Aerobic vs Anaerobic Training: Why Slowing Down Will Make You Faster

The Ingebrigtsen’s have made double threshold training popular across the running world. Josh Kerr is a famously low-mileage athlete. The late Kelvin Kiptum would run north of 300km per week. Parker Valby just became the first woman to win five NCAA distance titles in the same academic year while running less than all her competitors – just two or three times a week. For centuries, people have been trying to work out how to optimise human athletic performance. Unfortunately (or fortunately, if you’re a coach), the old adage that everyone is different is true: there just isn’t one right way to train.

That being said, there are some fundamental ideas that underpin every good training program, with the most important one being perhaps the least intuitive. Whether you look at Valby’s 50km weeks or Kiptum’s 300km ones, you will find that the best athletes in the world spend most of their training miles going slowly.

Parker Valby is one of the best runners in NCAA history – and she only runs three times a week.

The Energy Systems of the Body

To understand why running slowly is so important in the pursuit of becoming fast, we must first understand how the body uses energy. The adenosine triphosphate molecule (ATP) is responsible for unlocking all the stored energy you have, and while everyone has a small amount of ATP readily available, it can only supply the muscles for about 10 seconds. After that, the body needs to create ATP to keep moving, which it can do anaerobically (without oxygen) or aerobically (with oxygen).

The Anaerobic System

The anaerobic system is fuelled by glucose (sugar), which is broken down into a compound called pyruvate, which then generates ATP. Unfortunately, this process, known as glycolysis, releases protons that lower the pH of the muscles and lead to the extremely fatiguing phenomenon called acidosis. If you’re looking for a visual representation of this, find a 400m hurdles race on the internet. All those athletes who look like they’re going backwards in the final 100m are experiencing acidosis.

To counteract this, pyruvate is converted into lactate, commonly referred to as lactic acid. If “don’t shoot the messenger” was a molecule, it would be lactate. This compound actually consumes protons and shuttles them out of the cells and into the bloodstream, helping to stave off acidosis. While the buildup of lactate may not be the culprit of fatigued muscles, it does directly correlate to the number of protons in the muscle cells. When blood lactate levels rise above a certain point (approximately 4mmol/L), the body can’t clear it as quickly as it’s being created, meaning protons in the muscle cells can no longer be effectively removed. This is known as the lactate threshold, and if a person continues to exceed this level of effort, they will experience acidosis. Naturally, the higher the intensity above the lactate threshold, the shorter the duration it can be sustained.

400m hurdlers accumulate incredible amounts of lactate in their blood during a race

The Aerobic System

If the anaerobic system is effective, the aerobic system is efficient. This system uses oxygen to convert carbohydrates (which are turned into glucose) and fat (which is used in place of glucose) to create ATP in the mitochondria of the cells. The only byproducts of this process are water and carbon dioxide, both of which are exhaled each time we breathe. Of the two options, carbohydrates metabolise the quickest and are, therefore, the dominant energy source in high-intensity activity. This is why athletes sometimes consume gels during a run: they are bolstering their carbohydrate stores. If a person runs out of carbohydrates, they will hit that infamous wall as their body is forced to rely solely on fat, which is much slower to metabolise (but does produce three times as much ATP). Although the intensity of the activity dictates how much the body leans on either fuel source, both are always used to some degree. Even the likes of Eliud Kipchoge burn plenty of fat in a marathon!

Training the Anaerobic System

When an athlete is running at such an intensity that they are in an oxygen deficit, the body leans on anaerobic glycolysis to generate the additional ATP required, inevitably causing a buildup of protons in the muscles. Sessions like 400m repeats at 5km pace and fartlek sessions (intermittent bouts of fast and slow running) are both examples of typical workouts used to emphasise the anaerobic system. If the body is given sufficient chance to recover after such workouts, it will slowly develop a tolerance to the proton buildup in the muscle cells and learn to clear lactate from the bloodstream more quickly. This adaptation allows athletes to perform at higher intensities for longer before fatigue sets in. However, anaerobic workouts are demanding, and training too often at this intensity can lead to chronic acidity in the blood, resulting in overtraining. Therefore, high-intensity activity should only make up about 20% of your weekly training.

The Ingebrigtsen brothers famously monitor their blood lactate levels during workouts to ensure they maximise the anaerobic benefit

Training the Aerobic System

This brings us to the most counterintuitive idea in distance running. If only 20% of your weekly training load is at a high intensity, then the other 80% has to be at a low intensity. This is known as aerobic training and manifests in practice as any pace at which you can comfortably maintain a conversation.

A popular way to visualise aerobic capacity is as the base of an energy system pyramid: the wider you make your base, the higher you can build. Slow running teaches the body to metabolise fat more effectively, thereby conserving glycogen for when it is most needed. It also makes the mitochondria more efficient, meaning they can keep pace with the ATP demands of the activity without leaning on the anaerobic system. This allows an athlete to run faster while staying within their aerobic zone, producing energy more efficiently and avoiding the fatigue-inducing byproducts of anaerobic glycolysis.

Put simply, aerobic training raises a person’s maximum pace, and anaerobic training allows them to sustain that pace for longer. Think of it like a car: the Honda S500 can redline at 9500rpm but tops out at 130km/hr. Meanwhile, the Bugatti Chiron can get up to 500km/hr but can’t push above 7000rpm. Your ability to redline means nothing if your competitors can run at the same pace without strain. If I were to race someone like Beatrice Chebet – a three-time world cross country champion and the first woman to break 29 minutes for 10,000m – she would beat me because the pace where I am at my greatest effort would be entirely aerobic for her. She’s not hiding some Tony Stark-like arc reactor in her chest for extra energy; she can simply use the same energy more efficiently.

You might be wondering: if this aerobic work is so great, why not do all your training in this zone? The truth is, you kind of are. Unless you are doing something like 200m sprints with 10 minutes rest in between, you are unlikely ever to be working totally anaerobically. Something like a 20-minute run slightly above your lactate threshold is more typical of an ‘anaerobic session’, even though it relies on – and develops – both the anaerobic and aerobic systems. Nevertheless, these workouts still demand plenty of recovery: this is where slow running comes in.

Also known as active recovery, low-intensity running increases blood flow to the muscles, which helps to remove metabolic waste and deliver the nutrients necessary for repair and adaptation. Furthermore, because it doesn’t produce harmful byproducts, the aerobic system can be trained more regularly, meaning these active recovery days also greatly contribute to overall fitness.

Abraham Lincoln is quoted as saying, “Give me six hours to chop down a tree, and I will spend the first four sharpening the axe.” If you want to become a better runner, you need to dedicate most of your training to sharpening your aerobic axe.

Eliud Kipchoge (M) leads a group of professional athletes enjoying an easy run in Kenya