is a compound produced when glucose is broken down and oxidized. During intense exercise when oxygen levels are lower, more of this is produced, which can produce hydrogen ions and a burning sensation in muscles while they're active.
As our bodies prepare us for danger in a flight or fight response to stress, our adrenal glands release the hormone
Fast Twitch Muscle Fibers
Type II muscle fiber - Used in powerful burst, sprinting, jumping, etc...
Slow Twitch Muscle Fibers
Type I muscle fiber - Used in long distance, endurance training sessions (Marathon, bike ride, etc...)