How “Zone 2 training” went from temporary fad to science-backed gospel.
In the past two decades, exercise science and lab testing has caught up with the demand of endurance athletes, providing them with unprecedented knowledge about the ‘why’ behind their training. Born from this endurance enlightenment, if you will, is the widespread acknowledgment of the effectiveness of aerobic base training (aka, Zone 2 training). In this article, we’re going to dive into the science and physiology of aerobic base training and its application to endurance sports such as swimming, running, cycling, and more.
What is Aerobic Base Training?
Aerobic base training is specific training meant to increase your aerobic threshold, or your ability to perform steady-state work for a long period of time. Base training workouts are simple: go at a pace just below your aerobic threshold (the upper limit of Zone 2), and hold it. Your aerobic threshold is the exercise intensity at which blood lactate begins to increase substantially. Below your aerobic threshold — in Zone 1 and Zone 2 — the exercise intensity is quite low, and that’s why you can maintain these “easy” efforts for a long period of time. We’ll dive deeper into blood lactate and training zones in the next section.
Research has shown that almost all elite endurance athletes use aerobic base training as a part of their weekly routine, including sprint-distance triathletes, marathon runners, and Tour de France cyclists. The reason is simple. Endurance events are typically longer than a few minutes, sometimes longer than a few hours, and sometimes longer than a week (in the case of cycling’s Grand Tours). These events place physiological demands on the body for very long periods of time, testing the body’s ability to endure rather than its explosive energy output. Training to increase your aerobic threshold, therefore, will allow you to sustain activity for longer periods of time so that you perform better on race day. Only in one-off events lasting less than a minute could you completely forgo aerobic endurance training — think powerlifters or javelin throwers, neither of which are endurance athletes!
Now that you know the why behind aerobic training, let’s get a bit deeper into the science of it.
Aerobic Threshold and Anaerobic Threshold
In basic terms, aerobic refers to ‘with oxygen’ while anaerobic refers to ‘without oxygen.’ During an anaerobic effort, such as a 10-second sprint, your body is not using oxygen to fuel its main energy source. Conversely, during aerobic efforts like endurance events, your body is running on oxygen.
In fact, there are three different energy systems in the human body — the phosphagen system, glycolytic system, and oxidative system — that fuel muscle contraction. The phosphagen system (aka, the ATP-PCr system) is used for short and explosive bursts of energy lasting less than one minute; the glycolytic system powers high-intensity efforts lasting one to five minutes; and the oxidative system powers longer efforts lasting anywhere from a few minutes to multiple hours. These systems use different amounts of oxygen to fuel exercise, with the least amount of oxygen used for sprints and the most amount of oxygen used for long-duration endurance exercise.
Aerobic training utilizes your oxidative system and targets your aerobic threshold, which is the exercise intensity at which blood lactate starts to rise above resting levels (typically around 2mmol/L, or millimole per liter). Blood lactate is directly associated with muscle fatigue and the degradation of endurance performance, which is why it is so important that endurance athletes train their bodies to clear blood lactate.
As soon as your exercise intensity increases above aerobic threshold, your body isn’t able to clear blood lactate as quickly, leading to quicker and earlier muscle fatigue. Thus, by raising your aerobic threshold, you will be able to go further and faster with less blood lactate buildup, less fatigue, and longer time to exhaustion.
Anaerobic training, on the other hand, targets your anaerobic threshold, which is when blood lactate begins to build up very quickly. Your muscles will quickly fatigue in this state, and even the most highly-trained athletes can only hold an anaerobic effort for a few minutes. Endurance athletes use their anaerobic systems during sprints and other high-intensity efforts, but it’s their ability to recover using their aerobic capacity that is more often the determinant of performance.
How to Build an Aerobic Base
As we begin to look at training your aerobic base, we get to the term “Zone 2 training”, which is most often associated with aerobic threshold training because of the matching intensities between power output, heart rate, and blood lactate concentration. In other words, maintaining your power output in Zone 2 (based on Andy Coggan’s training model) will likely put your heart rate in Zone 2 and keep your blood lactate concentration below your aerobic threshold.
According to Dr. Iñigo San Millán, Ph.D., Director of the Exercise Physiology and Human Performance Lab at the University Of Colorado School Of Medicine, the purpose of Zone 2 endurance training is to improve lactate clearance “by increasing the number of mitochondria to clear lactate mainly in slow twitch muscle fibers as well as by increasing the number of MCT-1 and mLDH [lactate-specific transporters which transport lactate away from muscle fibers].”
The key point is that lactate is cleared mainly by slow twitch muscle fibers, not fast twitch muscle fibers. So, training at a high intensity will not exactly improve your aerobic threshold or your body’s ability to clear lactate because high-intensity exercise targets fast twitch muscle fibers. Instead, you need to train your slow twitch muscle fibers at low intensities (i.e., Zone 2) in order to improve your aerobic threshold.
The Components of a Good Base Training Plan
When constructing a base training plan, it’s important to maintain a balanced and repeatable schedule that will simultaneously increase your fitness while also giving you the necessary time to recover in between each session. Part of this balance is to not completely forgo high-intensity work — one or two HIIT (high-intensity interval training) sessions per week will help increase your aerobic and anaerobic thresholds, and you can tailor your workouts to target one or the other. Longer Tempo intervals will increase your aerobic threshold, and short 40/20s will increase your anaerobic threshold.
Here are the key components that I look for in a base training plan:
Weekly volume increases of 5-10%
1 or 2 HIIT sessions
1 or 2 long endurance sessions (>2 hours)
1 rest day
Rest week every fourth week (decrease weekly volume by ~50%)
These components emphasize the principles of progressive overload and structured rest above all, which will help you increase your fitness without being at great risk of injury or burnout. As opposed to the “build” or race season, the base season is more focused on consistent training and aerobic endurance rides than it is on race-specific HIIT sessions (for all the reasons mentioned above).
In my Base Training Plan (12 weeks, FTP-based), you can see how I fit a number of HIIT sessions and Zone 2 rides into the weekly calendar for a cyclist in their base training season. The main focus of this block is Zone 2 training, but that doesn’t mean you can’t include a couple of HIIT sessions to stimulate both the mind and the body’s anaerobic system.
In this article, we went over the endurance sport buzzwords “aerobic threshold training.” Unlike other fads, supplements, and myths, aerobic training has decades of research behind it, all leading up to the Zone 2 training principles that we know today. In the world of endurance sports, the aerobic threshold might be the most underrated and underappreciated metric of all. FTP and VO2 max get all the love, but it is more often your aerobic threshold that wins you races.
The aerobic threshold, or the upper limit of Zone 2, is the intensity at which blood lactate increases above resting levels. The best endurance athletes in the world are pushing 300 w, or running 6-minute miles below their aerobic threshold, and that’s what puts them a step above the rest. Training your aerobic threshold is arguably more important than training your anaerobic threshold when it comes to endurance sports. With long Zone 2 rides, runs, or swims, you are training your body’s ability to maintain low blood lactate and a given intensity. The higher you raise this floor, the further and faster you will go as an endurance athlete. The best way to improve your aerobic threshold is through a structured training plan with well-timed periods of rest. As you increase your training volume, make sure to include one or two HIIT sessions per week and long Zone 2 training sessions to get the most out of your weekly training.
Patel, H. et al. (2017, February 26). Aerobic vs anaerobic exercise training effects on the cardiovascular system. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5329739/
Stöggl, T.L. & Sperlich, B. (2015, October 27). The training intensity distribution among well-trained and elite endurance athletes. Retrieved from https://www.frontiersin.org/articles/10.3389/fphys.2015.00295/full