If I put you on a treadmill and ramp up the speed to a challenging level, you will eventually have to hop off or hit the emergency stop button. But why? What is it, precisely, that determines the moment where you have to give up? On the surface, this seems like a super-obvious question, but it's actually the focus of hot debate these days. Is it that your muscle fibers fail, or run out of fuel, or can't get enough oxygen? Or is it that your brain decides that you're pushing too hard? Or is there some more complicated mixture of effects?
There's a new paper in the Journal of Applied Physiology, from Markus Amann's group at the University of Utah, that offers some more evidence in favor of the role of something called "afferent feedback." Basically, Amann argues that it's not your muscles that fail per se; rather your muscles send a constant stream of signals back to the brain letting it know how they're doing. When that feedback reaches a critical threshold, the brain says "that's enough." (This is different from Tim Noakes's central governor theory, in that the brain's role is purely reactive: it doesn't try to predict and avoid coming problems.)
So how do you test this idea? Well, it's complicated... but here's the meat of Amann's latest study:
The study involved volunteers doing one-legged constant-load knee extensions (basically like one-legged cycling, but using only the quads). The open circles in the graph above show the perceived exertion as the volunteers do this exercise to exhaustion for one of their legs -- at the chosen load, they can last just under 10 minutes before their perceived exertion reaches 10 out of 10 and they give up.
The filled circles represent the same exercise with the same leg on a different day; the difference is that this time it was performed immediately after doing the exercise with the other leg. So they're doing this single-legged exercise with fresh muscles and not particularly exhausted (using just one muscle means they don't have the full-body exhaustion you'd get from running or even cycling). Still, the effort they perceive is higher right from the start, and they give up about 50% sooner. Why should this be? The researchers argue it's because the afferent feedback from muscles on both sides goes to the same part of the brain, so there's still lingering distress signals coming from the other (recovering) leg while the second leg exercises.
My take? This view of afferent feedback from muscle to brain playing a role in fatigue seems eminently reasonable and intuitive. That being said, I'm not particularly convinced that they've entirely eliminated other possible explanations. They only very briefly acknowledge the potential role that afferent feedback from other parts of the body -- like the heart and lungs -- could play (heart rate at the start of the second exercise session was over 130 bpm, for example), as well as the possible role of cognitive demand (it takes effort and concentration to exercise to exhaustion, and numerous recent studies have shown that these are finite and depletable quantities). The scientific debate seems to involve a lot of "either-or" thinking, but I suspect the eventual conclusion will be more along the lines of "all (or at least many) of the above."