Dimmer Switches and Zones

We want you to imagine a dimmer switch for a moment. You are in a dark room. Your hand reaches out to the wall, and there it is. A switch. As you slide it from bottom to top, the room gets progressively lighter until it is fully lit. It is a simple, observable change. Behind the wall, multiple processes are working to regulate the energy delivered to the light so it reaches exactly the level you set. 

Your body works the same way.

At rest, the light is off. The room is dark. Your body is operating at a level that requires very little energy. The key word is little, not none.

When we discuss energy, we will focus on two primary sources. Fat and carbohydrates.

Fat is your diesel fuel. It burns slowly, lasts a long time, and is one of our major advantages as a species. Your body likely stores over 100,000 calories of fat. That is a massive reserve of potential energy.

Carbohydrates are your premium fuel. They burn quickly, produce high energy output, and are limited. In contrast to fat, your body can store roughly 1,200 calories of carbohydrates at a time. If you have ever used cardio equipment that tracks calories, you know how quickly that can be depleted.

When the dimmer switch is off, your body relies almost entirely on fat for fuel. Again, almost. Your brain and liver always require some carbohydrates. Think of this as Zone 1. The body is stable, efficient, and operating with a high reliance on fat. This effort is sustainable for extremely long durations.

Now the light begins to turn on. Energy demand increases, and carbohydrate use begins to rise. Fat is still the dominant fuel source, but more systems are activated. Muscle recruitment increases. Cellular activity rises. Heart rate climbs.

As we continue increasing the intensity, fat and carbohydrate use begin to approach balance. This is the upper limit of Zone 2. Zone 2 exists between rest and moderate effort. It is often described as easy, but it ranges from very easy to moderately challenging. The upper end of Zone 2 is close to marathon effort for many athletes.

This intensity can be sustained for long periods. On the lower end, it can last for many hours. On the higher end, it may last two to three hours.

At this point, the dimmer switch is only halfway up. There is still significant capacity above Zone 2 before reaching maximum effort.

Next is Zone 3. This is a transitional space between moderate and hard effort. It can be sustained for roughly an hour in trained athletes. The limitation here is carbohydrate depletion and increasing muscular fatigue due to higher muscle recruitment. Zone 3 has a role in training, but it must be used deliberately.

As we continue increasing intensity, the room becomes bright. We enter Zone 4. Carbohydrates now become the dominant fuel source, accounting for a large portion of energy production. This effort is hard. Elite athletes may sustain the lower end of Zone 4 for up to an hour. For most, that duration is much shorter. At the upper end, effort may only be sustainable for several minutes.

At this level, you are operating on a narrow margin. Small increases in effort can significantly reduce how long you can maintain output.

Now turn the switch all the way up. This is Zone 5. The body is almost entirely reliant on carbohydrates. Stress is extremely high. Muscle recruitment is near maximum. This effort can only be sustained for a very short duration, often less than two minutes.

So in a five-zone system, why does Zone 2 get so much attention?

Let’s take a look.