The body runs on 3 distinct energy resource systems and it can tap into all of them at various times for athletic performance. Proper training should focus on the energy system(s) most important to baseball.

The Energy Systems

• Anaerobic Alactic (or phosphagen) system, for bursts of high-speed movement in less than 10 seconds) - that's baseball!
• Anaerobic Lactic (or glycogen-lactic acid) system for bursts of activity lasting 10-90 seconds, and
• Aerobic (or respiration) system for prolonged efforts)
  

Except for legging out a triple, baseball relies mostly on the anaerobic alactic - the start-up system that centers around ATP (adenosine triphosphate) and burns chemical-energy stores of  CP (creatine phosphate).
[The aerobic system burns fat and carbohydrate from outside the muscles. The anaerobic lactic system runs entirely on muscle carbohydrate/glycogen, the lactate producing/dissipating cycle.]

So baseball practice and nutrition must focus on training the anaerobic-alactic (aka phosphagen or ATP ) system on top of an aerobic base.

• All work/pause training must tap the right system.
• Interval training should be baseball specific.
• Intensity should exceed normal demands.
• Keep work time per rep under 10 seconds (otherwise the longer-time energy systems benefit more).
• A set or reps should be 60 seconds at most (i.e. 10 reps of 5 seconds each) then rest for 5 times as long - a work/pause ratio  of 1:5 - to let the ATP and CP recover.
• Maximum training is 3 times a week, for 8-12 weeks.

Overloading produces positive physical change, 
but overtraining produces burned-out athletes.

 

You might encounter other terms and definitions for the energy systems, depending on the source and the focus of the site. For example, the following is posted at http://www.healthlessonsonline.com...

The first system is the Adenosine Triphosphate (ATP) is the immediate source for energy that is stored in muscle cells. As the phosphate is broken down, energy is released. This system is only good for very quick and powerful movements. Then, ATP runs out and much be re-synthesized so the process can be repeated. Oxygen is not required for this to work, there are no waste products, but it is extremely short in duration. This system will last for about 10-15 seconds. Sprinters have a very well developed ATP system.

The second system is called the Anaerobic Glycolysis (lactic acid) System. This energy is obtained by breaking down glucose (either stored in muscles or from the blood stream). Through this process lactic acid (which causes the burning sensation) is produced. If the activity is too intense, and there is no time for the body to clear out the lactic acid, then the activity can not be maintained. This system lasts from 15 seconds to 3 minutes.

The third system is the Aerobic Glycolytic System, also called the Oxygen System. The main source of energy for this system is carbohydrates (from stores in muscles or from the blood stream) and fats (from stores). In order for this system to work there needs to be oxygen present, as it is part of the cycle. If oxygen is not present then this system simply will not work. The waste products are carbon dioxide (which we exhale) and water. This system, if the activity is not too intense, can continue indefinitely (assuming the body has glucose, fat, and oxygen. However, if the acuity is too intense then glucose may run out, and the body will produce lactic acid. This is referred to as oxygen deficit. Marathon runners have well developed aerobic systems. Sprinters usually don't, but they don't need to.