i cant ever remember pain as such when i first started double bass. i do feel strain sometimes after playing straight 16ths for a few minutes. the position is very important. i sit a little further away giving my legs room. doing calf raises on some stairs can build strength.
maybe the pain is coming from the muscles developing??
also, try playing that exercise ur doing heels down and play that exercise really slow. this helped me alot when i play heels up. makes playing heels up alot more comfortable
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