Background: Early mobilization on a tilt table with stepping versus standard care may be beneficial for patients with severe brain injury, but data from randomized clinical trials are lacking. This detailed statistical analysis plan describes the analyses of data collected in a randomized clinical feasibility trial for early mobilization by head-up tilt with stepping versus standard care after severe traumatic brain injury. Methods: Primary feasibility outcomes are the proportion of included participants who were randomized out of all screened patients; the proportion of participants allocated to the experimental intervention who received at least 60% of the planned exercise sessions; and safety outcomes such as adverse events and reactions and serious adverse events and reactions. Exploratory clinical outcomes are suspected unexpected serious adverse reactions; and functional outcomes as assessed by the Coma Recovery Scale-Revised at four weeks; Early Functional Ability Scale and Functional Independence Measure at three months. The description includes the statistical analysis plan, including the use of multiple imputations and Trial Sequential Analysis.