In the present study, we identified severe myofascial MSP (VAS score ≥ 70 mm) and the use of high-frequency shock waves as positive predictors of rESWT response. To our knowledge, this is the first study to address the predictors of a response to rESWT. Intense pain limits the use of kinetic therapy, and controlling it quickly is a way of preparing the patient for exercises that will modify the biomechanical factors that cause or amplify pain. This study demonstrates that rESWT is effective in reducing pain in a short time and can be used as the first-line analgesic intervention in musculoskeletal rehabilitation programs.
In a study conducted in 2017, Hong et al.  achieved a 48.0 mm reduction in the VAS score with three sessions of focal shock wave therapy in lumbar myofascial pain syndrome and a 29.3 mm reduction in the mean VAS score with trigger point injection. Another study on the treatment of trapezoid myofascial pain with focal shock waves reported a reduction of 26.4 mm in relation to the mean initial VAS score after four sessions . Comparatively, our study demonstrated that rESWT is superior to focal shock wave therapy and trigger point injection, because rESWT, which is noninvasive, reduced the mean VAS score by 43 mm with fewer sessions over a similar period using a more affordable radial ballistic pneumatic device than the focal device used in the two studies cited above. Given that there is still no evidence of any advantages of focal generators over radial generators [48–50], the present study contributes to leading health care facilities to prefer, for reasons of cost and access, pneumatic radial ballistic equipment for analgesia in patients with MSP.
A study comparing rESWT with therapeutic ultrasound showed that four sessions of rESWT at 2,000–3,000 pulses/session, a pressure of 1.60–3.00 bars, and a frequency of 10 Hz reduced the VAS score by 15.3 mm, compared with only 10.7 mm for therapeutic ultrasound . In another study, involving patients with shoulder tendinitis, seven sessions of rESWT, with a mean pressure of 1.70 bar, mean frequency of 5 Hz, and mean of 2,175 pulses/session, resulted in a 26 mm reduction in the VAS score . The fact that we achieved greater reductions using the same type of generator, with fewer pulses/session, higher pressures, and higher frequencies is likely attributable to the last (the higher frequencies).
The fact that the rESWT device employed in the present study must be serviced every one million pulses is important for the organization of health care facilities. If the dose is the same as that employed in our study (1,800 pulses/session), with a single service protocol, a larger number of patients can be served and the analgesic effect can be greater, thus reducing the direct and indirect costs, as well as increasing access to the treatment.
Although the present study did not involve the use of a placebo or control group, a review of the literature based on articles cited in systematic reviews and meta-analyses showed that, for the pathologies included in this study, placebo effects accounted for a 25–35% reduction in pain, as measured by VAS [53–56]. Despite its design, our study showed that rESWT promoted effective analgesia greater than any placebo effect reported recently in the literature.
Regarding the frequency of rESWT application, our findings are in line with explanations of the mechanical and chemical effects of the treatment. According to the literature, rESWT evokes dose-dependent histological reactions related to mechanotransduction phenomena—the conversion of mechanical stimuli into biochemical and biological signals [32, 36, 37, 57]. Higher frequencies increase the cycles of tissue compression and decompression, forming more cavitation bubbles that are implicated in the signaling that promotes mechanotransduction and the consequent biological effects, providing greater pain reduction [58–61].
One common feature of all pressure waves occurs when the energy displacement encounters an obstacle presenting resistance to its propagation . Every tissue interface (skin, fascia, muscle, or bone) or higher density structure (motor plate, myofascial trigger point, inflammation or calcification) functions as a reflective surface . The shock wave emitted by the equipment propagates towards the focus to be treated and thus undergoes increased reflection in these areas. The encounter between the next emitted wave and the reflected wave generates a secondary wave of greater intensity than the incident wave . Thus, increasing the frequency of pulse firing increases the number of these secondary waves, resulting in a greater number of cavitations [20, 23] and thus a greater therapeutic effect, precisely in the areas related to pain and analgesia, explaining the fact that the higher frequency used in our study increased the magnitude of the analgesic effect in comparison with that obtained with low frequencies by other authors [51, 52].
Radial shock waves present less penetration, and cavitations are therefore concentrated closer to the emitting source, presenting no propagation to act in deep structures . Thus, its analgesic effect in joint diseases, which require greater shock wave penetration, is inadequate, which is consistent with our finding of lower analgesic effect in our joint pain group [23, 25].
Although still speculative, other researchers have concluded that rESWT produces a series of reactions such as increased capillary blood flow at the site of application, decreased muscle tension, reduced substance P , and, especially important for our study, direct suppression of pain receptors [26, 62], which explains the fact that higher initial VAS scores correlated with greater treatment success. Greater pain increases the number of active nociceptors that increase local density and act as a reflective surface that generates more cavitations and a greater therapeutic effect. In addition, rESWT mechanically inhibits the nociceptors, generating immediate analgesia and minimizing pain afferents to the central nervous system [59, 63].
Although the mechanism involved in MSP analgesia by rESWT remains uncertain, studies show that the series of events triggered by the energy also promote the process of repair and regeneration of muscle, tendon, and bone in the target area [19, 44, 61, 65]. Regeneration is a challenge for modern medical science, and basic research has provided evidence of the potential of rESWT to stimulate regenerative activity. Studies have shown increased collagen synthesis and maturation, resulting in a more resilient scar [14, 43, 47, 50, 64].
Because rESWT is a physical therapy, the biological responses will be similar, regardless of the pathology or disease being treated, as reported in studies related to the neovascularization induced by this treatment in other conditions such as ischemic cardiomyopathy  The present study, based on the biological effect already demonstrated in other areas of medicine, grouped together a large number of diseases of the musculoskeletal system and showed that rESWT promotes analgesia, regardless of the anatomical site of the pain, patient gender, patient age, the duration of symptoms, the rESWT pressure, or the number of rESWT pulses per session.
This study proved that rESWT provides analgesia that is efficacious for at least one week after the end of treatment, which is important for patient well-being and for the prevention of chronicity (conceptually, all chronic pain was once acute). Although the rate of treatment abandonment was low in our study, it was not possible to identify the cause—whether due to improvement or worsening of symptoms, or to other reasons such as transportation difficulties or the treatment schedule.
Musculoskeletal pain is treated, regardless of cause or location, mainly in overburdened health services, in the same way: patients receive analgesics, anti-inflammatory drugs or opioids for up to 7 days. This work proposes the use of radial shock waves as a technique to promote analgesia quickly, effectively, in a non-invasive way, in which two doses are sufficient to give analgesia to the patient for up to 1 week after the end of treatment. Representing a response to manage the overload of services and the opioid crisis.