Background: Musculus gastrocnemius tightness (MGT) can be diagnosed by comparing ankle dorsiflexion (ADF) with the knee extended and flexed. Although various measurement techniques exist, the degree of knee flexion needed to eliminate the effect of the gastrocnemius on ADF is still unknown. The aim of this study was to identify the minimal degree of knee flexion required to eliminate the restricting effect of the musculus gastrocnemius on ADF. Methods: Bilateral ADF of 20 asymptomatic volunteers aged 18-40 years (50% female) was assessed prospectively at six different degrees of knee flexion (0 degrees, 20 degrees, 30 degrees, 45 degrees, 60 degrees, 75 degrees, Lunge). Tests were performed following a standardized protocol, non weightbearing and weightbearing, by two observers. Statistics comprised of descriptive statistics, t-tests, repeated measurement ANOVA and ICC. Results: 20 individuals with a mean age of 27 +/- 4 years were tested. No significant side to side differences were observed. The average ADF [95% confidence interval] for non weightbearing was 4 degrees{[}1 degrees-8 degrees] with the knee extended and 20 degrees [16 degrees-24 degrees] for the knee 75 flexed. Mean weightbearing ADF was 25 degrees[22 degrees-28 degrees] for the knee extended and 39 degrees[36 degrees-42 degrees] for the knee 75 degrees flexed. The mean differences between 20 degrees knee flexion and full extension were 15 degrees[12 degrees-18 degrees] non weightbearing and 13 degrees[11 degrees-16 degrees] weightbearing. Significant differences of ADF were only found between full extension and 20 degrees of knee flexion. Further knee flexion did not increase ADF. Conclusion: Knee flexion of 20 degrees fully eliminates the ADF restraining effect of the gastrocnemius. This knowledge is essential to design a standardized clinical examination assessing MGT.