48 Chapter 3 METHODS Study design The study was designed at the Erasmus MC University Medical Centre (NL) in collaboration with the University of Leicester (UK) and the University of Waikato (NZ). The local Medical Ethics Committee (Southwest-Holland, the Netherlands) approved the study protocol (MEC-2020-0585). The trial was prospectively registered (Netherlands Trial Register, NL9010) and we adhered to the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines.15 Protocol deviations In the initial study registration, 'total work' and 'total vertical displacement' were listed as secondary outcomes, but were reclassified as primary outcomes. Similarly, 'repetitions' and 'peak height,' were not initially registered, but were included as primary outcome measures. These decisions were based upon further clinical assessment and postregistration discussions within our research team. The correlation between structure and calf muscle strength-endurance was prospectively registered but can’t be reported at this stage as it requires specific software, which, as of now, has not been developed and validated by the manufacturer. Participants The study was conducted at the outpatient departments of two large universities (Erasmus MC University Medical Centre and University of Leicester) from October 2020 to June 2023. The study was paused from November 2020 to May 2022 due to Covid-19 restrictions. These restrictions also compelled us to limit sample size to 500, which is a reduction of 100 participants compared to the pre-defined protocol. We aimed to include a sample of participants that accurately reflects the general population, with an even distribution of both gender and across decades of life. To recruit participants, a comprehensive announcement was disseminated through internal websites and various social media platforms, including Twitter (now X), Facebook, and LinkedIn. Interested individuals were screened remotely. Those meeting eligibility during remote screening were scheduled for an appointment with a researcher, during which further screening assessments were conducted. Inclusion criteria were: (1) at least 18 years of age, (2) no current or prior history of Achilles tendon pain or stiffness, (3) no localized fusiform thickening of the Achilles tendon on palpation, and (4) a full score on the adapted (questions 1 to 5) Victorian Institute of Sports Assessment-Achilles (VISA-A) questionnaire. 16,17 Exclusion criteria were: (1) a history of Achilles tendon or ankle surgery, (2) any lower-limb injury requiring immobilization within
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