this study evaluates the addition of initial short term NSAID treatment to physiotherapy prescribed exercise treatment in patient with early phase tendinopathy. Half of the participants will initially receive NSAID (naproxen) for 7 days, while the other half will receive a placebo, where after both groups will be subjected to 3 months heavy slow resistance training.

NSAID is effective in short term pain relief in musculoskeletal disorders, but little is known about the long-term effect in early phase tendinopathy, when used as a supplement to treatment as usual, namely heavy slow resistance training and training load reduction. In this study, NSAIDs will be used, which works through COX inhibition, which again inhibits Prostaglandins, in this way investigators attempt to dampen the initial inflammation and pronounced cell activity that is thought to be present in early phase tendinopathy. With this project, it will be attempted to investigate whether initial dampening adds to the effect achieved through heavy load resistance training, which is thought to stimulate structural repair and regeneration in the tendon, by applying appropriate load to the tendon tissue, thereby stimulating the tendon cells to repair and regeneration.

Inclusion Criteria:

• Activity related pain in the achilles tendon
• Palpation pain in the achilles tendon
• Onset of symptoms within the last 3 months

Exclusion Criteria:

• Previous injury in the achilles tendon on the ipsilateral side.
• Recent infection around the achilles tendon
• Previous surgery in the achilles tendon.
• Contraindications for NSAID treatment.
• NSAID treatment for the current injury
• Medication with NSAID interaction