Exercise fatigue: It's all in your head!

CurioCity
23 January 2012

Jordan Querido M.Sc.

writer

May 31, 2009

Canada's Simon Whitfield fought through a 1.5 km swim, 40 km bike, and 10 km run to capture silver at the 2008 Olympics in Beijing. After crossing the finish line, Whitfield stood bent over with his hands on his knees, gasping for air. With his lungs burning, and his legs feeling like jelly, you can be confident that he was completely fatigued. Although the majority of us have not competed on the Olympic stage, we can still relate to the feeling of exercise fatigue. Have you ever thought of why you fatigue during exercise?

Did You Know?
In the 1904 Olympic marathon, Thomas Hicks drank alcohol with strychnine, the active ingredient in rat poison, in attempts to prevent fatigue. Of course this was extremely dangerous and could have been fatal.

Exercise physiologists have been intrigued by this extremely complicated question for over a century. In general, many have thought that not enough oxygen or glucose (used to fuel the muscles), insufficient blood flow to muscles, and the build-up of by-products that are produced from the working muscles are some of the factors that contribute to fatigue.So, near the end of the race when fuel for muscles was at its lowest,by-products from the muscles were high, and breathing couldn't seem to keep up, Whitfield should have been running his slowest. However, with200m left in the race, Whitfield actually increased his running speed.Does this not seem opposite to what you would expect considering the factors that cause fatigue? If Whitfield was capable of running faster,why didn't he do so at the beginning of the race? In fact, most runners are fastest near the end of the race, even when they should be their most tired.

Dr.Timothy Noakes, an exercise physiologist, has suggested a controversial theory that may help explain this. He has developed a theory called the 'central governor model'

, which essentially states that fatigue occurs because our brain tells us to slow down. For example, if you are to run 10km, his model would suggest that your brain would determine the running pace after calculating how much fuel is needed to complete the run. If you start running so fast that you start depleting your fuel for your muscles before the race is over, then your brain tells you to slow down by making you feel fatigued. The little boost you get near the end of the run when you see the finish line, is simply your brain saying 'the race is almost done, we can pick up the pace for this last little bit'. The brain is not worried about using up the last little bit of energy, since it can see the race is almost over.

Did You Know?
Dr. Noakes is a professor at the University of Cape Town, and has run over 70 marathons.

New research has suggested that the brain doesn't make the decision by itself. Although the brain may decide how hard we exercise, the muscles may influence the brain's decision. Marcus Amann and his colleagues from Wisconsin and Switzerland had cyclists bike 5 km as fast as they could. The cyclists then repeated the exercise test but the researchers blocked the ability for the leg muscles to 'speak' with the brain;therefore the muscles could not influence the brain.

Did You Know?
To stop the muscles from communicating with the brain, the researchers injected Fentanyl directly into the spinal cord. Messages from the muscles never reached the brain because Fentanyl stopped message transmission up the spinal cord.

When the muscles were blocked from influencing the brain, they found that cyclists biked harder during the first half of the 5 km, and were more fatigued after the test. Since the muscles were working extremely hard, there was an excessive amount of by-products produced from the muscles. By the end of the test, the accumulation of by-products was so great that the muscles were forced to slow down. From these results, the researchers suggest that during a race the brain tells us to work as hard as we can, but the muscles will communicate with the brain and tell the brain to make us slow down if we are working harder than what the muscles can maintain.

Although we all know the feeling of exercise fatigue, scientists don't all agree why and how it occurs.

Learn More:

Story of Thomas Hicks:

http://www.boston.com/sports/other_sports/running/articles/2009/02/22/one_mans_poison/

Simon Whitfield Olympic race:

http://www.cbc.ca/olympics/triathlon/story/2008/08/18/olympics-triathlon-men.html

The perception of carbohydrate ingestion and fatigue:

http://seedmagazine.com/content/article/energy_for_nothing/

Jordan hails from Ottawa and received his BSc from the University of Ottawa. He then traveled to UBC in Vancouver to obtain his MSc in Human Kinetics, and is now a PhD student at UBC investigating the effects ofhypoxia on cardiorespiratory systems. He is a huge sports fan, butstill can’t tell the difference between a triple salchow and a triplelutz.

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