Simple Understanding of our Energy Systems

We usually talk of energy in general terms, as in “I don’t have a lot of energy today” or “You can feel the energy in the room.” But what really is energy? Where do we get the energy to move? How do we use it? How do we get more of it? Ultimately, what controls our movements? The three metabolic energy pathways are the ATP-CP system, glycolytic and the aerobic system. How do they work, and what is their effect, and how can we train them?


ATP Resynthesis
Think of your body as the stock market, ATP is the currency that our body uses to exchange energy. The energy for all physical activity comes from the conversion of ATP (adenosine triphosphate) to ADP (adenosine diphosphate). During this breakdown of ATP, which is a water-requiring process, a proton, energy and heat are produced; and energy is made! But since our muscles don’t store much ATP, we must constantly resynthesize it. The resynthesis of ATP is a circular process—ATP is hydrolyzed into ADP and Pi, and then ADP and Pi combine to resynthesize ATP.

ATP is produced through three metabolic pathways that consist of many chemical reactions. Which pathway I use for my clients for the primary production of ATP depends on how quickly they need it and how much of it they need. Lifting heavy weights, for instance, requires energy much more quickly than jogging on the treadmill. However, the production of ATP is never achieved by the exclusive use of one energy system, but rather by the coordinated response of all energy systems contributing to different degrees.


During short-term, intense activities, a large amount of power needs to be produced by the muscles, creating a high demand for ATP. The phosphagen system (also called the ATP-CP system) is the quickest way to resynthesize ATP. Creatine phosphate (CP), which is stored in skeletal muscles, donates a phosphate to ADP to produce ATP. No carbohydrate or fat is used in this process; the regeneration of ATP comes solely from stored CP. Since this process does not need oxygen to resynthesize ATP, it is anaerobic. As the fastest way to resynthesize ATP, the phosphagen system is the predominant energy system used for all-out exercise lasting up to about 10 seconds. However, since there is a limited amount of stored CP and ATP in skeletal muscles, fatigue occurs rapidly. This system makes 1 ATP or like $1 dollar in my hypothetical bodily energy trade.

How do you train it?
Work intervals of 10-20 seconds
Work to rest ratio 1:3 – 1:6 (the long rest periods allow for complete replenishment of creatine phosphate in the muscles so it can be reused for the next interval)
Work volume of 2-8 minutes accumulated work time
Type of rest would be rest relief, light movement such as walking or dynamic stretching

Example: Sprints on the treadmill or bike lasting 10 seconds with 30-60 seconds rest, repeated 6 times for a total work volume of 60 seconds. Rest 3–5 minutes between each set and repeat to achieve 2-8 minutes of work volume.

2. Glycolytic
Glycolysis is the predominant energy system used for all-out exercise lasting from 30 seconds to about 2 minutes and is the second-fastest way to resynthesize ATP. During glycolysis, carbohydrate—in the form of either blood glucose (sugar) or muscle glycogen (the stored form of glucose)—is broken down through a series of chemical reactions to form pyruvate. For every molecule of glucose broken down to pyruvate through glycolysis, 2 molecules of usable ATP are produced (or $2, lol) very little energy is produced through this pathway, but you get the energy quickly. Once pyruvate is formed, it has two fates: conversion to lactate or conversion to acetyl coenzyme A (acetyl-CoA), which enters the mitochondria for oxidation and the production of more ATP. Conversion to lactate occurs when the demand for oxygen is greater than the supply (i.e., during anaerobic exercise, the burn you feel!). Conversely, when there is enough oxygen available to meet the muscles’ needs, pyruvate (via acetyl-CoA) enters the mitochondria and goes through aerobic metabolism.

In this system, when oxygen is not supplied fast enough to meet the muscles’ needs, muscles lose their ability to contract effectively, and muscle force production and exercise intensity ultimately decrease.

How do you train it?
Work intervals of 20 seconds – 2 minutes
Work to rest ratio 1:2 – 1:5
Work volume of 2-12 minutes accumulated work time
Type of rest would be work relief such as marching or jogging in place

Example: Hill climb on the treadmill or bike lasting 30 seconds with 90 seconds rest, repeated 6 times for a total work volume of 3 minutes. Rest 10 minutes between each set and repeat to achieve 2-12 minutes of work volume.

3. Oxidative a.k.a Aerobic
The metabolic reactions that take place in the presence of oxygen are responsible for most of the cellular energy produced by the body. However, aerobic metabolism is the slowest way to resynthesize ATP. Oxygen, as the king of metabolism, knows that it is worth the wait, as it controls the fate of endurance and is the sustenance of life. “I’m oxygen,” it says to the muscle, with more than a hint of superiority. “I can give you a lot of ATP, but you will have to wait for it.”

The aerobic system uses blood glucose, glycogen and fat as fuels to resynthesize ATP in the mitochondria of muscle cells. When using carbohydrate, glucose and glycogen are first metabolized through glycolysis, with the resulting pyruvate used to form acetyl-CoA, which enters the Krebs cycle. The electrons produced in the Krebs cycle are then transported through the electron transport chain, where ATP and water are produced. Complete oxidation of glucose via glycolysis, the Krebs cycle and the electron transport chain produces 38 molecules of ATP for every molecule of glucose broken down ($38!).

Fat, which is stored as triglyceride in adipose tissue underneath the skin and within skeletal muscles, is the other major fuel for the aerobic system, and is the largest store of energy in the body. When using fat, triglycerides are first broken down into free fatty acids and glycerol. The free fatty acids, are transported to the muscle mitochondria, where the carbon atoms are used to produce acetyl-CoA. Fat metabolism is identical to carbohydrate metabolism, with acetyl-CoA entering the Krebs cycle and the electrons being transported to the electron transport chain to form ATP and water. The oxidation of free fatty acids yields many more ATP molecules than the oxidation of glucose or glycogen. For example, the oxidation of the fatty acid palmitate produces 100 molecules of ATP ($100 !!!). No wonder clients can sustain an aerobic activity longer than an anaerobic one!

How do you train it?
While the first two systems are best trained with intervals, because those metabolic systems are emphasized only during high-intensity activities, the aerobic system can be trained with both continuous exercise and intervals.
Work intervals of 2+ minutes
Work to rest ratio 1:.5 – 1:1
Work volume of 20-60 minutes accumulated work time
Type of rest would be rest relief such as sitting

Example 1: Walk or run on the treadmill for 60 minutes at 70%–75% maximum heart rate. Rest 10-15 minutes between each set and repeat, or not, to achieve desired work volume.

Example 2: Any equipment, 15-20 minute tempo workout at lactate threshold intensity (about 80%–85% maximum heart rate). Rest 7-10 minutes between each set and repeat to achieve 20-60 work volume.

Example 3: Any equipment, 3 minutes at 90% maximum heart rate. Rest 3 minutes between each set and repeat to achieve 20-60 work volume.


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