When athletes do their hardest sprints, how fast they go and how much lactic acid builds up in their blood can explain most of why their growth hormone spikes — it’s closely tied to how intensely...

Sprint athletes get a much bigger spike in their growth hormone after a hard 30-second sprint than endurance athletes do — their bodies react differently because of their training.

Doing really intense sprints gives your body a big, quick boost in growth hormone—up to 17 times more—which helps protect your muscles and burn fat.

During prolonged exercise, the intensity of the workout and whether a person is a trained runner or not account for about one-third of the differences in how much growth hormone their body releases.

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.