Statistical analysis
The SPSS program version 22 (IBM SPSS Statistics V22.0 for Windows, SPSS Inc., USA) was used for all statistical analyses. The descriptive statistics was used to analyze the demographic data as well as the median and interquartile range (IQR) of outcomes. The Shapiro-Wilk test was used to test for the normality of the data. If the data were found to not have a normal distribution, the Wilcoxon signed-rank test and the Friedman’s test were used to investigate differences between tablet tilt angles and among time intervals, respectively. The significant level was set at α = 0.05 for the primary analyses. If a significant difference among time intervals was found, pairwise comparisons by the Wilcoxon signed-rank test would be conducted. The adjusted significance level for the 10 pairwise comparisons was set at α = 0.005.