A321 Captain reported they stopped the aircraft quickly during taxi which resulted in a flight attendant being injured.

Narrative

On taxi out from the ramp in ZZZ airport; we had to stop the aircraft quickly to avoid encroaching onto taxiway 1. Our taxi speed at the time of stopping quickly was approximately 5 mph. We continued our taxi to runway XXR per ATC clearance and departed for ZZZ1 airport without incident. The First Officer and I took a bathroom break at three hours into our four-and-a-half-hour flight. During our bathroom break; a Flight Attendant mentioned to me that they may have injured their shoulder earlier during the quick taxi stop on taxi out in ZZZ airport. I inquired into their condition; they said they were okay; and I told them I would check on their condition when we arrived and after deplaning the passengers at ZZZ1 airport.After arriving in ZZZ1 and saying farewell to passengers; I waited for the Flight Attendant in the forward galley. I was going to help them remove their roller bag from the overhead bin; but I had to wait for the last few remaining passengers to walk off the plane. The Flight Attendant removed their roller bag from the overhead bin on their own without difficulty. On final consultation with the Flight Attendant; they informed me their shoulder felt a little sore; but they were okay.Cause: Both pilots were heads up during taxi out. When bringing the aircraft to a quick stop; the brakes were manually applied with toe pressure grabbing and stopping the aircraft quicker than I anticipated.