For an ECG or EKG, how to explain shortening of P wave, Q-T Interval after activity/exercise vs. when resting?

I'm doing an A&P lab, and can't explain why mechanical changes like increased contractility (or the contractile force exhibited by cardiac cells) will shorten distance between deflection waves on an EKG. I thought that I understood, but now I'm finding that when I'm trying to explain, I cannot. I know increased contractility comes from more catecholamines like epinephrine or norepinephrine, but how is the contractility increased by exercise? Why exactly is more blood being pumped out? I get why the QRS complex isn't affected, but how can I explain the mechanical changes?