This a randomized controlled clinical trial with first primary end point at four weeks and second primary end point at two months.

Sample size was determined through power analysis. The Minimal Clinically Significant Difference (MCID) of our main outcome measure, Kujala Anterior Knee Pain Scale, was used for the estimation. The MCID of the Kujala scale has been reported to be 10 points with an estimated standard deviation of 13,5 points. For a power of 0.80 and a level of significance a=0.05 and considering a two tailed t-test the sample size was calculated to be 60 participants total (30 in each group). The GPower 3.0.10 software was used to calculate the sample size. By estimating a possible 20% drop out we concluded to a sample size of 75.

The participants of this study will be volunteers and considered a sample of convenience since no official records or data base exists in Cyprus with all PFP patients. Participant recruitment will begin in November 2019. Potential participants will be informed for the study through printed posters around the European University Cyprus (EUC) campus, local social media online posters and will volunteer for inclusion. Screening will be carried out by an orthopedic surgeon, at the musculoskeletal laboratory of the EUC to determine their eligibility based on specific inclusion and exclusion criteria.

Volunteers who meet inclusion criteria will receive oral and written information and give informed consent to participate in the study by signing a consent form. They will be randomized into one of two treatment groups. Group 1 will be the Hip and Knee Exercise group and Group 2 the BFR training Hip and Κnee exercise group. Randomization will be carried out using a block sequence of 4 by an external statistician to maintain a balance in sample size across groups over time and to ensure the assessor was blind to the participants allocation.

All interventions will take place in the musculoskeletal laboratory and the training center lab of the EUC. Time consistency in treatments will be a goal, in order to train participants at similar circadian rhythm. Room temperature for both labs will be set at 25 °C. Two experienced physiotherapists will each be randomly assigned to supervise one of the two intervention groups and provide the treatment. The therapists role is to supervise treatments and their progression, keep track of compliance with the study design, adverse effects and drop outs. If patients do not comply with the study (seek other treatment for PFP, exercise during the study, inability to attend treatments) design they will be excluded. The two therapist will be blinded as to the other groups treatment protocol and the studies outcomes. They will be given written information for all procedures of their intervention group in order to ensure consistency throughout the study. Data collected by the therapists will be stored at the E.U.C laboratories and will only be available to them. Additionally, the therapists will be assisted by fourth year physiotherapy undergraduate students in supervising participants during treatments. Participants who fail to complete at least 10/12 therapy sessions will be excluded and their data will not be analyzed. All therapy sessions will be supervise by a physician who will be in charge of participants safety. Any adverse events will be reported with publication of final results. Participants are free to withdraw from the study at any given moment. In case of any complaints, participants have the right to file official complaints to the university bioethics committee.

Planed statistical analysis: Statistical analysis will be performed by the studies assessor who is blinded to the participants treatment group using the IBM SPSS "Statistical Package for the Social Sciences" (SPSS, Version 20.0). Descriptive statistics will be analyzed with the calculation of means, standard deviations, minimum and max values for each variable. A normality check will follow using the One Sample Kolmogorov-Smirnov test (p\<0.05). The chi-squared test will be used to test for difference between the two groups with respect to the categorical variable gender. To check for differences within groups (baseline-4weeks, baseline-2months follow up and 4weeks-2months follow up) we will use the paired t-test for each comparison. In case of nonparametric data, the Wilcoxon signed rank test will be used. In order to estimate differences between the two groups (baseline-4weeks, baseline-2months follow up and 4weeks-2months follow up) we will use the mixed two way ANOVA with time as a within-subjects factor and treatment as the between subjects factor. Homogeneity of variance will be checked with the Levene test \<0.05. In the case of not normally distributed data we will use the generalized mixed effect model with treatment as a fixed factor, time as a random factor, and inverse as the link function. Possible correlations will also be checked for our outcome measures using the Pearson correlation coefficient. In case of not normally distributed data the Spearman correlation coefficient will be used if there seems to be a linear correlation from a scatter plot. Missing values in the Patient Reported Outcome Measures will be imputed using Multiple Imputation (MI). Only upon agreement by the authors that there will be no further changes and/or analysis, results of the study will be published.