Statistical analysis
Statistical analysis was performed by the software SPSS v. 22.0 for Windows (IBM, Armonk, NY, USA). The Kolmogorov–Smirnov test was used to determine whether the assessed variables followed a normal distribution, and therefore, if parametric or nonparametric statistical tests had to be applied. The significance of differences between study and placebo groups (two sample t-student or Mann–Whitney U test) and between pre and post-training visits (one sample t-student or Wilcoxon test) was analysed with the adequate tests depending on the normality of the compared variables according to the result from the Kolmogorov–Smirnov test. Likewise, the correlation between different evaluated variables was analysed, calculating the Pearson correlation coefficient for normal variables and the Spearman correlation coefficient for non-normally distributed variables. Differences in percentual variables between placebo and study groups were assessed with the Chi-square test, whereas differences between pre- and post-visual training visits were analysed with the McNemar test. Likewise, the raw QoV questionnaire data were Rasch-scaled onto an interval level scale. All statistical tests were 2-tailed, and 