Physiology of Fatigue: What are we fighting when we try to push through a challenging workout?

Why are workouts so hard?

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We are fighting a lot when we push ourselves through workouts that are challenging. There are times we may be fighting a bad attitude, discouragement, lack of confidence, drive or our own personalities, but we are, at times, also fighting a very complex regulatory system designed to protect us from severe self-induced damage.

Fatigue and Temperature

Fatigue can be defined as reaching a point where the body seeks to slow down or stop. Exhaustion is that point where a person (or animal) is unable to continue. The most important factor driving suppression of motor activity is believed to be brain temperature. In an untrained person, exhaustion may occur when core body temperature reaches 100 to 102F(~38 to 39C) while a highly trained person may not reach exhaustion until body temperature has reached 104F (~40C).

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Interestingly, it’s not only humans who are stopped at these temperatures. Internal temperatures of ~104 oF will stop other animals whether sprinters (Cheetahs) or the generally more placid and possibly endurance-oriented (Goats) (Taylor and Rowntree 1973). And yes, I’m sure you’re wondering: temperatures were measured rectally, and the animals ran on a treadmill while wearing masks so oxygen and carbon dioxide levels could be assessed. The research team also cranked the heat up. Cheetahs ran for shorter periods when the room was hot. The authors of this paper concluded that the duration of a Cheetah’s sprint is limited by core temperature, which is influenced by air temperature. Keep this in mind when you are working out in the summer with no air-conditioning. There are other factors that are also thought to play roles in regulation of intense physical output. Working muscles send feedback to the brain, and in most of us, they are not yelling “Go! Go! Go!” At first they are saying things like “we need more oxygen over here” and “pump the heart faster.” Unfortunately you maximum output can only go on for as long as you have the necessary materials to keep the system running. Your maximum obtainable heart rate will matter. That may be one you cannot make “just do it.” although you can improve your ability to pump blood with training.

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Blood concentrations of important factors or metabolites, and depletion of working materials, are also monitored by the brain. Changes in concentrations and availability of neurotransmitters, endorphins, cytokines, along with a build-up of ammonia in the brain, occur during continued intense exercise. Cerebral energy use increases requiring more oxygen, while blood flow will decrease by about 20% due to constriction of brain arterioles. Low oxygen, loss of neurotransmitters, and accumulation of waste products can cause a problem that is truly “all in your head” but a real problem none the less. An increased need for oxygen and fuel in the brain may be part of what causes someone to want to slow down or stop.

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Practice improves physiology and performance.

Increasing oxygen intake may improve performance not necessarily by providing muscles with additional oxygen, but in providing the brain with what it needs to keep the system running. Depletion of brain glycogen and excessive use of lactate as an alternative brain fuel may also signal fatigue. This may happen faster in untrained athletes. Physical training is, after all, about much more than simple strength and endurance. It includes getting all systems, including subtler aspects of physiology like the ability to dissipate heat, produce lactate, carry oxygen and oxygenate the brain, to work as efficiently as possible. We can reach our limits, but our brains rarely stupid enough to allow us to go beyond them and recklessly run our bodies off the edge of a cliff. The brain also likes to know what’s going on and practice (going through the motions) and rehearsal are important to performance. Rehearsing movements before a WOD may be as important as traditional warming up. It preps your system for what it is about to do and lets it know what is coming. Even imagining movements may help improve strength output and performance (Jeukendrup et al. 1996).

CrossFit training, rational mental toughness.

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We can improve performance intelligently rather than fight what we imagine to be a lack of mental toughness, or allow ourselves be discouraged. We can keep cool and well-hydrated. We can be patient enough to recognize that our physiological and biochemical systems are becoming more efficient as we train, even if our speed or strength has plateaued, and not give up on long-term goals. Finally, encouragement and cheers can help people achieve their maximal level of oxygen consumption (Nybo & Secher 2004) and maximum performance. This may be especially true if they are new to Crossfit and have type A personalities. New Crossfitters may be putting superhuman efforts into their workouts and should be congratulated and admired for these as much as our seasoned champions.

Taylor CR, & Rowntree VJ (1973). Temperature regulation and heat balance in running cheetahs: a strategy for sprinters? The American journal of physiology, 224 (4), 848-51 PMID: 4698801

JEUKENDRUP, A., SARIS, W., BROUNS, F., & KESTER, A. (1996). A new validated endurance performance test Medicine & Science in Sports & Exercise, 28 (2), 266-270 DOI: 10.1097/00005768-199602000-00017

Nybo, L., & Secher, N. (2004). Cerebral perturbations provoked by prolonged exercise Progress in Neurobiology, 72 (4), 223-261 DOI: 10.1016/j.pneurobio.2004.03.005

Taylor CR, & Rowntree VJ (1973). Temperature regulation and heat balance in running cheetahs: a strategy for sprinters? The American journal of physiology, 224 (4), 848-51 PMID: 4698801 Nybo, L., & Secher, N. (2004). Cerebral perturbations provoked by prolonged exercise Progress in Neurobiology, 72 (4), 223-261 DOI: 10.1016/j.pneurobio.2004.03.005ResearchBlogging.org

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