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Home / Training Science Series #5 - Aerobic vs Anaerobic Fitness

Training Science Series #5 - Aerobic vs Anaerobic Fitness


This is an article series designed to help further educate my Performance Coaching clients. I am releasing it in this article series to help educate more people to create a life of health and adventure. If you are interested in getting fitter -- irrespective of whether you are a novice or regular athlete -- then please read through this series and learn more about the endurance training process. I welcome you onboard as your performance coach to help guide you to the summit of your athletic potential!

Training Science Series: Why We Focus on Capacity Training…to Eventually Go Really Fast

Part One - Introduction

Part Two - The Utilization Problem

Part Three - Overtraining Syndrome (OTS)

Part Four - Advice for Beginner Runners

Part Five - Aerobic vs Anaerobic Fitness

Part Six - The Aerobic Base - Capacity Training 101

Part Seven - The New Science of Fat Adaptation

Part Eight - Introduction to Threshold Training

Part Nine - A Brief Introduction to Heart-Rate Zone Training

Part Ten - How to Choose Which Zones to Train in Regularly

Part Eleven - Peaking: When to Enter a High Intensity Training Phase

Part Twelve - Keeping the Ego in Check and Sticking to a Long-Term Plan



Part Five - Aerobic vs Anaerobic Fitness


When people talk of increasing fitness, often this is thought of as an all or nothing thing, without consideration of the completely different aspects of fitness. Are you talking about speed, power, endurance? A combination of some of those, or all of these components? The fitness of an Olympic sprinter versus the fitness of an Olympic marathoner are very different, but both of these athletes are considered extremely fit. The sprinter has optimised the performance of his anaerobic fitness and fast-twitch muscle fibers, while the marathoner has optimised the performance of his aerobic fitness and slow-twitch muscle fibers. Each of these different forms of fitness respond to different types of training stimulus with their varying pros and cons. Your body is amazing and will quickly adapt to whatever you do routinely, but only to a point, and likely compromises something else. If you’re always training at your endurance limit, you dial up the metabolic systems related to anaerobic pathways (“glycolytic” sugar burning pathways) that completely decondition your aerobic metabolic pathways (fat-burning pathways).

High-intensity interval training (HIIT) proponents sometimes say their approach makes you the fittest athlete (and in half the time!), but what they are really talking about is anaerobic fitness. Anaerobic fitness equates more to speed and power; think sprinting less than 20 seconds or lifting a really heavy weight. That is anaerobic fitness. This form of fitness utilises predominantly fast-twitch sugar-fuelled muscle fibers.

HIIT proponents tend to have woefully underdeveloped aerobic fitness levels which utilises primarily slow-twitch muscle fibers. The major fuel for slow twitch muscle fibers are triglycerides (fats) as this energy source can provide an abundance of ATP to fuel muscles. Very fit HIITer’s can be so aerobically deconditioned, even a slow jog can push them above their aerobic threshold. So, irrespective of their strong muscles and their high levels of anaerobic fitness, these athletes will run out of puff quite quickly in any endurance event.

I saw this regularly in stair climbing events that last (for me) typically less than 10 minutes. Many people consider stair-climbing an anaerobic power-based event, so often athletes with massively strong muscular legs with barely any bodyfat show up to compete. While they can achieve relatively decent times off of leg strength alone from their high power to weight ratio, they still can’t climb anywhere near as powerfully as I could, nor any of the other elites who were all lean and relatively skinny in comparison. Most of them “pop” (redlining) after only 20-30 floors, a third of the way (and effectively ~2-4 minutes) into a 90-floor race.

High intensity training creates adaptations in the capacity of the anaerobic metabolic system, which is really helpful for events lasting less than two minutes, but for anything longer they are not getting the most effective training stimulus to perform at the highest level. A 10-minute stair climb is still an endurance event, but it also requires a strong capacity of anaerobic fitness than an ultra-marathon would. Solely focusing on anaerobic fitness won’t cut it, which is why stair climbs (for example) are dominated by endurance mountain runners, rowers and cyclists, not HIIT and gym enthusiasts.

Ultimately, training at very high intensity does not train both the aerobic and anaerobic systems at the one time, although you will often find that pitched by those in the HIIT industry. Low-intensity training and high-intensity training cause completely different endurance adaptations and you need to properly train both to maximally optimise endurance performance. There are no world class athletes in any major endurance sport (running, cycling, rowing, cross-country skiing) using an exclusive HIIT training approach, which closes the case on that debate.

Because anaerobic pathways decondition far more rapidly than aerobic ones do, HIIT advocates require a regular dose of high-intensity training to maintain their anaerobic fitness, putting them at constant risk of OTS. While the aerobic system takes a lot longer to develop, it also takes a lot longer to decondition than the anaerobic system. This means endurance athletes can maintain their aerobic base for many months with little deconditioning, allowing them to switch over to more anaerobic utilization training to peak for events without losing any fitness at all. In fact, the opposite happens, this supercharges their fitness to even higher levels.

The science is clear on this, the aerobic system has to be optimised using a large volume of capacity training…and then—only then—do you invest any substantial time into training the anaerobic system. That is why no one is suggesting that high-intensity utilization training should never be performed, but only used as icing on the cake of a more regular training approach.

Next Article -> Part Six - The Aerobic Base - Capacity Training 101