5 Reasons Why Zone 2

If you are here, you have likely heard of “Zone 2” before. It has, maybe unfortunately, become shorthand for easy training. It can be. But remember, it exists on a dimmer switch. It ranges from very easy to marathon pace. Anyone who has run a marathon knows that is not easy. What matters is understanding what is happening inside the body in this zone. 

There are five reasons you should know.

First, fat is the primary fuel source. We covered this in the last article, but why does it matter? Your body stores a massive amount of energy as fat, often over 100,000 calories. This is nearly unlimited compared to carbohydrates. We want the body to rely on this fuel source as much as possible. By doing so, we preserve carbohydrates and teach the body to sustain work for longer durations and, over time, at higher intensities while still using fat.

Second, you develop more mitochondria. Think back to high school science. The mitochondria are the powerhouse of the cell. In Zone 2, the body increases mitochondrial density through a process called mitochondrial biogenesis. These structures help process energy, especially carbohydrates. The more mitochondria you have, the more efficiently you can produce energy when intensity increases.

Third, capillary expansion. Capillaries are the endpoints of your cardiovascular system. Your heart pumps oxygen-rich blood to your muscles, and capillaries deliver it where it is needed. In Zone 2, these capillaries expand and reach deeper into muscle tissue. This improves your ability to deliver oxygen and fuel during higher intensity efforts. The system that transports blood in your body literally grows.

Fourth, maximum stroke volume. Your heart can only pump a certain amount of blood with each beat. In Zone 2, you reach near maximum stroke volume. The heart can beat faster, but it cannot move significantly more blood per beat at this point. Training at this level improves the efficiency of the heart and can increase how much blood it pumps with each contraction over time. This means more oxygen delivered to working muscles.

Finally, force equals mass times acceleration. The faster you move, the more force you place on the body. More force increases the risk of injury. Think about how many Soldiers develop stress fractures or overuse injuries. These often come from moving too fast, too often. By reducing the average force of training sessions, we allow the body to recover and adapt. Over the course of a year, this allows for more consistent training. And consistency is what builds fitness.

There is a clear theme here. We train easier to create the conditions to train harder later. Zone 2 builds the system required to perform at high intensity. The tradeoff is that it takes time.

Some of these adaptations may take years to fully develop. In many cases, it can take seven to eight years of consistent training. Most Soldiers will never have uninterrupted time like that. That makes it even more important to prioritize this type of training when the opportunity exists.

The goal is to apply the minimum effective dose of training to achieve the desired adaptation, and to do it consistently.

Zone 2 sometimes gets a bad reputation in the force. It is seen as an excuse to go easier and avoid challenging training. That misses the point.

First, it builds camaraderie and teamwork. Many problems are solved during long, easy runs that would never be addressed in formal settings. You learn what your Soldiers are thinking, what they are dealing with, and what they need. And they enjoy it.

Second, this type of training builds the foundation for meaningful fitness. It creates the base that allows you to later push intensity when it matters most.

There is one caution to keep in mind. We improve fitness from below, not from above.

Let’s explain what that means.