Exercise physiology is the study of how the body responds and adapts to physical activity. It involves understanding the basic concepts or principles of muscle physiology, cardiovascular function, and connective tissue function that provide the basis for the adaptations we call fitness. These adaptations can be categorized into skill, speed, power, strength, hypertrophy (growth of muscles), and various forms of endurance. Endurance, for instance, can be muscular endurance, anaerobic endurance, 3 to 12 minute endurance, and 30 minutes or more endurance type exercise. Each of these categories taps into different fuel systems and aspects of muscle physiology. Understanding these principles allows us to develop optimal fitness protocols tailored to individual needs and goals.