The exercise program is initiated at an individual clinical visit. An experienced physiotherapist introduces the participant to the exercise program and provides instructions to the individual exercises. An experienced physiotherapist provides the monthly supervision visits.

The program details be requested rom the responsible party. In brief the HE-program consists of:

- A short warm-up and 3 specific exercises for the hip abductor and extensor muscles

Progression of the exercise intensity is done by adding (additional) external loads, such as elastic rubber bands, body weight etc.

Isometric muscle strength of hip abductors, hip external rotators, quadriceps, and hamstrings is performed by using a handheld dynamometer following a previously published testing protocol.

The Dynamic Assessment of Pain Test is a simple performance test with an integrated pain score, designed to provide useful information for monitoring treatment progress and evaluating treatment effects in clinical physiotherapy practice. The patient is asked to perform as many squatting movements (both legs) as possible within 30 seconds. The knees should reach approximately 90 degrees of flexion and full extension for each squat. This is supervised by the rater. There outcome of the test is the knee pain during the test on a 0-10 Numeric Rating Scale (NRS) rated immediately after the test.

The pain self-efficacy questionnaire is a 10-item questionnaire developed to assess the confidence people with pain have in performing activities while in pain. It is applicable to all persisting pain presentations, and covers a range of functions. Confidence in performing activities are rated on a 7-point (0-6) Likert scale with 0 representing not at all confident and 6 representing completely confident. A total score is calculated by summing the answers producing a score between 0 and 60. Higher scores reflect stronger self-efficacy beliefs.

The EuroQoL questionnaire is a standardised patient-reported instrument for use as a measure of health related quality of life.

The Transition Questionnaire of global perceived effect is an adaptive questionnaire in which the participants initially answer if their current state is "unchanged, worse" or "better" compared to the baseline visit. An "unchanged" equals a transition score of 0. If the participant answers "worse", he/she is asked to rate the degree of worsening on a 7 point Likert scale, and the corresponding scores range from -1 to -7. Correspondingly, if a participant answers "better", he/she is asked to rate the degree of improvement on a 7 point Likert scale, and the corresponding scores range from 1 to 7. Thus the Transition score range from -7 (worsening) to 7 (improvement), with the mid-point - 0 - representing no change. The transition scale is used to assess overall knee related health status.

Self-reported presence of pain in the feet, ankles, and/or hips lasting more than 3 months, recorded as "Yes" (pain present) or "No" (No pain) for feet, ankles and hips, respectively. The participants are asked to estimate the frequency of pain in other lower extremity joints (feet, ankles, and hips) during that last 3 months. Predefined frequency options are available:

• Almost daily
• Several times during a week
• Weekly
• Monthly
• Rarely

We will a priori define a dichotomisation of the scores as "frequent [foot/ankle/hip] pain" ("Almost daily" and "several times during a week") and "infrequent [foot/ankle/hip] pain" ("Weekly", "Monthly", and "Rarely"). We will make sensitivity tests of this a priori cut-off by changing the cut-off value by +/- frequency option.

Self-reported presence and frequency of low back pain. The participants are asked to estimate the frequency of low back pain during that last 3 months. Predefined frequency options are available:

• Almost daily
• Several times during a week
• Weekly
• Monthly
• Rarely

We will a priori define a dichotomisation of the scores as "frequent low back pain" ("Almost daily" and "several times during a week") and "infrequent low back pain" ("Weekly", "Monthly", and "Rarely"). We will make sensitivity tests of this a priori cut-off by changing the cut-off value by +/- frequency option.

The painDETECT questionnaire (PDQ) is a patient reported questionnaire developed and validated to assess presence of signs of neuropathic pain. It comprises questions regarding pain intensity, course of pain, subjective experience of a radiating quality of the pain, and the presence and perceived severity of seven somatosensory symptoms of neuropathic pain. A validated algorithm is used to calculate a total score ranging from 0 to 38. A score ≤12 indicates that the presence of neuropathic pain is unlikely, a score ≥19 indicates that a neuropathic pain component is likely to be present, while a score of 13-18 points towards unclear screening conclusion.

Exercise self-efficacy in relation to the two different exercise programs are assessed by asking the participants to rate their confidence in performing the two different exercise programs on a 7-point (0-6) Likert scale with 0 representing "Not at all confident" and 6 representing "Completely confident".

The Pain Catastrophizing Scale (PCS) will used as a measure of pain-related catastrophic thinking. The PCS instructs participants to reflect upon past painful experiences, and to indicate the frequency with which 13 pre-specified thoughts or feelings occur while they are experiencing pain. The frequency is scored on a 5-point scale ranging from 0 (not at all) to 4 (all the time). The PCS measures 3 distinct components: rumination, magnification, and helplessness.

Hypermobility is assessed by the Beighton Score (28) applying the revised criteria for the diagnosis of benign joint hypermobility syndrome. The Beighton score ranges from 0-9. In this study we define hypermobility as a score of 4 or more.

Knee joint alignment is assessed by clinically observation of the participant while he/she performs a forward lunge movement. The observation is made by a trained physiotherapist that classifies each participant's knee as "varus", "neutral", or "valgus".

Knee joint alignment is assessed by clinically observation of the participant while he/she performs a single-leg mini-squat movement. The observation is made by a trained physiotherapist that classifies each participant's knee as "varus", "neutral", or "valgus".

A trained physiotherapist's estimate of each patient's potential for a successful outcome based on professional appraisal. The therapists are instructed to score each patient on a continuum of 1 (suggesting a very poor projected outcome) to 10 (suggesting an excellent projected outcome). The therapist must score each patient following their complete encounter with the patient. This may include the physiotherapist's assessment of the patient's resources (personal, material, social, etc.), personality, medical history, comorbidities, surgery reports, physical examination(s), physiotherapeutic assessments, and more.

Midfoot mobility is calculated as the change in midfoot width from non-weight bearing to weight bearing. Midfoot width is measured at 50% of total foot length using a caliper during non-weight bearing and during standing with equal weight on each foot. The mobility is recorded as the difference between non-weight bearing and weight bearing measurements.