Why Zone 2?
Nearly all athletes training towards a specific goal follow some form of structured training.
Training is spread out using easy and hard sessions. The intensity of these sessions tends to be prescribed using training zones.
Everyone loves a good hard training session. You know the sessions when your lungs are bursting, and legs are screaming stop.
But the most important interval that we fall down on is our Zone 2 interval.
Why? It can feel too easy and that means we didn’t do any work right? Wrong.
The purpose of training zones is to elicit a specific physiological and metabolic adaption.
Spending time in any training zone elicits these adaptions by varying intensity. We use different training zones to target a desired adaption in training.
Let us peak behind training zones and get a basic understanding of what is happening:
At the cellular level: how cells transform energy (Bioenergetics)
Muscle metabolism: energy production to support muscle contractions
Your ability to exercise ultimately is determined by how your cells transform energy into mechanical energy.
Your cells create Adenosine Triphosphate (ATP) which generates energy in the cell.
During exercise muscles contract to produce power (e.g. quads pushing big gears). Muscle contraction needs ATP to be produced and transported to your muscles. ATP is therefore produced constantly during exercise.
ATP production is achieved by two mechanisms – anaerobic and aerobic metabolism. Carbohydrates and fats are primarily used with a small contribution from protein to create ATP.
Our bodies store carbs and fats differently. Fat is stored mainly in the adipose tissues but is also stored in small amounts in muscles. Carbohydrate is stored as glycogen in muscle and liver. Approx. 80% of all glycogen is stored in the muscle and approx. 10 to 15% in the liver.
Why is this important?
Exercise intensity and muscle fibre recruitment dictates the energy system (Anaerobic or Aerobic) and substrate (Fats or Carbs) activated.
We use training zones to help target different energy systems and corresponding substrate utilisations.
At most exercise intensities ATP is generated through aerobic metabolism. At easy to moderate efforts, Zones 1,2 & 3 (see below), energy comes from a combination of fat and carbohydrates. Most of the energy comes from fat in Zone 1 & 2 and as intensity increases more and more carbohydrate is used along with fat (Zone 3).
At higher intensities ATP generation needs to be quicker to maintain demands of exercise (Zones 4,5 & above).
Fat metabolism is too slow and carbohydrate utilisation increases becoming the predominant source of fuel.
At these higher intensities ATP cannot be generated quickly enough by aerobic metabolism. The anaerobic system provides the energy demanded.
At the Muscular Level
Skeletal Muscle is composed of 2 muscle fibre types.
Type I: slow twitch
Type II: fast twitch
Muscle fibres are recruited in sequence. Type I are recruited first. As exercise intensity increases so does the demand placed on muscle contraction.
Type I fibres cannot support the demand at the higher intensities. As intensity increases Type IIa begin to kick in and eventually Type IIb will be recruited.
Type I muscle fibres are very efficient at utilizing fat for energy.
Type IIa fibres have a higher capacity to use carbohydrate in the form of glucose.
Type IIb have a very high capacity to use glucose and ATP stored for anaerobic energy.
Exercise intensity implies different metabolic responses and muscle fibre recruitment patterns. These correspond to training zones shown in the table below:
Table 1: Training Zones/Associated Substrates/Muscle Fibre Types
So why is training in Zone 2 so important?
It simulates Type I muscle fibres. This increases mitochondrial growth and function, also known as mitochondrial density. Increasing mitochondrial density improves your ability to utilise fat. For endurance events this is a key performance enhancer. The more fat you can utilise preserves your glycogen stores for later in the event.
Type I muscle fibres also clear lactate. Lactate is a by product of the breakdown of glucose used during high intensity efforts. When intensity eases lactate is shuttled into the aerobic system to be cleared. Lactate is used by the mitochondria as energy as it is cleared.
By now you should see the value of Zone 2 training. It helps improve:
· Fat utilisation
· Spares glycogen for when it is needed
· Increases the clearance of lactate