When guys are bent over grabbing their shorts, they are exhausted, it doesn't mean they aren't somewhat tired before that. Leaning over and grabbing your shorts is a sign to the other team that your gassed, and even if you are gassed you aren't going to send an obvious sign to your opponents that you're tired. Watch how many guys stand with their hands on their hips, it makes breathing easier and when players get tired they do that, the next game you watch count how many times you see players standing with their hands on their hips.