This trial will be designed as a randomised, participant-blinded, superiority trial, with a 2-group parallel design to be conducted in Denmark. Reporting of the trial will follow CONSORT guidelines for reporting non-pharmacologic treatments and TIDieR for intervention description. Reporting of the protocol will follow the SPIRIT statement. The planning of the trial was done in accordance with the PREPARE Trial guide (25).

In addition to the exercises, both groups receive patient education on their condition. They will be informed about what is known about the condition in terms of risk factors and aetiology, the pathology, activity modification, and the rationale for why their specific exercise programme (self-managed or predetermined) could lead to recovery. The participants of the predetermined group will be informed that this specific exercise and exercise programme has been found to be superior to stretching but it is important to follow the protocol as closely as possible. The participants of the self-managed group will be informed that this specific exercise has been found to be superior to stretching but based on previous research of other tendinopathies we believe that doing the exercise as heavy as possible but not heavier than 8RM and with as many sets as possible will increase the odds of recovery. Both groups are told that compliance to their protocol is very important and that compliance to the exercises are associated with their recovery. They are also informed about other types of evidence-based treatments however, they are asked to refrain from seeking other treatments during the course of the study. A silicone heel cup will be given to all participants. If the participant already uses an insole or any other type of foot orthosis they will be allowed to continue wearing this if they do not want to use the heel cup.

All statistical analyses will be performed according to a pre-established analysis plan in consultation with a statistician. STATA ver. 14 will be used as statistical software. The primary intention-to-treat analysis will test the between-group difference in the FHSQ pain domain at the 12-week follow-up. In addition to this, between-group comparisons of the other FHSQ domains, PSEQ, IPAQ, and plantar fascia thickness will be performed using a repeated measures ANCOVA with the outcome as the dependent variable, time (baseline, 4 weeks and 12 weeks) as the within-subjects factor, group allocation as the between-subjects factor and the baseline value as the covariate. If any interactions are found, post hoc Bonferroni adjustments for multiple comparisons will be made. The between-group difference in time to PASS among participants who report PASS, and the number of training sessions performed during the trial using independent t-tests. The relative risk (RR) will be calculated for the dichotomized GROC to determine the probability of being improved and for the dichotomized PASS (Yes/No) to determine the probability of achieving a satisfactory result within the 12 weeks of intervention. The difference in mean training sessions performed per week from before to after PASS among participants who report PASS no later than during week 11 of the intervention will be investigated using a paired t-test. The number needed to treat will be calculated as 1/risk difference for the primary outcome. The association between the PSEQ score and compliance will be investigated using Pearson's correlation coefficient.