## 1 Introduction  
Deep friction massage (DFM) is a therapeutic resource used in the treatment of tendon injuries based on its potentially positive effects on function and pain [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]. When applying DFM, there is a set of basic principles of applications that should be adjusted, depending on the severity or condition of the injury [10]. In addition to the accurate identification of the injured tissue, the physiotherapist should take into account the direction (transversal) and the pressure of the massage (\"deep enough\") [1].  
Clinically, its effect on pain is confirmed by the immediate feedback of the patient with tendinopathy, which describes an analgesic effect during and immediately after the application of the technique [10; 11]. The positive effects of DFM on pain are often explained by the induction of hyperaemia produced by mechanical stimulus [5; 10; 11; 12; 13], the modulation of nociceptive impulses at the spinal cord (\"gate control theory\") [4; 5; 10; 13; 14; 15], and also the descending mechanisms of pain modulation [5; 10; 11; 16; 17; 18; 19]. Notwithstanding, regardless of the mechanism beyond the analgesic effect of DFM, the mechanical stimulus seems to be a common trigger of this response [16], which in DFM corresponds to the pressure applied during the technique execution.  
The principles of the DFM application are quite vague with respect to pressure, once it is described as \"deep enough\". During DFM, the intensity of the pressure is managed according to the feedback of the patient, and clinically the physiotherapist grades the intensity of the pressure by asking the patient to report whether the pain is bearable [1; 11; 12; 20; 21]. The pressure aims to guarantee indirect access to the target tissue, which may lead to an alteration of the morphological properties of the tendon and influence pain modulation [11; 12; 13; 21; 22; 23; 24].  
It has been suggested that the lack of the standardization of the DFM protocol, namely the pressure applied, could be one of the potential confounding variables concerning DFM's effectiveness [21]. Notwithstanding, taking into account that pain is a symptom used by the physiotherapist as a reference for the management of the pressure applied, the authors question if the standardization claimed for this technique is possible or even useful. Therefore, despite DFM being a widely used technique [10; 25; 26; 27], there is no study characterizing the pressure applied during DFM, remaining at the discretion of the physiotherapist. Additionally, it is not known how the amount of pressure applied may influence the abovementioned mechanisms and consequently how it affects the time needed to promote an analgesic response. Before further mechanistic or clinical studies are undertaken, it is important to accurately describe the pressure application conditions that elicit an analgesic effect, further enlightening if the standardization of the pressure applied may add clinical value. The purpose of this study was, therefore, to determine if a dose-response relationship existed between the pressure applied during DFM and the time to the onset of analgesia in an asymptomatic patellar tendon. In order to accomplish this, we first characterized the pressure applied during DFM, describing the mean pressure applied by physiotherapists and determining if it was applied uniformly during the DFM execution (study 1). Because pressure may influence the effects of DFM on pain, we further studied the effects of different pressures, based on the mean pressure found in study 1, applied during DFM on the time to the onset of analgesia (study 2).