Data and statistical analysis
Median values of cardiopulmonary parameters of CPET and ergometry were calculated during the last three minutes of each 6-min load after reaching the physiological steady state. Measurement values are presented as boxplots (box: median, 25th-75th percentile; whiskers: 3.5-fold interquartile range). In tables, data are expressed as medians (range).A generalized linear mixed (GLM) model, along with generalized estimating equations (GEE), was used on the logarithmized values. In all GLM models, the load level (pre, E1, E2, E3, post) and the time of measurement (30, 60, 90, 120, 150, 180, 210, 240 min) were included as influencing factors. This allows intraindividual comparison at the different examination times (i.e., each subject is compared to him/herself), and a consideration of repeated measurements. Geometric means (G.M.) as well as least-square means were calculated based on these models. The situation without mask was used as reference. We adjusted for further influencing factors, including sex (dichotomous), age (continuous per 10 years), and height (continuous per 10 cm). Pearson correlation coefficients (r) (+ 95% confidence intervals (CI) and Fisher transformation over all examination times in each module) were calculated to predict the monotone association between parameters. A 