3 51 Normative Calf Muscle Strength-Endurance Values: An International Study metrics. The median (50.0th percentile), lower (2.5th), and upper (97.5th) percentile values of the regression models were extracted to present HRET metrics as median with a 95% (2.5th to 97.5th percentiles) reference interval (RI). A covariate was excluded from the multiple regression model if the following two criteria were met: 1) it exhibited a weak/negligible (r<0.3)20 correlation coefficient and 2) the removal of the covariate did not impact the model's 'accuracy,' as indicated by the stability of the R2 value. To evaluate the influence of each covariate on HRET metrics, the 95% confidence intervals (CI) were extracted to allow estimation of the impact of each covariate. We adhered to the CHecklist for statistical Assessment of Medical Papers (CHAMP) statement for the statistical analysis and presentation of results.21 IBM SPSS Statistics (version 28.0.1.0) were used for all analyses, and significance was set at p < 0.05. RESULTS Flow of participants through the study A total of 547 asymptomatic persons were screened for eligibility and 500 of these participants were included (n=300 at Erasmus MC and n=200 at University of Leicester). A flowchart of the study is presented in online supplementary file 1 (supplementary figure 1). In five participants, the HRET metrics could not be extracted due to recording/technical errors; there was a complete dataset of 495 participants. The participant characteristics are displayed in Table 1. More than half of the participants were female (55%) and the majority (88%) participated in sports activities. Eight participants did not want to disclose their gender. The age of the participants ranged between 18 and 81 years.
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