In early phase tendinopathy, isometric exercises are seen as ideal to provide pain relief to patients. This approach is mainly based on a paper by Rio et al (2016), where they found that isometric exercises of a certain load magnitude and time (5 repetitions of 45 second hold at 70% of maximum) gave 100% pain relief for 45 minutes in patients with patellar tendinopathy. This then helps patients to perform their more heavy load exercises during rehabilitation, which would otherwise be too painful.

Unfortunately, the study of Rio et al only consisted of 6 participants, and recent papers have contradicted the findings. In Achilles tendinopathy, plantar fasciopathy and lateral elbow tendinopathy, the pain relief was not consistently present, with "responders" and "non-responders" being found in these studies. Also, a study yet to be published (poster at conference), replicating Rio et al, also found a heterogeneous response, debunking the "one size fits all" approach that seemed to work.

However, in our understanding, isometric exercises do have a crucial role in early tendinopathy management, but the way the exercise is performed, in which position, what magnitude of load, time under tension, … has an important influence. The same protocol (5 repetitions of 45 second hold at 70% of maximum) might lead to big inter-individual differences. Therefore, there might be a mechanistic reason why some patients respond, and others do not.

Fortunately, the P.I. of this current trial application has recently optimized an ultrasound-based method to quantify local tendon deformation during exercises. The main purpose of this trial is therefore to evaluate the local tendon deformation pattern of patients with tendinopathy during isometric exercises and evaluate whether there is an interindividual difference in pattern between "responders" and "non-responders".

Inclusion Criteria:

- Clinical diagnosis of patellar tendinopathy, consisting of

• pain located at the proximal part of the patellar tendon
• painful at palpation the proximal part of the patellar tendon
• Numeric Rating Scale > 1/10 with squat on decline (20°) board

Exclusion Criteria:

• previous treatment for patellar tendinopathy
• rupture of patellar tendon on ultrasound
• unclear differential diagnosis with patellofemoral pain
• concomitant neuromuscular disorders

Patients visiting the sports medicine consultation of the University Hospitals and presenting with complaints of patellar tendinopathy will be screened for eligibility.