As we progress to higher energy systems, we generate more and more ATP and consume more oxygen. Once the ATP demand cannot be supported by fat alone, the fuel source is switched, and type II muscle fibers are recruited. VO₂ max has not been reached. Efficiency decreases because we move to a less efficient but faster fuel source (glucose). We see a big drop in fat oxidation and an increase in glucose oxidation.

If the ATP demand exceeds the capacity described in zone 2, pyruvate is converted into lactate (a faster fuel source) to generate additional ATP. Fatty acids and carbohydrates are mixed as fuel sources, but the muscle starts using more carbohydrate and more lactate is therefore produced.

Zone 3 sees a transition to a glycolytic energy system. Fat decreases and glucose increases as a substrate for energy as exercise intensifies in zone 3. At the end of zone 3 we are almost purely burning glucose. Energy demands exceed the mitochondrial capacity to be the sole provider of ATP. The cell becomes dependent on glycolysis in the cytosol rather than the mitochondria. 

High Intensity Training: zone 4, 5 and 6

Zone 4

The lactate threshold (LT) is the point during exercise at which blood lactate levels begin to rise rapidly, leading to muscle fatigue and an inability to continue exercising at that intensity. The lactate threshold represents a different output depending on the distance and pace of the exercise. It is the point at which the body can no longer effectively clear lactate from the skeletal muscles, and the muscles begin to shut down.

An exercise test can be used to determine an athlete's lactate threshold. This is typically done by riding a stationary bike or running on a treadmill at a given intensity for 45 minutes in a laboratory setting. A small sample of blood is collected and analysed for lactic acid levels. The workload is then increased and the sampling process is repeated every few minutes. When lactic acid levels begin to rise exponentially, this indicates the lactate threshold.

Improving lactate clearance capacity and lactate threshold involves training at the right intensity. It is a common misconception that training at the lactate thrshold will improve lactate clearance capacity, but this is not entirely accurate. Lactate is primarily cleared by the mitochondria in slow-twitch muscle fibres, so it is important to train these fibres to increase the mitochondrial lactate oxidation complex (mLOC). Endurance training, particularly in Zone 2 (low-intensity endurance training), is essential for improving the function of slow-twitch muscle fibres, mitochondrial capacity, and mLOC.

In summary by training at a lower intensity, we improve the support system for when the body is overwhelmed at lactate threshold and beyond. 

Zone 5

Zone 5 is designed to be at an intensity that elicits V̇O2max. In high intensity training the goal for entering zone 5 is to inhale, diffuse and metabolise the maximum amount of oxygen possible. The speed and efficiency of this process determines your V̇O2max.

Oxygen consumption plateaus despite an increasing intensity of exercise. Training at this intensity will force the body into adaptations that equip you to deal with zone 5. 

A main adaptation that zone 5 training elicits is an increase in the heart's stroke volume, thereby pumping more blood (and thus oxygen) to the muscles and organs. Training in zone 5 will typically lead to these adaptations, particularly in a younger healthy population. 

Glucose is considered the predominant substrate to be converted into pyruvate for energy in zone 5. However, the demand of the muscle for ATP exceeds the body's capacity to regenerate it. 

In zone 5 we are utilising type IIX muscle fibres that evolved to work in anaerobic environments. Therefore, a transition to anaerobic metabolism occurs in the cytosol of the cell— outside of the mitochondria. This is a much faster process of producing ATP but forms increasing levels of lactate. This is partially due to NADH being oxidised to NAD+ to enable further ATP production, forming lactate in the process (see figure below). The production of lactate is beneficial for NAD+ regeneration (pyruvate is reduced to lactate while NADH is oxidised to NAD+)

Lactate can be converted back into pyruvate to undergo aerobic metabolism in the mitochondria. Type I fibres work to mop up lactate spilling out of type II fibres, and in the highly adapted muscle, can metabolise significant amounts of lactate before it spills out into the blood. 

This is an important concept known as the lactate shuttle theory. It accounts for huge gains made by humans in increasing their overall aerobic base. As mentioned earlier, elite athletes can generate huge amounts of power whilst exporting very little lactate into the blood. This was discussed in the zone 2 section, where training at that intensity develops the capacity of type I muscle fibres to metabolise lactate.

The burning sensation in muscles is not attributed to increasing lactate levels, it is the result of a build-up of acidity in the muscle cells. More specifically this acidity is caused by the release of hydrogen ions during the fast turnover and conversion of ATP, which releases hydrogen protons. This release of hydrogen protons within the muscle cells causes an interference with the ability of the muscle fibres to contract properly. Unless the intensity of the exercise decreases, the high level of burn (acidosis) will eventually cause cessation of the exercise.

The lactate exported can be used in several ways:

• The oxidation of lactate back to pyruvate in well-oxygenated muscle cells, heart cells, and brain cells, which is then directly used to fuel the Krebs cycle.
• The conversion of lactate to glucose via gluconeogenesis in the liver, which is then released back into circulation through the Cori cycle.
• When blood glucose concentrations are high, the glucose can be used to build up the liver's glycogen stores.