I think because our aerobic system is up the creek without a paddle, we rely on our anaerobic energy system, this system is only used for short bursts of energy etc so if we are using this system more then we should, we will become fatigue easier. Supplement creatine is helpful for anaerobic energy as it helps us recycle ADP into ATP.
marathon runners have well developed aerobic systems, 100mtr sprinters have well developed anaerobic systems, even the best sprinter cant keep up going full steam for any great length of time. So we are like the sprinter, we have energy to sprint from something life threatening or do activity in short spurts but we also have a poor ability to recover from this. I think its also this that makes us appear well to others as they may see us vacuuming the floor but we dont have the energy to keep cleaning for a few hours etc etc so we rest and do things in short bursts. PEM is because our body is trying to recover from this type of activity.
thats my 10 cents worth.
cheers!!!