Not every set in powerlifting is a single. When you push a hard set of five, or grind through back-off work, the energy comes from a different source than your max attempts. The glycolytic system fuels those longer, brutal efforts, and it is the reason a heavy set of reps leaves you gasping in a way a single never does.
What It Does
The glycolytic system breaks down carbohydrate to produce energy quickly, without needing oxygen. It takes over once an effort runs longer than the few seconds your ATP-PC system covers, powering work that lasts roughly ten seconds up to a couple of minutes. A tough set of squats for reps lives right in this zone.
The Burn and the Fatigue
This system produces byproducts that build up fast and create that deep burn and heavy fatigue you feel during a hard rep set. That fatigue is normal and it is part of what makes higher rep work so demanding. It is also why you need real rest after a tough set, because your body has to clear the byproducts and restock fuel before you can do it again well.
How It Fits Powerlifting
You train this system anytime you do moderate rep work, back-off sets, and accessory training that pushes into higher reps. That work builds muscle and work capacity that support your max strength. It also explains why carbohydrate matters so much for a powerlifter, since this system runs on stored carbs. Keep your fuel topped off and these sets feel stronger.
Your singles run on the phosphagen system, but your hard rep work runs on glycolysis. Both have a place in a complete program, and feeding them with enough carbohydrate keeps both firing.