As I have previously shown...
...the type one muscle fibre responds well and GROWS when stimulated by overloading it with resistance in the form of high reps as seen above. Ok, now that we have stimulated growth there, we've got to deliver some top notch blood full of nutrients in order for those stimulated fibers to really take off. That's where the ultra high reps with very light weight come in to do their job on the capillaries by increasing them in numbers as well as in density.
So in short Rasika, I'd go for higher reps (50-100) if my aim was to hit the capillaries and I'd leave the 21s to work their magic on the type I muscle fibers.
PS: when the capillaries are "primed" through your performance of ultra high reps, that then will serve as foundation for muscle growth in the type II A and type IIB fibers as well since there will be blood supply GALORE thanks to the "priming" you've achieved in your capillaries.
I hope that answers your question Rasika and I thank you for it my friend.
Fadi.