Dr. Mike from RP Strength explores the optimal body fat percentage ranges for maximizing muscle growth. He categorizes body fat into four ranges: ultra lean, pretty lean, kind of fluffy, and full, providing specific percentages for men and women. For men, ultra lean is 5-10%, pretty lean is 10-15%, kind of fluffy is 15-25%, and full is 25%+. For women, ultra lean is 17-23%, pretty lean is 23-30%, kind of fluffy is 30-40%, and full is 40%+. He discusses the downsides of being ultra lean, such as low energy, poor hormone levels, and joint issues, which can hinder muscle growth. However, starting a bulk at this range offers a long runway before reaching higher body fat levels. Pretty lean offers optimal conditions for muscle growth with good energy, hormone levels, and manageable hunger. Kind of fluffy might lead to strength gains but can obscure visual progress and require more dieting to return to a lean state. Full body fat levels can lead to health issues, poor sleep, and reduced insulin sensitivity, making it less ideal for bulking. Dr. Mike advises experimenting with different ranges to find what works best for individual goals and preferences. He emphasizes the importance of considering personal lifestyle, psychological comfort, and health markers when choosing a bulking range. He also suggests creating a bulking runway by initially losing some fat to allow for a longer bulking phase without exceeding desired body fat levels.