volume (area x depth) of the tissue in question. As a result, very superficial areas, such as an elbow where there is very little soft tissue, need a very small dose. Larger muscles need a significantly larger dose, and deeper areas need an even larger dose.

Energy = power x time

Energy is measured in Joules, which is measured as Watt x seconds. Consequently, the energy dose can be adjusted either by varying the power (Watt) or the time (seconds) – or a combination of both.

Not enough to reduce the time
For very superficial areas, however, it is not enough to reduce the time if the dose is too large. Here, another factor to consider is the power density, measured in Watt/cm2. If the power density or the intensity of the laser is too large, the cells will be overstimulated/inhibited, no matter how long the treatment time is. WALT* has looked into a number of common ailments and has compiled a list of recommended doses, both in terms of Joules and milliwatt (mW). The following is an extract of their list:

Max 100 mW!
For very sensitive areas, it is important not to exceed 100 mW/cm2. If the power density and/or the dose is exceeded, the laser loses its otherwise positive effect and may make the tissue more irritated than it already was, with unnecessary pain as a result.

*) WALT
WALT, the World Association for Laser Therapy, is an international organization that focuses on promoting
the awareness of biostimulating laser therapy. See Walt.