Updated: Feb 27, 2020
Kinematics, mechanics and the world of Physics…phenomenona so simple, and yet so complex to observe and express in mathematical form. Sir Isaac Newton, arguably one of the best in the matter of mechanics, summarized as the third law of motion that for every action there is an equal and opposite reaction. Forces always come in pairs - known as "action-reaction force pairs." Does this principle apply to shock waves? The answer is without exception, yes.
It is a fact that shock waves are capable of generating the “action” force via compressive pressure of its positive pressure signal, and high velocity jets (another “action” force) during the implosion of cavitational bubbles produced by tensile/negative pressure of the shock wave pressure signal (only when shock waves are traveling through liquids).
However, it is even more complex when living organisms are involved, and toned-down medical shock waves are acting at the cellular level or tissue level. The action-reaction principle is still applicable at the moment shock waves pass through tissue, which gives the instantaneous “action” of the shock waves and “reaction” at the macro level of the tissue and the micro level of the cell. The interesting and less typical part is the “secondary reaction” or “delayed reaction”. Nicknamed by medical professionals as the Mechanism of Action, or MOA, it has far more implications on the tissue and cells.
When produced by an explosion, the shock waves’ action-reaction effect is noticed immediately in close proximity to the point of origin, as seen below in the archived photos of early nuclear explosion tests. The compressive force generated by acoustic pressure shock waves is the “action” that rolled/pushed the school bus for about 50 feet, and the “reaction” is the rolling motion of the bus that consumed completely the “action” force until the bus came to a complete rest. At the time when the bus stopped from its rolling motion, the shock wave front that started the “action” on the bus was further away, due to the fact that shock waves travel through air at 300 m/s (0.186 mile/s). Thus, the shock wave “action” was then felt in other places.
What happens at large distances away from the explosion’s epicenter? It’s fast and furious.
When the military tried to monitor one atomic bomb explosion, from the air and at a considered safe distance from explosion’s epicenter, the high energy-generated shock waves showed action farther than expected. To general surprise, the zeppelin used for observation turned into a “victim” of the shock wave action, with the shock wave crushing it and easily sending it down, as seen from picture below.
When they specifically travel through liquids and not atmosphere, the other possible “action” forces of shock waves are the cavitational jets produced by the collapse/implosion of the cavitation bubbles generated by the negative pressure of the shock waves in its tensile phase, as seen below.
In many cases, cavitation action forces can generate undesirable consequences. In devices such as propellers and pumps, cavitation causes a great deal of noise, damage to components, vibrations, and a loss of efficiency. In domestic plumbing, when a pipe is suddenly closed at the outlet (downstream), the mass of water before the closure is still moving, thereby building up high pressure and resulting in a shock wave that manifests as a loud banging resembling a hammering noise, known as a “water hammer”, which, if the pressure is high enough, can cause pipelines to break. The action forces produced by cavitation can produce “reaction forces” in materials surrounding the fluid that can exceed the strength of the material. This can be devastating. The below picture depicts the total destruction as a result of cavitation of a headrace cement tunnel from a hydroelectric dam.
In medicine, shock waves are used either to destroy kidney stones or to stimulate living tissue to repair and regenerate. The immediate “action” of the shock waves stretches the tissue and produces tissue strain, thus generating the immediate tissue “reaction” (macro level, immediate reaction). When cavitation bubbles produced in any of the body’s fluids (blood, interstitial fluid, urine, etc.) collapse, they produce micro-jets (the “action force”) that interact at micro-level with individual cells from the fabric of the tissue or the adjacent structures.
Regarding kidney stones destruction, the shock wave “action” forces exceed the kidney stone’s strength, thus producing stone fragmentation. For tissue regeneration medical application, the shock wave “action forces” are reduced in intensity in order to produce “reaction forces” at the macro and micro level and generate a cascade of “secondary body reactions” such as the reactive oxygen species (ROS) inside body fluids, expression of growth factors, angiogenic factors, inflammation modulation, improved microcirculation and oxygen supply that ultimately produce cell proliferation and differentiation.
The summary of all these reactions, demonstrated with numerous scientific publications results, are part of the shock waves mechanism of action or “MOA” inside living tissue, as presented in the following movie.
In conclusion, shock waves definitely follow the nature principle of action and reaction, although shock waves have their own nuances when it comes to living tissue reaction: a “double reaction” (instantaneous and delayed) can be seen, can regenerate cells/tissues and constitute a non-invasive mean to add to our armamentarium of ways to keep one healthy and repair damaged tissue, which ultimately translates to a more productive life for personal benefit and for society.
Keywords: ESWT, extracorporeal shock wave technology, extracorporeal shock wave therapy, cavitation, mechanism of action, wound care, diabetic foot ulcers, DFU, shockwave therapy, amputation prevention, dermaPACE, chronic wounds, SANUWAVE, PACE Technology