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Fatigue And The Muscle-Mind Connection

  • By Dr. Robert Portman
  • Published May 1, 2013
  • Updated May 1, 2013 at 1:59 pm
Photo: ITU Media / Delly Carr

For triathletes, fatigue is the great equalizer. Fatigue will ultimately determine how well you perform and how far you can push your personal limits. There is a spirited argument among exercise physiologists about the causes of fatigue. Thirty years ago, scientists believed that fatigue resulted primarily from the buildup of lactic acid in the muscles. However, research conducted over the last five years has revealed that the true causes of fatigue are far more complex.

There is general agreement that fatigue has two components: a muscle component and a brain component. The question is whether they act in concert or independently. The best available evidence strongly suggests that fatigue begins in the muscles. If it’s true that muscle is the primary source of fatigue, then the outdoor adventurer has a number of nutritional strategies at his disposal to minimize, or, at the very least, delay the onset of fatigue.

As muscles work harder to generate the energy required for muscle contraction, there is a natural change in the levels of four critical constituents:

1. Glycogen
2. Free radicals
3. pH
4. Branched-chain amino acids

The changes in these four constituents initiate a series of physiological and biochemical responses that cause fatigue.

Glycogen is the form in which muscles store energy. Glycogen supplies in the muscles are limited. When glycogen falls to a critical level, the muscle begins to lose its ability to maintain energy output and we bonk. To preserve glycogen and stave off the bonk, the muscle cell increases its use of blood glucose as an energy source. Since glucose is the brain cells’ primary energy source, any drop in circulating glucose impacts the brain, making us feel more tired. A drop in blood glucose also causes a release in the brain of IL-6, an immune regulator, that has been shown to increase feelings of fatigue.

We all know about free radicals. These are highly reactive molecules that can damage muscle proteins and are one of the causes of post-exercise muscle soreness. Free radical buildup is a natural consequence of exercise. The more oxygen we breathe, the more free radicals we produce. Free radicals cause fatigue in three ways. First, free-radical buildup interferes with the mechanism responsible for maintaining oxygen and nutrient delivery to the muscles. Second, free radical formation also interferes with the mechanism that controls muscle contraction. Third, the muscle damage generated by free radicals may release brain fatigue signals.

During strenuous activity, blood pH is lowered when the muscle is forced to generate energy in the absence of cellular oxygen. This process produces lactic acid, which at one time was believed to cause a lowering of blood pH and generate fatigue. We now know that the pH effect is a result of the energy-producing reaction, not lactic acid. Regardless, a lowering of blood pH produces multiple symptoms associated with fatigue such as muscle burning and reduced work capacity.

Branched-chain amino acids, of which there are three–leucine, isoleucine and valine– are highly concentrated in muscle cells. Unlike most amino acids, branched-chain amino acids can be efficiently utilized by muscles as an energy source. In fact, during extended exercise, up to 20% of the muscles’ energy may come from branched-chain amino acids, particularly leucine. As branched-chain amino acid stores are used up, their blood concentration falls, which sets in motion the release in the brain of a neurotransmitter called 5HT. 5HT is often called the “fatigue hormone” since it creates a feeling of tiredness.

Although more research is needed to complete the picture, it is certainly clear that changes in these four key constituents, which are a direct result of increased muscle metabolism, can cause brain fatigue and bonking. This unified model of fatigue also gives us an excellent blueprint for how we can use nutrition to combat fatigue.

Ideal Anti-fatigue Fuel
The ideal anti-fatigue nutrition to consume during strenuous activity contains:

• Carbohydrate in the form of high-glycemic sugars, such as sucrose, fructose and glucose (preferably two or more). High-glycemic sugars preserve muscle glycogen, maintain brain function during exercise and prevent the release of fatigue neurotransmitters.

• Antioxidants such as Vitamins E and C. Antioxidants recharge the body’s natural antioxidant system, which is essential in combating the effects of free radicals on muscle function and the brain’s perception of fatigue.

• Protein, particularly protein high in branched-chain amino acids. BCAA’s serve as an additional source of muscle energy and prevent the release of fatigue signals from the brain. The ideal ratio of carb to protein is 4:1, which is found in sports drinks like Accelerade and gels like Accel Gel. Numerous studies have demonstrated that sports drinks and gels that contain a 4:1 ratio are far more effective in not only preventing mind and muscle fatigue, but also minimizing post-exercise muscle damage.

This brain-muscle nutritional strategy won’t eliminate fatigue completely, but it certainly will have a positive impact in your next triathlon.

Dr. Robert Portman is a well-known exercise scientist and author of Nutrient Timing and The Performance Zone. His latest book is Hardwired for Fitness. To create your own personal nutrition program visit www.portmancalculator.com

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