When I discuss fitness and conditioning, I use the categories of Strength Training, Endurance Training, General Athleticism Training, and Nutritional "Training."
Naturally, different regimens address each fitness category with some specificity but with some overlap. To get stronger, you have to stress nerves and damage muscle fibers by moving weight. For endurance training, 20 mins of intense interval exercise seems to beat out longer low-intensity aerobic cardio. All categories feed into General Athleticism which is most peoples' real goal - Fitness for whatever life brings - but we address it specifically with calisthenics and by playing recreational sports and other outdoor vigorous activities.
These categories of activity use different combinations of energy systems. Animal bodies have three energy systems, each with different purposes.Just as nerve pathways can improve with stress and challenge, cellular energy systems can be bolstered with stress. When you think about it, a fair amount of stress is good for both body and soul.
Your energy systems are the Phosphagen System, the Gycolysis System, and the Aerobic Oxygen System. In a balanced Conditioning regimen of Strength, Endurance, and General, all three energy systems adapt to being stressed and, ideally, exhausted. Your body will build up those systems, over time, to meet the challenge. That is called "more energy."
For one example, the Phosphagen metabolic system can be stressed or depleted by ten heavy ball floor slams or by a 30-second full-out sprint. That system is for quick bursts of maximum power.
This site, The Three Metabolic Energy Systems, explains the basic physiology of the three energy systems with a discussion of how each is best stressed (but unfortunately that discussion only addresses things like running rather than the other exercise categories which also make specific demands on the 3 energy systems).
