126 Chapter 7 evaluated in the transverse plane and in this view, we estimated the maximum thickness and measured it. The insertional region was defined as the area from the lowest Achilles tendon insertion on the calcaneus to the upper border of the posterior calcaneus. The midportion region was defined as the area proximal to the upper border of the posterior calcaneus. The inter- and intra-rater reliability for AP-measurements have been shown to be excellent for AT patients (intra-class correlation coefficient (ICC) 0.93 and 0.95 respectively) as well as for asymptomatic participants (ICC 0.91 and 0.94).25 Figure 1. UTC image of the Achilles tendon. In the longitudinal plane (A and C), the thickest part of both the midportion and insertional region of the tendon were estimated. Subsequently, those regions were assessed in the transversal plane (B and D). A and B; midportion region of the Achilles tendon. C and D; insertional region of the Achilles tendon. Statistical analysis To establish normative equations for maximum AP distance of Achilles tendon thickness, the data from 500 subjects were inspected using scatter- and boxplots to identify outliers. Descriptive statistics were used for presentation of personal characteristics. Quantile regressions were used for analysis, given the expected skewed nature of the data and the aim to establish normative values. Quantile regression allows for estimations of medians and does not make distributional assumptions.26 Potential differences in tendon thickness between the right and left leg were analysed using a Wilcoxon signed-rank test (in the case of non-normal distribution) for the midportion and insertion region. When no statistically significant differences were observed, data is presented as the mean tendon thickness for the midportion and insertional region. Bivariate models were constructed for each covariate (age, sex, height, weight, BMI, activity level, leg dominance, smoking, alcohol
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