Photo Of A Cyclist On Zone 2 Training Ride Through Lush Green Forest Peeking Through Trees

Zone 2 Training: Build Your Aerobic Capacity

BY Iñigo San Millán, PhD

Almost everyone training with a goal and a purpose has some form of structured plan based on different training zones. While training in all zones is needed, Zone 2 training should be one of the most important parts of any training program.

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 to 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.

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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 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 phosphorylation. Depending on the level of fitness of an individual and up to 55-75% of VO2 max intensity, ATP synthesis (energy) is generated from fat and carbohydrates, although CHOs are used at small rates during low and moderate exercise intensities. At higher exercise intensities beyond 75% of VO2 max, 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 aerobic glycolysis, so ATP needs to be generated through the anaerobic mechanism also called substrate phosphorylation. 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 two kinds of muscle fibers — Type I, also known as slow twitch, and Type II, fast twitch. Fast twitch fibers are also divided into 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 metabolic responses and muscle fiber recruitment patterns, which also correspond to different training zones, summarized below:

Training Zone Type/Energy Substrate Mainly Used/Type of Fiber

The Many Benefits of Zone 2 Training

In this training zone, we stimulate Type 1 muscle fibers. Therefore, we stimulate mitochondrial growth and function, which will improve the ability to utilize fat. This is key in athletic performance, as by improving fat utilization, we preserve glycogen utilization 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 utilization.

Besides fat utilization, type I muscle fibers 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 called MCT-4, export lactate away from these fibers. However, lactate needs to be cleared, or else it will accumulate. This is when Type I muscle fibers play the key role of lactate clearance. Type I muscle fibers contain a transporter called MCT-1 which is 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 MCT-1 transporters. By training Zone 2, we will not only improve fat utilization 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 two days of maintenance once the season is in full-blown.

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About Iñigo San Millán, PhD

Dr. Iñigo San Millán, Ph.D., is the Director of the Exercise Physiology and Human Performance Lab at the University of Colorado School of Medicine and also Assistant Professor of Family Medicine and Sports Medicine Departments at the University of Colorado School of Medicine.’Dr. San Millán is considered one of the most experienced applied physiologists in the world. He has worked with many elite athletes and teams in sports including track and field, running, triathlon, rowing, basketball and cycling; including eight professional cycling teams. Follow Iñigo on Twitter.

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