Zone 2: Endurance training
Zone 2 training is absolutely essential to improve performance. To quantify this, elite athletes time dedicated for zone 2 training is somewhere between 60-75% of their entire training time. Very similar data across many different sports has been described by coaches worldwide as well as in the scientific literature.
The purpose of each training zone is to elicit specific physiological and metabolic adaptations in order to improve performance. It’s important to know what physiological and metabolic adaptations occur while at different intensities and how they can be improved in training. To know this, we first need to have some understanding of basic bioenergetics and muscle metabolism.
Basic Exercise Bioenergetics
The capacity of an athlete to exercise ultimately depends on the ability to transform chemical energy into mechanical energy. The exercise intensity or metabolic and physiological stress as well as muscle fiber recruitment pattern will dictate the energy system and substrate that is activated, which will then correlate with different training zones. Essentially, going slowly lets your body use fats as fuel and as you increase the pace you increase you demand on glycogen.
Skeletal muscle is composed of 2 kinds of muscle fibres – Type I, also known as slow twitch, and Type II, or fast twitch. Fast twitch fibres are also divided in two subgroups called Type IIa and IIb. Muscle fibre contraction obeys a sequential recruitment pattern where Type I muscle fibres are the first ones to be recruited. As exercise intensity increases muscle contractile demands increase and Type I muscle fibres cannot sustain the necessary demand. Type IIa muscle fibres kick in and eventually as intensity keeps increasing Type IIb will finally be recruited. Simply put, slow twitch fibres are used at slower speeds and fast twitch at faster speeds. Each muscle fibre has different biochemical properties and thus different behaviours during exercise and competition. Type I muscle fibres have the highest mitochondrial density and capacity and therefore are very efficient at utilising fat for energy purposes. Type IIa fibers have a lower mitochondrial density and a higher capacity to utilise glucose. Type IIb muscle fibres have a little mitochondrial density and a very high capacity to use glucose as well as ATP stored in these fibres for instant anaerobic energy. Therefore, each exercise intensity implies different metabolic responses and muscle fibre recruitment patterns which also corresponds to different training zones which are summarised below:
Training Zone Type/Energy Substrate Mainly Used/Type of Fiber
In this training zone we stimulate Type 1 muscle fibres, therefore we stimulate mitochondrial growth and function which will improve the ability to utilise fat. This is key in athletic performance as by improving fat utilisation we preserve glycogen utilisation throughout the entire competition. Athletes can then use that glycogen at the end of the race when many competitions require a very high exercise intensity and therefore a lot of glucose utilisation.
Besides fat utilization, type I muscle fibres are also responsible for lactate clearance. Lactate is the byproduct of glucose utilization which is utilized in large amounts by fast twitch muscle fibers. Therefore, lactate is mainly produced in fast twitch muscle fibers which then, through a specific transporter, export lactate away from these fibers. However, lactate needs to be cleared or else it will accumulate. This is when Type I muscle fibres play the key role of lactate clearance. Type I muscle fibers contain a transporter which are in charge of taking up lactate and transporting it to the mitochondria where it is reused as energy. Zone 2 training increases mitochondrial density as well as the transporters. By training Zone 2 we will not only improve fat utilisation and preserve glycogen but we will also increase lactate clearance capacity which is key for athletic performance.
An endurance athlete should never stop training in zone 2. The ideal training plan should include 3-4 days a week of zone 2 training in the first 2-3 months of pre-season training, followed by 2-3 days a week as the season gets closer and 2 days of maintenance once the season is in full swing.