The surprising reason our muscles get tired – Christian Moro

As muscle fatigue sets in during exercise and your ATP stores are fast diminishing, your muscles begin to produce an acidic environment known as acidosis according to Len Kravitz writing for the University of New Mexico.This is why your muscles burn during an intense bout of exercise. The release of lactate helps neutralize this burning effect and as your body naturally removes this lactate can actually help regenerate energy.By increasing your training volume and intensity, you can make this entire process in your body more efficient over time. Your muscles become more efficient at using lactate to reduce acidosis and converting lactate waste to energy.The optimal recipe for muscle growth includes exercising a muscle group with 12 to 20 total sets per workout, six to 12 reps per set, train to failure, eat 10 to 35 percent of your calories from protein according to the Academy of Nutrition and Dietetics, and sleep about eight hours per night.
https://www.livestrong.com/article/475652-why-do-your-muscles-get-tired-when-you-exercise/