Dr. Mike from Renaissance Periodization explains the critical role of warming up in fitness routines. He outlines that warming up physically prepares the body by increasing tissue temperature and activating energy systems, which enhances performance and reduces injury risk. The warm-up process also primes the nervous system, ensuring that muscles are ready to perform at their best. Dr. Mike highlights that a proper warm-up involves both general and specific exercises, with the latter focusing on the muscles and movements to be used in the workout. He suggests a two-step warm-up process: starting with light, high-repetition sets to engage muscles and technique, followed by heavier, low-repetition sets to fully prepare the body for intense exercise. This approach not only improves performance but also ensures that the most effective muscle growth occurs during the workout. Dr. Mike advises against overly long warm-ups, suggesting that once the body feels warm and ready, it's time to proceed with the workout. He also emphasizes the importance of practicing good technique during warm-ups to maximize benefits.