In this 2009 lecture at Midwestern State University for the American Society of Exercise Physiologists, Greg Glassman lays out a rigorous, first-principles framework for understanding fitness, health, and longevity through the lens of work capacity. He argues that fitness is best defined not by isolated physiological markers or sport-specific skills, but as the ability to perform work across broad time and motor domains—a quantity that can be measured, tracked, and graphed over a lifetime. From this foundation, he proposes that fitness and health are not separate concepts but different expressions of the same underlying reality, with most clinical health markers mattering primarily insofar as they predict declines in functional capacity. Glassman connects this model to real-world outcomes, suggesting that maintaining high work capacity preserves functional independence, delays morbidity, and meaningfully extends quality of life, while its loss—not disease labels themselves—often precipitates institutionalization and death. He further defines functional movement as the capacity to move large loads long distances quickly, emphasizing power as the unifying metric behind strength, speed, and coordination. Throughout the lecture, he defends a data-driven, open-source approach to training, argues that general physical preparedness transfers across all sports and professions, and contends that effective fitness must balance mechanics, consistency, and intensity—accepting managed risk as the price of meaningful adaptation.