Are you going to eat or be eaten?
When your bursts of effort are too long or if your rests are too brief—as in HIIT or metcons—acid cuts your power. No longer able to contract quickly and powerfully, fatigued muscles compensate with longer and weaker contractions. This messes up movement coordination. It could mean failing to catch dinner—or becoming dinner yourself. Even if you have avoided such a fate, you are learning slowness and poor form for the next time.