Metabolism of Energy
Before we can begin attempting to improve how you function through training, we have to consider how you create the energy you're about to expend.
There are three ways you can create energy in your body. While they overlap, each plays a dominant role at its peak:
1. Phosphagen – stored ATP in the muscle, plus as much ATP as the body can quickly generate through the combination of ADP and CP. Good for about 3-4 seconds (or one hard lift) with ATP alone, and about 30 more seconds when the secondary cycle (ADP plus Creatine Phosphate) is operating efficiently. This secondary cycle can be enhanced through Creatine Supplementation and the training effect.
2. Glycolytic – Stored blood glucose / muscle glycogen goes through a chemical process to create pyruvic acid, which is necessary for the generation of more ATP. Since this process takes time and generates lactic acid, performance drops considerably during this phase. Important: potential in the glycolytic phase isn't limited by supply, as in the phosphagen phase, but by the ability to sustain activity in the presence of its byproduct, lactic acid. This effect is lessened by training: you're able to buffer lactic acid better AND delay the point at which its presence becomes performance-limiting. Moreover, you can break lactic acid down for more energy. This is where CrossFit shines.
As point of illustration, it's useful to consider the value of extending the glycolytic phase for sport. If a hockey player can metabolise energy effectively in the glycolytic phase for two minutes, he's going to perform better. Shifting into the aerobic phase early (ie 30 seconds into a shift) means that the player has to slow down, as the aerobic pathway dictates a lower heart rate and the inability to create energy quickly.
3. Oxidative (we call it 'aerobic'): breaks down fatty acids for fuel. However, this does not necessarily guarantee the breakdown of adipose tissue (fats) during exercise. Most exercisers doing 'aerobics' or 'cardio' exercise at too low an intensity, or at too short a duration, to trigger the breakdown of fat for energy. An hour on the elliptical trainer – if you can stand the mind-numbing boredom – will predominantly use up blood-borne glucose, muscle glycogen, and also metabolise muscle for energy. It's a metabolism-killer in the long term, and may actually harm weight-loss efforts. At the very least, an athlete seeking to perform better in 1-2 minute time intervals will waste time in the offseason training at this low level of intensity, and may actually see their performance suffer.
In Part II, we'll consider the overlap between energy systems, and what that means for CrossFitters and non-specialized athletes.