When your muscles don’t have enough stored sugar (glycogen), you can’t sprint as fast or go as long during all-out efforts like cycling or running hard.

Trained athletes performing repeated sprints show a measurable reduction in performance when following low-carbohydrate or ketogenic diets, linked to lower muscle glycogen levels that limit ATP production through glycolysis.

During prolonged exercise, fatigue is not caused by low muscle glycogen stores; instead, low blood glucose is the only metabolic change consistently seen when exhaustion occurs.

Even when eating almost no carbs after a race, athletes who’ve been on a high-fat, low-carb diet for months can still rebuild their muscle sugar stores just as well as those eating lots of carbs.

Even though the person gets much weaker during their 10th sprint compared to the first, their muscles aren’t producing more lactic acid — which is weird because their stress hormone (epinephrine) is high, and you’d expect lactic acid to rise. So something is shutting down the energy system that usually makes lactic acid.

After a short, super-hard workout, your muscles refill their energy stores way faster than after a long, slow one—even if you eat the same amount of carbs.