What is the least amount of time you should stay in your target heart rate

Aim to be at your THR for at least 15 to 20 minutes workout time, and ideally 35 to 45 minutes. If you're a beginner, start at 65 percent to 70 percent of your maximum heart rate and gradually increase the intensity.

Hope this helped...

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.