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.

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.

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.

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.

When you work out really hard and your muscles burn, the buildup of lactic acid might help your muscles refill energy faster—but after lifting weights, less lactic acid means slower recovery.