Not only can this be an issue in fast pitch, but it can be in slow pitch as well. I was observing a slow pitch tournament a few years ago where a women's team had a rather obese catcher, and she got to the point where she was s-l-o-w-l-y chasing balls to the backstop. It was over 100 in the air, and probably 125 or more on the field. Blue working turned to the coach and said, I think she needs to come out. Coach wasn't too happy, because his only sub was a couple of months pregnant, and was having trouble dealing with the heat too. So Blue was generous (the team was getting killed anyway), and said he would allow playing short-handed.