Heard of the 10% “rule” from several sources (not unnamed, I just don’t have them in front of me) but most times it has to do more with visual observations in those studies i.e. seems to have been about “average” amount being used when good sprint form deteriorated.
Same thing applies when talking about the angle of the hill but thinking has changed on that depending on exact detail(s) you are trying to work.
The other often modified factor, compared to studies, is the distance/length of the pull or hill. IRRC most talk about a 20m to 30m “limit” but again having to do with breakdown of form. Does this mean if using say 5% without form breaking down one should go as far as possible before the form does change?
The difficulty I often have is with these studies themselves. There are so many variables, the number of studies appears to be quite small and the groups studied quite varied (so one often questions validity in application to “my” specific group).
FYI: I personally mistook the title as breaking a real rule…this is far more “suggestion” than rule as far as I am concerned