Exactly. And the high weight/low reps works the ATP energy system (ATP (Adenosine Tri-Phosphate) is the energy source for muscles that they use for exerting high force. As the ATP is depleted (loses a phosphorous, turning to ADP), the body turns the ADP back to ATP with the Creatine Phosphate reserves - which is how creatine supplementation helps out. It can let you get an extra rep or two, working the muscles harder, etc.). As the weight decreases and reps increase, you start shifting to the lactic acid system. Decrease resistance further and increase reps more, and you start moving into the aerobic energy system.