Slowing Down to Go Fast: The Undervalued Aerobic System

Cyclist on empty road approaching a turn, pine trees in background

Physiological indicators imply much about the preparedness and performance of endurance athletes. To effectively train for an endurance event, or coach an athlete who is competing in one, accurate physiological data is paramount to guide and monitor training performance and intensity. Aerobic system development, and understanding the aerobic threshold (AeT), is perhaps the single best determiner of performance for an endurance athlete. The basis of the training consists of prescribing load intensity and volume in cycles that will, over time, result in the permanent adaptation of the body and thus enhance endurance (Lipárová & Brodáni, 2015). 


The Fundamentals of the Aerobic System 

The aerobic system development primarily involves increased stroke volume, increasing enzymes that use oxygen to convert fat to energy, increasing readily available fuel stores at the muscle level, increasing the size and number of mitochondria, increasing capillary density in the muscle to improve oxygen transport. In more science-specific terms, aerobic threshold represents the level of effort at which anaerobic energy pathways start helping out with energy production. For endurance athletes, having an increased AeT is critical for an athlete to train longer and further. AeT refers to the intensity after which the body begins to rely increasingly on carbohydrates, and decreases its use of fat as energy. At this effort level, the body relies mainly upon Type 1 endurance muscles. When working below AeT, the body contains enough oxygen to function without producing significant amounts of lactate and other associated byproducts that build up with higher intensity exercise. Above AeT, breathing increases, and lactate levels start to rise, along with a bit more muscle damage. That is: above-aerobic-threshold training takes longer to recover from. 

How can an athlete improve performance through aerobic threshold?

In most endurance sports, the development of aerobic endurance is essential, and represents 60-85% of the total training volume (Lipárová & Brodáni, 2015). The training zone associated with the aerobic system is between 75-85% of the individuals' maximum capacity (Gabrys et al., 2017). When an athlete trains too often above his or her AeT and below his anaerobic threshold (AT) there can be repercussions both in health and long-term aerobic development. The increased stress of training above AeT often causes an increase of stress in the body through the production of the stress hormone cortisol, which can cause higher levels of muscle breakdown. The training benefits between the two thresholds are similar to those of below AeT but the stress on the body is higher. If the stress on the body continues without adequate recovery, the body often breaks down through fatigue and injury associated with overtraining. By comparison, the benefits of a higher intensity effort do not kick in until an athlete is training above AT. Training at AeT or just below strengthens endurance muscles and improves the body's ability to use fat as fuel, which enables the athlete to utilize this efficient energy type at higher effort levels.

A common misconception related to the aerobic system is that long slow distance (LSD) workouts are the best way to develop the aerobic system. By itself, LSD workouts are not enough to effectively improve the aerobic system; in fact, an athlete must challenge his or her AeT to develop the aerobic system to its best potential. A typical week for an endurance runner would consist of only two days spent above AeT with all other days spent at it. Though this depends on the individual athlete, phase of training cycle, and time of race season. Using this approach allows the athlete to build aerobic volume while minimizing injury risk.

Aerobic system development, and knowing the AeT, along with the Anaerobic Threshold, is perhaps the single best determiner of performance for an endurance athlete. It can seem counter-intuitive to focus on lower intensity aerobic endurance; however, performance for endurance races overwhelmingly relies on aerobic fitness and effective aerobic metabolism. Improving aerobic fitness is a long-term project and can initially feel slow, but when training adaptations begin to accumulate, the payoff is evident. 

  1. Gabrys, T., Garnys, M., Szmatlan-Gabrys, U., Stanisz, L., Rzepka, R., & Lukasiewicz, A. (2017). Results of intensity variable effort on condition of polish national team cross country MTB cyclists. Journal of Science and Cycling, 6(3), 25-27.
  2. Lipárová, S., & Brodáni, J. (2015). Dynamics of the aerobic and anaerobic threshold in relation to training load in cross triathlon. Journal of Physical Education and Sport, 15(4), 619-626.

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Kristen Hench

Kristen Hench, Ph.D., is a certified coach through USAT, USAT Para, ACE, USAC, ASCA, ASFA Yoga certified, and is a RRCA certified race director. She has trained beginner through elite athletes helping many to reach podium finishes as well as meet their personal goals in triathlon, running, track, and swimming. Kristen coaches adaptive sports with the USAF AFW2 program as a cycling coach and also works with a variety of able-body and parasport athletes through TRIMotion3. She enjoys helping children and youth get healthy, learn new life-long skills, and build confidence. She has coached internationally and was selected to coach in the 2018 and 2020 Invictus games. Kristen has also directed numerous family races and competed in several triathlons herself, including the Ironman distance events. She placed in the top three for her age group in the inaugural year of the Mountaineer Half Ironman and was one of the top swimmers in the 2004 Lake Placid Ironman. Besides triathlons, Kristen also enjoys marathons (with a PR of 3:15), triathlon, swimming, and a multitude of boot camp activities.