The first, the phosphagen, fuels the highest-powered activities, those that last less than about ten seconds.
The second pathway, the glycolytic, dominates moderate-powered activities, those that last up to several minutes.
The third pathway, the oxidative, dominates low-powered activities, those that last in excess of several minutes.
Total fitness, the fitness that CrossFit promotes and develops, requires competency and training in each of these three pathways or engines. Balancing the effects of these three pathways largely determines the how and why of the metabolic conditioning or “cardio” that we do at CrossFit.
--From "What is Fitness," CrossFit Journal, October 2002