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High Intensity Interval Training and Fatigue: It’s all in Your Head

Many lay people, and personal trainers, think high intensity training is limited by physical characteristics of muscle physiology. It is commonly thought that lactic acid accumulation is a prime culprit for “hitting the wall” when doing high intensity interval training (HIIT). Lactic acid has gotten a bad rap for many aspects of high intensity training as well as delayed onset muscle soreness (DOMS).

In addition, there is a theory that oxygen debt is a limiting factor of high intensity training. However, both theories have recently has been found not to be the case. In fact, thanks to the work of Tim Noakes, Ph.D., (Noakes, 2012) and his “central governor theory” we now have an understanding that fatigue is not physical rather, it’s all in our head.

Learning Objectives

  1. Explain how the central nervous system and acts as a “governor” to protect the body from damage during maximal exercise
  2. Describe the misunderstanding that an increase in lactate production is one of the causes of muscle fatigue during intense exercise.
  3. Review Noakes (2012) model of exercise regulation and how it contradicts the understanding that fatigue is based of the body’s physiological and metabolic responses to high intensity exercise.
  4. Review the research showing that exercise fatigue is largely caused by psychological parameters.

Theories of Exercise Fatigue

Oxygen Debt -- The first researcher to investigate what causes fatigue was A.V. Hill from Manchester University in the 1920’s. Bassett (2002), who wrote a review paper of Hill’s contributions, indicates he defined the terms “maximum O2 intake,” “O2 requirement,” and “steady state of exercise.”  In addition, Basset (2002) indicates Hill’s research was focused on finding the physiological factors that limit exercise performance, specifically running. Hill’s answer to the question about what limits exercise was the inability to process oxygen to help with muscle contract. Basset (2002) indicates that what Hill hypothesized was that once a limitation has been reached at the muscular level, there is a breakdown that leads to an inability of the muscle to do anymore work.

Lactic Acid -- Robergs, Ghiasvand, Parker, (2004) conducted a review of literature on the biochemistry of exercise-induced metabolic acidosis and suggest the thought was that lactic acid is a product of muscle contraction in the absence of oxygen, and its accumulation was the cause of muscle fatigue.   It is a misunderstanding that lactate production causes acidosis and an increase in lactate production is one of the causes of muscle fatigue during intense exercise. There is no biochemical support for lactate production causing acidosis. Lactate production retards, not causes, acidosis.

McClusky (2015) indicates that lactic acid is present during muscle fatigue, but this does not mean it is the cause of the fatigue. McClusky (2015) suggests the best understanding of lactate now is it is produced by the muscles during both anaerobic and aerobic exercise. The cells produce more lactate as the intensity of exercise increases. Brooks (1984) indicates some of the lactate is shuttled to other parts of the body to be used as energy for muscle contraction. In fact, during high intensity exercise, most of the fuel used by the heart is lactate.

Physical or Psychological?

It was Tim Noakes Ph.D. who first presented the idea that fatigue during high intensity exercise was not physical (McClusky, 2015). Noakes (2012) proposed a model of exercise regulation in which exercise performance is not limited by a disruption of homeostasis in muscles rather, it is regulated to insure no failure can occur. The regulation originates within the central nervous system and acts as a “governor” to protect the body from damage during maximal exercise. The title of the paper in which Noakes describes his hypothesis sums it up well “Fatigue is a Brain-Derived Emotion that Regulates the Exercise Behavior to Ensure the Protection of Whole Body Homeostasis.”  Sensations of fatigue are different in each person or client. And the fatigue is misleading because it is largely independent of the real physiological state of an athlete or client when they start to get “tired.”

Noakes (2012) suggests his model of exercise regulation contradicts the understanding that fatigue is based of the body’s physiological and metabolic responses to high intensity exercise. This is because subconscious and conscious mental decisions are made by winners and losers of running races and are the ultimate causes of fatigue and athletic performance, or performance when doing HIIT.

Research to Suggest Fatigue is in Your Head

The perception of effort during exercise has been defined as the subjective awareness of the intensity of effort, discomfort, and fatigue experienced by a person during any intensity of exercise (Borg, 1982; West, et al., 2005).

Tucker, Lambert, and Noakes (2006) analyzed pacing strategies and lap times during men's world-record performances for 800-m, 5000-m, and 10,000-m races. They analyzed races as follows: 800-meter - 26 world-record performances from 1912 to 1997, 5000-meters and 10,000-meters times for each kilometer for 32 (1922 to 2004) and 34 (1921 to 2004) world records. In the 800-meter races, the second lap was slower than the first lap (except for two races). In the 5000 and 10,000-meter races, the two fastest kilometers for each runner were the first and last kilometer. The first kilometer makes sense to be fast when the runners are fresh and not fatigued. But the last kilometer being fast when the athletes are supposedly physiologically tired, suggests the brain is acting to help the athletes run

Carter, Jeukendrup, and Jones (2004) investigated the effect of a carbohydrate receptors in the mouth having an influence on exercise performance. Nine endurance cyclists performed two cycling trials in which they had to do a set amount of work as quickly as possible. During one trail, 6.4% carbohydrate solution was rinsed in the mouth at every 12.5% of the trial completed. During the other trail, water was rinsed. Subjects were not allowed to swallow either solution and each mouthful was rinsed for 5-seconds then spit out. Performance time was significantly improved with carbohydrate solution compared to water. The researchers suggest the mechanism responsible for the improvement probably has to do with an increase in central drive or motivation rather than having any metabolic cause.   Chambers, Bridge, and Jones (2009) conducted a similar study and suggest the improved exercise performance with a carbohydrate rinse may be due to the activation of brain regions believed to be involved in reward and motor control.

Winchester, R., et al., (2012) conducted a unique study on rating of perceived exertion (RPE) during exercise. 10 moderately active men completed three 20-minute running trials at 60% of their peak treadmill running speed. Each subject did three trials in random order: control, male-observed, and female-observed (4 – 7 days apart), where either a male or female observer came to watch the trial at the 10-minute mark. The male and female observers were introduced as research assistants. The position of the observer was the same for all trials and subjects. Both male and female observers talked to the subjects, as they ran, about sports, education and/or employment. The male and female observers were rated as highly attractive using a Likert-type scale by their peers (undergraduate sport science students).

Rating of perceived exertion was assessed at 6, 12, and 18-minutes. The introduction of a female observer caused a significant decrease in RPE, whereas the introduction of a male observer caused a significant increase in RPE compared to the control trial. The researchers conclude there is a social, interpersonal, psychological dimension to RPE during exercise.


Whereas it was previously thought that physical parameters were solely responsible for fatigue during high intensity exercise, it is now being presented as a psychological phenomenon. It was first thought that a lack of oxygen was the cause of fatigue. Then lactic acid was blamed for fatigue and muscle failure. But now, Noakes (2012) suggests fatigue is in our head, and that fatigue is caused by the central nervous system “protecting” the body from exercising too hard.


Bassett Jr., D.R. (2002) Scientific contributions of A. V. Hill: exercise physiology pioneer. Journal of Applied Physiology. 93(5):1567-1582

Brooks, G.A., (1984) Exercise physiology: human bioenergetics and its applications, New York, John Wiley & Sons.

Borg, G. A. V. (1982) Psychophysical bases of perceived exertion. Medicine & Science in Sports & Exercise, 14(5), 377-381.

Carter, J. M., Jeukendrup, A.E., and Jones, D.A. (2004) The effect of carbohydrate mouth rinse on 1-h cycle time trial performance. Medicine and Science in Sports and Exercise. 36(12):2107-2111.

Chambers, E. S., Bridge, M.W., and Jones, D. A. (2009). Carbohydrate sensing in the human mouth: effects on exercise performance and brain activity. Journal of Physiology 15(587)(Pt 8):1779-1794

Noakes, T. D. (2012) Fatigue is a Brain-Derived Emotion that Regulates the Exercise Behavior to Ensure the Protection of Whole Body Homeostasis. Frontiers in Physiology. 11;3:82. doi: 10.3389/fphys.2012.00082. eCollection

Robergs, R.A., Ghiasvand, F., and Parker, D. (2004) Biochemistry of exercise-induced metabolic acidosis. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. 287(3):R502-516.

Tucker, R., Lambert, M.I., and Noakes, T.D. (2006) An analysis of pacing strategies during men's world-record performances in track athletics. International Journal of Sports Physiology and Performance. 1(3):233-45.

West, S. J., et al., (2005) Submaximal force production during perceptually guided isometric exercise. European Journal of Applied Physiology, 95(5-6), 537-542.

Winchester, R., et al., (2012) Observer effects on the rating of perceived exertion and affect during exercise in recreationally active males. Perceptual & Motor Skills: Motor Skills & Ergonomics, 115(1):213-227