7 125 Normative Ultrasound Values for Achilles Tendon Thickness: An International Study questionnaire was completed and 4) the Ultrasound Tissue Characterisation (UTC) procedure was performed. Patients completed a digital questionnaire prior to their appointment at the outpatient department. The questionnaire included information on demographics, lifestyle habits, comorbidities, work, injury characteristics, and physical activity level. The VISA-A questionnaire was also completed.18 A single senior sports physician (RJDV) performed complete history taking and physical examination, which included assessing pain on tendon palpation and the presence/absence of tendon thickening. The clinical diagnosis was made based on history and physical examination. The clinician established the clinical diagnosis of AT if the pain was 1) located to the Achilles tendon region, 2) associated with Achilles tendon-loading activities AND 3) provoked on Achilles tendon palpation.1,7,10,22 If the pain was localised at the level of the posterior calcaneus, insertional Achilles tendinopathy was diagnosed and if the pain was localized above the superior border of the posterior calcaneus, midportion Achilles tendinopathy was diagnosed. Hereafter, the UTC procedure was carried out on the symptomatic side. In case of bilateral symptoms, the side with the most severe complaints was scanned. Outcome measures Ultrasound Tissue Characterization (UTC) Primary outcome measure was the maximum anterior-posterior (AP) distance of the Achilles tendon in transversal view (also referred to as thickness) using UTC. Achilles tendon thickness can be depicted with UTC.8,11,23 The UTC is a customized tracking and ultrasonographic data-collection device that allows for objective, standardized measurements, which can be translated to conventional ultrasound.8 The UTC Imaging version 2020 (UTC Imaging, Stein, The Netherlands), consisting of conventional ultrasound equipment (multi-frequency 5-16 MHz linear-array transducer) and a tracking device, was used. At each site, one single trained researcher (TSV and SO) performed all UTC scans. Participants were positioned prone on an examination table with a maximum tolerable dorsiflexion angle of the ankle.8 The transducer was placed in a transverse position to the Achilles tendon and moved automatically from proximal to distal over a distance of 12 cm.24 Images were stored using a specific code and analysis of the images was performed in a subsequent stage of the research project. Previous studies have described the UTC procedure in more detail.8,11,12,23 Image analysis of all UTC scans was performed by one trained researcher (TSV). This researcher was blinded to patient characteristics while performing the analyses. The maximum anterior-posterior distance was measured with the UTC software. In the longitudinal plane (sagittal view), we screened for the area of maximum thickness in the midportion and insertional region of the tendon (Figure 1). Hereafter this area was
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