Any studies on functional impairment by levels of severity?

I'm looking for studies that assessed levels of functional impairment and levels of activity (ideally objective - steps per day, hours of upright activity or some other measure if possible) for different levels of severity.

I have the 2011 Hvidberg article comparing quality of life to other diseases and also the 2018 Kingdon/Nacul article comparing ME to MS and HC using the SF-36. CDC also has a similar report. But these are not categorizing patients at different levels of severity - e.g. people who are completely bedbound are not taking any/many steps and also may not sit up much while people who are homebound but not bedbound would presumably be walking more.

Does anyone know of studies that break down these functional measures by severity levels?

Thanks in advance for any pointers you have. Much appreciated