Treat your heart rate zones as different energy systems

I recently watched an interview with physiologist and performance coach Inigo San Millan on GCN. You can find it here. Inigo San Millan is one of the foremast experts on exercise physiology and zone 2 so I tend to listen to his advice.

In this interview he talks about each of the heart rate zones as energy systems with different outputs and therefore different demands on the body.

This was really interesting, the key takeaway for me is that we need to train each of the systems based on our long term goals. We know that zone 2 is needed to build a strong aerobic foundation, but’s it’s also the zone you should aim to be in during endurance training or events. For exampe ,if you are going to be racing for a max of one hour you should spend some of your time training in zone 4.

In summary the way he defines zones is:

• Zone 1 – uses slow twitch muscle fibres, fueled by fat and glucose. This is the recovery zone.
• Zone 2 – uses slow twitch muscle fibres, fueled by fat and glucose. This is where the highest fat oxidisation takes place (sometimes called FatMax). We also see an increase in lactate production. This provides the highest stimulus to improve mitochondriol efficiency.
• Zone 3 – slow twitch muscles are fully expressed and we start to use fast twitch fibres. Fat oxidisation decreases rapidly as fast twitch muscles need energy faster, so more glucose is used.
• Zone 4 – fat oxidisation can no longer so energy is coming from glucose. This is generally where lactate threshold and FTP are
• Zone 5 – is sometimes called VO2Max, no oxygen is needed, energy is from glucose. These high intensity effort can only be kept up for 2 to 4 minutes.
• Zone 6 – uses energy stored in muscles and can only be kept up for 20 to 60 seconds

Defining the zones as energy systems really helps understand why zone 2 is really important. The stimulus to improve mitochondrial efficiency will help anyone get fitter and burn more fat. For endurance athletes training zone 2 will build the engine that will allow you to maintain effort for longer by burning more fat and less glucose while keeping lactate production at a level that your body can maintain.

For more information on heart rate zones, click here.