This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Ok, not ALL in your head. But certainly PARTIALLY in your head.
I don't know about you guys, but Sci does a little bit of weightlifting. I'm a distance runner, you see, and part of training yourself to run your best means lifting a few weights now and then. My weightlifting sessions can be intense, and sometimes I come home with arms so tired and wussy I feel like a little T. rex in gym shorts. Some days, when I've just increased weight or reps (or both), or I'm trying a new and particularly grueling exercise, the last few lifts can just be torture. It's an exercise of will to look at my puny little tired arms and the heavy weights, and then to lift them from here to there and back again. But I don't think much about my brain during these sessions, unless I suddenly lose count of my reps. After all, this is about my muscles, and tiring them out, not about my brain, right?
Wrong! It turns out that your brain may have something rather interesting to do with muscle fatigue, and the way you look at yourself while you lift might make you feel worse, rather than better.
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(Sci's muscles are featured on the right. Source)
Tanaka et al "Central inhibition regulates motor output during physical fatigue" Brain Research, 2011.
(Much thanks to Drugmonkey and Isis for pointing out this paper)
Now, there are two different kinds of fatigue as we think of them in physiology. The first is the straight up physical fatigue, where the muscle does not have the ability to contract fully. This could be due to depletion of energy sources in your muscle, such as glycogen, and so your muscle doesn't have enough energy available to contract fully. There's more to physical fatigue than just this, however. Muscles contract in response to stimulation from nerves, which come very close to muscle fibers at the neuromuscular junction. When the neuron is stimulated, and your muscle needs to contract, the nerve releases neurotransmitter toward the muscle which signals it to contract. Decreases in membrane potential in these neurons and other failures of neurotransmission have also been linked to physical fatigue. Either way, physical fatigue means that the muscle doesn't have the energy to get the full contraction going on.
Then there is central fatigue. This is a decreased ability of your nerves to contract the muscle. This could be because they are sending smaller signals due to inhibition, or due to signalling which is inhibited due to signals coming from your muscles associated with pain (a sign of muscle fatigue). But it could also come from your BRAIN. After all, the nerves are all coming from the central nervous system to begin with, and the brain can certainly inhibit the actions of peripheral nerves. So is your brain contributing to your muscle fatigue?
To figure this out, the authors in this study used something called Ramachandran's mirror box.
The above is a demonstration of the mirror box, which is usually used for things like stroke rehabilitation or phantom limb pain. The box 'fools' the brain into thinking your dominant hand and non-dominant hand are the same hand. This can be very useful when you are trying to, say, recover from a stroke which affects one hand. You can perceive it as the other hand, which is still working well, and this can fool your brain and actually increase the rate of your recovery. In this case, the authors used the mirror box to see if they could fool one hand into thinking it was less fatigued.
They started with a simple task. With hands outside of the box, the participants used their dominant hand to squeeze a ball as hard as they could. They then asked then how tired their hand was, and had then do it again. Each time the hand contracted, they measured the signals coming from the motor cortex of the brain using magnetoencephalography, which records the magnetic fields associated with the electrical firing events taking place in your brain. It's not very anatomically specific, but it can differentiate out the motor cortex, and show how much activity is taking place there.
What they saw was that, after the squeezing, the people said their hands were tired, and when they tried again, there was a small (though it looks like non-significant) decrease in activity in the motor cortex as measured by MEG. The motor cortex itself looking like it was sending smaller signals during physical fatigue. But then, they added in a mirror box.
This time, they had them conduct the fatiguing ball squeezing task with their dominant hand, but they couldn't SEE it happening. Instead, they saw their nondominant hand, which LOOKED like their dominant hand, just lying there, while the dominant had did the squeezing task unobserved by the participants view.
This time, when asked how tired their hand was, the participants stated that they felt a lot LESS TIRED than they felt before.
And when they measured the activity in the motor region of the cortex and asked them to squeeze the ball again (the bars on the far right) they found no difference in the level of the magnetic field evoked by the activity during the second squeeze. When you can't see your hand doing the work, you end up less tired and your brain sends stronger signals the second time around. It appears that there is input from your brain in how much muscle contraction to put into a task, and that it's more than just your muscles that determine when your muscles are fatigued.
Now, of course, this study did not prove that the fatigued hand actually contracted less. Unfortunately they did not measure that, which I think would have been a really cool addition to the study. I also think it would have been interesting to see what role the signaling from the motor cortex plays in the original hand contraction, does the hand contract more strongly when you're looking at it vs when you are unobserved? Could this have contributed to less of a sense of physical fatigue? And of course, while MEG is nice, I could have used something more specific in this case, like fMRI, which would have had much better anatomical specificity and where they might have gotten significant differences.
But what they DID show is that you can fool your brain with altered visual feedback. Your brain sees the effort put in by the hand performing the tiring task, and interprets that as fatigue. If the visual cortex is "fooled" and does not "see" the hand performing the task, you will feel less tired, and you may be capable of more effort. How exactly this takes place will have to be shown in future studies.
But this makes me wonder: does looking at myself as I lift weights increase my muscle fatigue, as my brain sees what I am doing and begins to inhibit future muscle contraction? Is my brain what's making me tired? Of course, it's obviously a good thing to be looking at yourself as you lift weights, this can be extremely important to learn proper form, make sure you're working the right muscles, and ensure you're not on the route to injury-ville. Of course this is all incredibly preliminary (and we don't even know how it works!), but it's certainly an interesting thought. Could we really do more for our muscles, if our brains didn't get in the way?
Tanaka, M., Shigihara, Y., & Watanabe, Y. (2011). Central inhibition regulates motor output during physical fatigue Brain Research, 1412, 37-43 DOI: 10.1016/j.brainres.2011.07.021