HD-FIT is a randomized controlled clinical trial that will analyze the impact of high volume online HDF in comparison to high-flux HD on measured physical activity (number of steps measured in the dialysis day) as a primary endpoint. The analysis of additional accelerometer data, HRQOL and time to recover from a dialysis session will be used as exploratory outcomes. The study will also capture safety data, based on intradialytic events, hospitalization and mortality. Biochemical (according to the local requirements) and drug prescription data will be monitored for pharma-economic analysis. Serum samples will be collected for future analysis of additional biomarkers.