Why your muscles don't get more tired even when you're exhausted
Human muscle metabolism during intermittent maximal exercise.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Your muscles use two energy sources: a fast-burning battery (PCr) and a sugar-burning system (glycolysis). At first, both are used equally. But after many sprints, the sugar system shuts off—even though your body still signals to use it. Your muscles keep going by using the battery more and breathing harder.
Surprising Findings
Muscle lactate didn’t increase during the 10th sprint despite high epinephrine and near-maximal effort.
Epinephrine normally triggers sugar breakdown and lactic acid production—so the fact that muscles ignored this signal contradicts textbook physiology.
Practical Takeaways
Use shorter rest periods (30s) in HIIT to train your muscles to recover PCr faster—this boosts repeated sprint performance.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Your muscles use two energy sources: a fast-burning battery (PCr) and a sugar-burning system (glycolysis). At first, both are used equally. But after many sprints, the sugar system shuts off—even though your body still signals to use it. Your muscles keep going by using the battery more and breathing harder.
Surprising Findings
Muscle lactate didn’t increase during the 10th sprint despite high epinephrine and near-maximal effort.
Epinephrine normally triggers sugar breakdown and lactic acid production—so the fact that muscles ignored this signal contradicts textbook physiology.
Practical Takeaways
Use shorter rest periods (30s) in HIIT to train your muscles to recover PCr faster—this boosts repeated sprint performance.
Publication
Journal
Journal of applied physiology
Year
1993
Authors
G. Gaitanos, Clyde Williams, L. Boobis, S. Brooks
Related Content
Claims (5)
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.
Even though your body is still pumping out a stress hormone after nine sprints, your muscles stop breaking down sugar and making lactic acid during the tenth sprint — like your body’s alarm system is still on, but your muscles decided to ignore it.
Even when your muscles are tired from doing ten all-out sprints, they're still able to quickly recharge a special energy fuel called phosphocreatine during the short breaks—so you can keep pushing hard, even if you're not as powerful as before.
When a person does their first super-hard 6-second sprint out of ten, their muscles get energy from two sources at the same rate: one that uses up a stored chemical called phosphocreatine (which drops by over half), and another that makes lactic acid (which jumps up to a high level).
When athletes do their 10th all-out sprint, they still keep about 73% of their top speed even though their body isn’t burning sugar as much anymore—so they must be using other energy sources like creatine and oxygen to keep going.