January 9, 2012

How Your Brain Manages Muscle Fatigue

Via Dan Peterson

Endurance athletes, both competitive and weekend warriors, know the feeling. Out on a long run or bike ride, their muscles start to feel a lot heavier the closer they get to their training distance goal.  While it makes sense that your muscles would get more tired the longer you go, sometimes it feels as if your brain is convincing you not to have any illusions of going past the agreed upon training distance.  Now, researchers at the University of Zurich have discovered that there is a control valve mechanism in the brain that actually decreases muscle performance and sends increased fatigue signals to your conscious mind to provide overload protection for your body.

In a previous Axon article, we looked at the brain’s role in managing your fatigue.  Traditionally, fatigue used to be considered a breakdown of biochemical balances with the build-up of lactic acid or depletion of glycogen for fuel.  However, research in the 1980s showed that this breakdown did not always occur and that athletes were still able to push through at the end of a race even though they should have been physically exhausted.

A new theory of the brain as a “central governor” emerged.  Like a warning light in your car, the brain calculates the time to physical catastrophe or total exhaustion based on the current pace and feedback signals from the body.  When it feels you won’t make it to your desired finish line, it begins to lower muscle output and sends messages to your conscious brain that its time to quit.

Led by neuropsychologist Kai Lutz, the Zurich team actually completed three different but related experiments to tie together the entire exercise process.  First, they wondered if you could “hide” the bad news of exhaustion from the muscle to the brain by blocking the warning signals.  They asked a group of volunteers to perform a thigh muscle contraction exercise until they could not continue.  Then, using spinal cord anesthesia, they were able to block all messages coming back from the thigh to the primary motor cortex, the movement control area of the brain.  As predicted, the athletes were able to perform significantly more contractions once their brain was not monitoring their fatigue.

Credit: University of Zurich

Of course, knowing exactly where in the brain these signals are received would be more useful to future research, so the Lutz team, with help from Urs Boutellier from the Institute of Human Movement Sciences and Sport at ETH Zurich, used functional magnetic resonance imaging to narrow down the location.  While the athlete volunteers performed exhausting exercises, fMRI of their brains showed increased activity shortly before fatigue in the thalamus and the insular cortex — both areas of the brain which analyze information that also guard against outside threats like pain and hunger.

To pinpoint the exact source of fatigue signals, a third experiment, conducted by Lea Hilty as part of her doctoral thesis, has now been published in the European Journal of Neuroscience. Using a biking exercise with 16 volunteers, she noticed specific communication activity between the insular cortex and the primary motoric area increased as the riders fatigue progressed.

“This can be regarded as evidence that the neuronal system found not only informs the brain, but also actually has a regulating effect on motor activity,” said Hilty.

Now knowing this specific communication path, sports science researchers hope to be able to develop new training regimens that can influence this process for better results.  According to Lutz, “The findings are an important step in discovering the role the brain plays in muscle fatigue. Based on these studies, it won’t just be possible to develop strategies to optimize muscular performance, but also specifically investigate reasons for reduced muscular performance in various diseases.”

This research may give a whole new meaning to the “power song” on our iPods when we’re running on empty but need to get to the finish line.

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