We were talking about tough practices and puking OL and such in another thread. It's funny the things that stick with you over time. My senior year, we had a small team, I want to say 26 or 28 of us. A half dozen of us rarely left the field that year. I wasn't on the punt return team, and that was about it. The summer two-a-days had been pretty tough, endless bear crawls, sprints and the like. We had gotten into shape. I remember running sprints the last practice before we went back to school (which would be the first game week). We just knew it was going to be another marathon session. We started out with a couple of 40's then coach backs up ten yards, a few 50's, then a couple of 60's. The only sound was the sound of equipment sloshing about. No one was breathing heavy. We're on the line to head back, just pure silence, not a breath. Coach hesitated, then gave us an "everybody up!" and we just exploded with relief and excitement. We had mastered our conditioning, ready to take on the world.
Over the years, when I've found myself getting a leg up on something - finding my trail legs well into a hike, getting a handle on a physically demanding construction/yard/auto project - I remember the sound of that equipment flailing about, and no one breathing heavy. I guess it's the sound of satisfaction.