Extracorporeal Shockwave Therapy of Healthy Achilles Tendons Results in a Conditioned Pain Modulation Effect: A Randomised Exploratory Cross-Over Trial

Background. This study investigated whether ESWT applied to healthy Achilles tendons results in a conditioned pain modulation (CPM) effect. Methods. 3000 and 6000-shock dose radial ESWT was performed on dominant side Achilles tendon (random order over 5-9 days) among 20 healthy participants (75% male). Pressure was modulated throughout, producing >4/10 pain (10=worst imaginable). Bilateral pressure-pain thresholds (PPT) were assessed pre-post ESWT at Achilles tendon, tibialis anterior and elbow lateral epicondyle (random order). PPT change per site and PPT change scores for each site were compared between doses (repeated ANOVA), PPT reliability (intraclass correlation coefficients [ICC]), and PPT change variability (each site, coefficient of variation) were investigated. Dose order was investigated (first vs second) (t-tests, each site). Results. Test-retest reliability was good for all sites (ICC [3,2]=0.76-0.89). PPT increased significantly (p<0.05) at all sites bilaterally following 3000-shock dose, and at bilateral Achilles tendon, and left sided tibialis anterior and lateral epicondyle following 6000-shock dose (right 6000-shock tibialis anterior, lateral epicondyle p>0.05). Dose order wasn't associated with outcome (p>0.05). Conclusion. ESWT produces immediate CPM effect when applied to Achilles tendon in healthy populations. This appears more consistent at 3000-shock dose. Determining if this mechanism occurs in Achilles tendinopathy will inform appropriate modality use.