Wall sits help muscular endurance but would they be counterproductive to a sprinter? If not what would be a good rep/scheme for them on a GPP/Off day?

I’ve used them for young athletes or those who can’t squat etc as simply a method of building some leg strength easily. However, I can’t see why you would want to use them unless there are no other options. I mean is isometric strength in that position really that useful for anyone other than martial artists in Kararte or something?

But if you have to use them i’d go with something like one minute on with a set rest period. Then you can either increase duration or decrease rest or both for progression.

As for being counter productive, its only counter productive if it wastes energy and recovery resources. E.g. if it impeeds on some other important part of training. Other than that i doubt it does much harm.