132 Chapter 7 pre-dominantly military recruits or elite fencers)13,14 or relatively small samples (ranging from 6 to a maximum of 100 individuals)8,11,12. These studies reported different mean values of tendon thickness ranging from 4.2 to 7.1 mm, without adjusting for personal characteristics.8,11-15 The relatively small and/or selected study populations in these studies may account for the variation in findings and no studies differentiated between the midportion and insertional region of the tendon, while these are considered separate clinical entities based on the current guidelines.7,28 The influence of personal characteristics on Achilles tendon thickness has been evaluated once in the past. A larger study (n= 267) by Koivunen-Niëmela et al. in 1995 evaluated the influence of personal characteristics on Achilles tendon thickness in an asymptomatic population. A large proportion of the population were military recruits who were predominantly male between the ages of 18-29.14 This study found that there was a significant correlation between tendon thickness and age, height, and weight, with tendon thickness increasing from 5.9 mm in those aged 10-17 years to 6.7 mm in those aged >30 years.14 These findings are consistent with those of the current study that is performed on a larger scale and without a clear selection, which also found that tendon thickness is largely influenced by age and height. Clinical implications Imaging techniques have been found to aid in the diagnosis of Achilles tendinopathy, particularly in challenging cases where not all clinical diagnostic criteria are met.7,10 It is, however, important to note that imaging may present a potential drawback, as findings suggestive for tendinopathy can be detected in 25% of asymptomatic Achilles tendons.10,16 Additionally, our study shows that 27% of the patients with clinical diagnostic criteria for AT do not have increased Achilles tendon thickness outside the 95% reference interval. While abnormal imaging might increase the likelihood of AT, these findings challenge the use of imaging as gold standard for diagnosing AT. Clinicians can benefit from having knowledge of reference values and parameters that impact on tendon thickness, which can help to distinguish between AT and normal morphological changes (www.achillestendontool.com). Strengths and limitations This study has several strengths. To our knowledge this is the largest cross-sectional study on this subject. We used strict methods, a pre-defined protocol and included an international cohort drawn from the general population which improves generalizability of the findings. Next to this, the outcomes of the quantile regression model are openly available, serving as a calculator for normative tendon thickness. The study also has limitations that must be acknowledged. First, only the maximum AP distance was used as an outcome measure in this study. While this is the most frequently used outcome
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