A tendon disorder (tendinopathy) is a literal Achilles' heel for patients and healthcare providers. Achilles tendinopathy is characterized by local pain in the Achilles tendon that worsens with loading, for example, during walking or sports activities. A risk factor of Achilles tendinopathy is weakness of the calf muscles. The underlying issue with Achilles tendinopathy is the lack of a proper linear structure of the tendon fibers. The Achilles tendon is thickened, and the structure of the tendon fibers is disrupted. Achilles tendinopathy is a common condition, observed in 3 out of 1,000 adults in Dutch general practitioner practices.

The Achilles tendon can be well visualized through ultrasound examination. In patients with Achilles tendinopathy, the ultrasound shows tendon thickening and a reduced organization of the tendon structure. According to the Dutch multidisciplinary guideline, a thickness of more than 6 mm of the Achilles tendon is considered abnormal. This knowledge is based on small studies in very specific populations, and currently, there is no consideration for factors that might influence this ultrasound image.

The cornerstone treatment for Achilles tendinopathy consist of exercise therapy. According to the current guidelines, strengthening exercises for the calf muscles are recommended. This has the potential to improve tendon structure and regain calf muscle strength and thereby prevent recurrence of tendinopathy once a subject is recovered. Currently, this treatment is advised to all patients, but it is likely that this one-size-fits-all approach is not beneficial for all patients.

An important knowledge gap is the lack of normative values for thickness and structure of the Achilles tendon and calf muscle strength of the general population. In this international study, we will examine 600 individuals without (previous) symptoms of Achilles tendinopathy and 200 patients with symptoms of Achilles tendinopathy. We will use the new standardized Ultrasound Tissue Characterization (UTC) technique. This ultrasound technique, developed in racehorses and recently applied in humans, can reliably measure tendon thickness and structure. We will also collect other participant data that may influence the imaging of the Achilles tendon, such as age, gender, weight, and sports activity. With this information, we aim to provide a better estimate of the normal thickness and structure of the Achilles tendon in different lifestyles and life stages. We will also measure calf muscle endurance strength using the standardized Heel Rise Endurance Test (HRET) in the asymptomatic group.

We aspire to publish an accessible online tool on the Erasmus MC website for estimating normative values for ultrasound measures and calf muscle strength (the 'Achilles Tendon Tool'). This tool will assist healthcare providers across the world in estimating the normal geometry of the Achilles tendon and calf muscle strength. We will be able to differentiate normal imaging findings from findings consistent with Achilles tendinopathy. We plan to promote this tool through existing networks.