Is Fatigue All in Your Head?

Rowing - USA Lwt 4 @ World Champs 2003 I just heard of a new series of studies (hat tip to Diane Jacobs) which provide a very good example of how the brain acts as a central governor on physical performance.*

For a long time, research on the causes of muscle fatigue have focused exclusively on changes in the muscle itself, while ignoring the fact that the brain has control over whether the muscle can contract. More recently, researchers such as Tim Noakes have been advancing the idea that the brain acts as a governor on muscle performance. The idea is that the brain makes you feel tired as a way to prevent you from the dangers of overexertion. In other words, when your muscles fail, it’s not because they can’t “just do it”, it’s because the brain just won’t let them do it.

A recent series of studies has identified some of the neural processes that are responsible for implementing the central governor. Here is a brief summary of the findings.

In one study, researchers showed that motor commands to engage in intense exercise eventually create a sensory feedback loop that inhibits those same commands. It goes like this. The primary motoric area of the brain tells the muscles to contract repeatedly. This results in sensory signalling from the body that is read by parts of the brain that act to inhibit the primary motoric area from continuing with the contractions.

This is very similar to the process by which pain inhibits muscular activity. Which is not surprising, since both pain and excessive muscle activity are a potential threat to the body. In each case, the brain elects to protect against the threat by essentially shutting down the ability to continue with the threatening activity - in this case, repeated contraction of tired muscles.

Researchers were able to determine through the use of a functional MRI that right before muscle failure, the areas of the brain that became more active were the thalamus and insular cortex, two areas that analyze information indicating a threat to the body. Another study confirmed that the insular cortex is in fact the area that inhibits the primary motoric area.

Another study showed that pain medication which blocks feedback from the body also prevents the muscular inhibition that comes from fatigue.

Put these findings all together and a simple picture merges. When the brain receives information from the body indicating that a particular form of exertion is a threat, it decides to shut the activity down. It is surprising to me that such a common sense idea takes so long to gain acceptance, and is ignored all too often.

This dovetails very well with a central principle of training recommended on this blog (which is also one of the basic ideas in Z-Health): threat not only causes pain, it limits performance. If you want to increase performance, work to reduce threat.

This is why the “no pain no gain” mentality of exercise is so counterproductive, and why giving the brain “good news” about the body should be a primary training strategy.

Let me know what you think in the comments.

*I have been meaning to finish my series on central governors, which will include a more extensive discussion of the theories of Tim Noakes. Not sure when I will get to that though.