Zone 2 Training for Endurance Athletes

Dr. Iñigo San Millán, PhD for TrainingPeaks

This article originally appeared in the Training Peaks blog

Almost everyone training with a goal and a purpose has some form of structured training which is based on different training zones, intensities and workouts spread through a week or a training block, something that could also be called microcycle and macrocycle. While training in all zones is needed, zone 2 training should be one of the most important parts of any training program. Unfortunately, many novice or young athletes barely train or are prescribed zone 2 training and therefore don’t develop a good “base”, thinking that the only way to get faster is by always training fast. By doing this they won’t improve nearly as much as if they trained zone 2 in large amounts.

For the past 18 years working with professional and elite endurance athletes like cyclists, runners, triathletes, swimmers and rowers I have been able to see that zone 2 training is absolutely essential to improve performance. By quantifying their training I have seen that their 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 is 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. Skeletal muscle needs to synthesize Adenosine Triphosphate, or ATP, for muscle contraction.  ATP is a nucleotide responsible for the energy processes in human cells. It is often called the “molecular unit of currency” for the cells and needs to be synthesized constantly during exercise. ATP generation is achieved by two mechanisms- anaerobic and aerobic metabolism. Fats and carbohydrates (CHO) are the two substrates mainly used, with some contribution from protein. Fat it is stored primarily in the adipose tissue but it is also stored in skeletal muscle in small amounts. CHO are stored in the form of glycogen in skeletal muscle (about 80%) and in the liver (about 15%). 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.
 
The majority of exercise intensities generate ATP through aerobic metabolism, also called oxidative phosphorilation. Depending on the level of fitness of an individual, and up to 55-75% of VO2max intensity, ATP synthesis (energy) is generated from fat and carbohydrates, although CHO’s are used at small rates during low and moderate exercise intensities. At higher exercise intensities beyond 75% of VO2max, ATP generation needs to be faster in order to maintain muscle contractile demands. Fat cannot synthesize ATP fast enough so CHO utilization increases and starts being the predominant energy substrate as the rate of energy synthesis derived from CHO is faster than that from fat. CHO becomes the major energy substrate used by skeletal muscle at exercise intensities up to 100% of VO2max. Beyond this intensity, ATP cannot be generated by the aerobic glycolysis, so  ATP needs to be generated through the anaerobic mechanism also called substrate phosphorilation. Essentially, going slowly lets your body use fats as fuel and as you increase the pace you increase the demand for CHO.
 
Types of Skeletal Muscle Fibers

Skeletal muscle is composed of 2 kinds of muscle fibers- Type I, also known as slow twitch, and Type II, or fast twitch. Fast twitch fibers are also divided in two subgroups called Type IIa and IIb. Muscle fiber contraction obeys a sequential recruitment pattern where Type I muscle fibers are the first ones to be recruited. As exercise intensity increases muscle contractile demands increase and Type I muscle fibers cannot sustain the necessary demand. Type IIa muscle fibers kick in and eventually as intensity keeps increasing Type IIb will finally be recruited. Simply put, slow twitch fibers are used at slower speeds and fast twitch at faster speeds. Each muscle fiber has different biochemical properties and thus different behaviors during exercise and competition.

Type I muscle fibers have the highest mitochondrial density and capacity and therefore are very efficient at utilizing fat for energy purposes. Type IIa fibers have a lower mitochondrial density and a higher capacity to utilize glucose. Type IIb muscle fibers have a little mitochondrial density and a very high capacity to use glucose as well as ATP stored in these fibers for instant anaerobic energy. Therefore, each exercise intensity implies different muscle fiber recruitment patterns and metabolic responses which also correspond to different training zones which are summarized in the table below: