There are three metabolic pathways that provide the energy for all human action. These “metabolic engines” are known as the phosphagen pathway, the glycolytic pathway, and the oxidative pathway.
The first, the phosphagen, dominates 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. These are the “anaerobic” pathways.
The third pathway, the oxidative, dominates low-powered activities, those that last in excess of several minutes. This is the aerobic pathway.
Here from the University of Connecticut is an excellent starting point for additional information on bioenergetics.
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.
Favoring one or two pathways to the exclusion of the others and not recognizing the impact of excessive training in the oxidative pathway are arguably the two most common faults in fitness training. Time domain matching of task or sport to training is the first step to effective, legitimate strength and conditioning.