by Aaron Friday
Movement is possible only via muscle contraction, and muscles can use only ATP (adenosine triphosphate) to contract. In a state of rest, we have enough stored ATP to fuel only a few seconds of activity. When that is gone, we need to make more. How that happens is the subject of this admittedly oversimplified article.
The body makes ATP via three energy systems, also known as energy pathways. We depend on each of these systems in different situations, depending primarily on the intensity of our activity. They are:
- Phosphagen System—For very high-intensity activity
- Glycolytic System—For moderately high to high-intensity activity
- Oxidative System—For low-intensity activity
An exercise program that targets each of these systems is important for developing and maintaining a strong and healthy metabolism. All of it can be done with kettlebell training, as described at the end of this article.
Phosphagen System
Maximal-effort activities like jumping, sprinting, throwing, and lifting heavy weights are fueled by the phosphagen system. When your brain senses that you are placing a very high demand on your body, it will start making ATP from the creatine phosphate stored in your muscles. This is the quickest possible way to make ATP, but it is very inefficient and lasts for only a few seconds.
In fact, after about six seconds of very high-intensity activy, your body will require you to “downshift” to a lower level of intensity and switch “fuel tanks” because you’re getting low on creatine phosphate.
Glycolytic System
For moderately high to high-intensity activity, we rely on the glycolytic system, which creates ATP from sugar (i.e., the glycogen stored in muscles and liver, and the glucose in blood). It is slower than the phosphagen system, but it supports activity longer (at a sub-maximal intensity level). When you do moderately high-intensity exercise lasting between 30 seconds and a couple minutes, your body uses this system extensively.
Oxidative System
For low-intensity activity, we rely on the oxidative (or “aerobic”) system for energy. This system is the slowest of the three, so it cannot support high-intensity activity. It can, however, support low-intensity activity for hours. It uses oxygen plus glucose and/or fat to make ATP. The glucose comes from the usual places: stored glycogen and your blood stream. The fat comes from, you guessed it, stored bodyfat and the fat in your blood stream.
The following table shows how the three systems work together to meet the demands of your exercise. As you can see, there are intensity/duration intersections where two energy systems share the burden. Also shown are examples of exercises that stress those systems.
| Energy System | Intensity | Duration | Exercise Example |
| Phosphagen | Very High | 6 seconds or less | One-repetition maximum lift, short sprint, explosive movements |
| Phosphagen + Glycolytic | High | 6 – 30 seconds | 10 – 20 heavy kettlebell swings, 10 full squats |
| Glycolytic | Moderately High | 30 seconds – 2 minutes | Complexes taking between 30 seconds and 2 minutes to complete |
| Glycolytic + Oxidative | Moderate | 2 – 3 minutes | 3 minutes of continuous kettlebell snatches |
| Oxidative | Low | More than 3 minutes | Continuous, light circuit training with short rest periods, walking, jogging, cycling |
Training the Metabolism
The energy systems respond to repeated stress by getting more efficient, much like a muscle gets stronger through progressive overload. We can focus our training on these systems to improve our health, our athletic ability, or both.
Some athletes require highly specialized metabolic adaptations. For example, a powerlifter needs repeated bursts of maximal energy interspersed with long periods of rest. This type of training stresses the phosphagen system, making it more efficient over time. Conversely, a marathon runner needs a much lower level of energy, but for hours at a time with no rest at all. This can be accomplished only with an efficient oxidative system, which is achieved by stressing it repeatedly over time.
Distance running will prepare an athlete for a maximum bench press about as well as powerlifting prepares someone for a marathon — terribly in both cases. And both of these athletes are likely to be sucking wind after a hard, 2-minute set of kettlebell snatches unless they also perform glycolytic training. Exercise adaptations are specific to the stresses placed on the body, so we shouldn’t be surprised when a person who is obviously in great shape gets their butt kicked doing something they’re not used to.
If your focus is on health instead of a competitive sport, I believe the best approach is to target all three of these energy systems on a regular basis. Work your metabolism from multiple angles and don’t specialize. You can do it all with kettlebell training, as long as the intensity and duration are controlled properly. See the following table for guidelines and examples. There is a method behind the madness!
| Energy System | How to Target (Examples) |
| Phosphagen | Use the heaviest weights you can use safely and with strict form, for about 5 or fewer reps. Applies to whole-body exercises like double-kettlebell cleans, squats, and push-presses as well as more isolated exercises like chinups and rows.
Rest as much as needed. Note: If you’re not resting at least a couple minutes between sets, you are not going heavy enough. It takes a couple minutes for creatine phosphate to be resynthesized after you use it up. |
| Phosphagen + Glycolytic |
The work:rest ratio is about 1:3 for interval training. |
| Glycolytic | Sets done with moderately heavy weights that take between 30 seconds and 2 minutes to complete (e.g., complexes)
The work:rest ratio is about 1:2 for interval training. |
| Glycolytic + Oxidative |
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| Oxidative | Light, continuous circuit training with a 1:1 work:rest ratio.
Note: Any interval training that does not allow full recovery between work intervals also stresses the oxidative system. |