Learn the body parts, muscles and bones and how human body moves.
Learn about how metabolism, energy systems, hormone, nerve, brain, blood, organs, or so, work and affect each other. Also you need to know some diseases like diabetes, high blood pressure, sarcopenia and so on.
Know the difference between different sports and their elements. For example, what are the abilities required for basketball players and swimmers? What are the differences for their sports and training? How do we jump higher or move faster and safely?
What are protein, carbohydrate and fat and something else? How they work? What we should take before/during/after exercise or sports? What are the principles to lose weight or gain muscles?
How to arrange training program? What are the skills of different movement? What's different between squat and deadlift? How to strengthen mind-muscle connection?