Which Fatigues First, Your Brain or Your Legs?

We all know what it feels like to reach our limits—to be unable to maintain pace for even one more step. But what defines those limits? Turns out there are lots of different factors that kick in depending on the situation, as I explored in a feature on fatigue earlier this year.

Researchers are still trying to sort out all these different factors, and a recent study in Medicine & Science in Sports & Exercise offers some interesting new insights. The study, from Kevin Thomas and his colleagues at Northumbria University in Britain, explores the relative contribution of the central nervous system compared to the muscles after different durations of all-out cycling.

Some background: If you cycle (or run, or pogo-stick, or whatever) to exhaustion, your muscles will be tired after you finish. This can be measured as a decline in “maximal voluntary contraction” (MVC), the greatest force you can produce with a given muscle by contracting it as hard as you can.

The current study involved 12 well-trained cyclists who, on different days, performed three different time-to-exhaustion rides at constant power. In one ride, the power was set so that exhaustion occurred after about 3 minutes on average; in another, it took 11 minutes; and in the third it took 42 minutes.

If you look at the change in quadriceps MVC after each of these three trials, this is what you see (the shortest trial is on the left, the longest on the right):

Neuromuscular fatigue data.









The muscle fatigue, as measured by the ability to produce the biggest force possible, is pretty much the same in each case, with a decline of around 15 percent or a little more.

But that doesn’t mean that the same things are occurring inside the muscles. The researchers used sophisticated techniques to investigate where, exactly, the fatigue was taking place.

You can apply electrical stimulation to the relevant nerve to make muscles twitch directly, which tells you how the function of the muscle itself has changed. This is referred to as “peripheral” fatigue, since it’s in the muscles.

And you can also apply stimulation farther upstream, using “transcranial magnetic stimulation” to produce an impulse in the brain area that sends a signal to the quadriceps muscle to twitch. This (with some additional calculations) gives you a measure of whether the maximum signal traveling from the brain to the muscle has changed. This is referred to as “central” fatigue, since the reduction is somewhere in the brain or central nervous system.

Here’s what the direct muscle twitch looks like after the three trials:

Neuromuscular fatigue data.










In this case, the short three-minute bout clearly produces the highest levels of peripheral fatigue. The muscle is no longer contracting as powerfully.

Here’s the change in “voluntary activation” measured with brain stimulation, showing the reverse trend:

Neuromuscular fatigue data.










When the subjects are trying to contract their quadriceps as hard as possible, their brains are betraying them by sending a weaker signal to the muscle when they’re tired, and the effect is greatest after the longest (but slowest) cycling trial.

So the simple headline here is that short, intense efforts produce greater fatigue in your muscles, while longer, sustained efforts produce greater central fatigue in your brain and central nervous system.

In reality, it’s not that simple. This is the latest in a long series of studies on central and peripheral fatigue, and one thing that’s clear is that the results depend on the precise details of what you’re measuring—different types of exercise, different durations, and other factors all make a difference.

Still, there are a couple of interesting points worth noting. First is that these results argue against the idea that we each have a specific “threshold” of peripheral fatigue that we’re willing to tolerate.

Intense exercise causes a build-up of metabolites like lactate, protons, and ATP, which produce a sensation of discomfort in the brain and may also interfere directly with muscle contraction. But there doesn’t seem to a be a specific threshold for these metabolites that causes you (or your muscles) to say, “Okay, that’s it, I’m done.” The cyclists were able and/or willing to tolerate higher levels of peripheral fatigue in the shorter trials.

The second interesting twist is that this study, which involved constant-power rides to exhaustion, produced slightly different results compared to an earlier study by the same group using self-paced time trials.

In this study, peripheral fatigue led to a modest decline of 11 percent in muscle function in the longest trial. In the self-paced study, a 20K time trial that lasted 32 minutes produced a 31 percent decline, while a 40K time trial that lasted 66 minutes produced a 29 percent decline—both far greater than in the new study.

Why the difference? The researchers suggest that it’s because in a self-paced time trial or race, you can (and most people do) launch a finishing sprint at the end. It’s that intense finishing sprint that drives up levels of peripheral fatigue. This is another example of why it’s so difficult to interpret these measurements: They can change dramatically based on how the subjects pace themselves.

The practical implications? I don’t have any pithy training insights to draw from this study, but I’m really interested in these attempts to unravel the various elements of fatigue. And it’s also fun to compare these results with my own sensations from racing: I knew I wasn’t imagining that 800 meters is the most painful race of all!

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