The Mystery of “Nonlocal” Fatigue

“Nonlocal fatigue” is one of those phenomena that seem either completely obvious or totally mystifying, depending on how you look at it. Consider, for example, a recent experiment by Michael Johnson and his colleagues at Nottingham Trent University, UK, whose results appear in the Journal of Applied Physiology.

The basic task in the experiment was simple: A cycling test to exhaustion at fixed power, which took about four minutes on average. Using EMG and electrical stimulation, the researchers were able to measure how much fatigue was in the muscles themselves as opposed to the brain or central nervous system.

The twist is that the subjects did this test twice, and before one of the tests they did eight minutes of vigorous “arm-cycling,” a tiring exercise that only involves the arms. Should that matter? With only six minutes of rest before the cycling test, their heart rates and lactate levels were still elevated. On the other hand, their legs were perfectly fresh. Which is most important?

As it turned out, the subjects reached exhaustion in the cycling test 38 per cent earlier after the prior arm exercise. But fatigue levels in the leg muscles themselves (as measured with electrical stimulation) couldn’t explain why the subjects reached failure: in the arm-leg test, the subjects gave up when their legs were still less fatigued than in the leg-only test.

(They also returned for a third test, designed to last the same amount of time as the arm-leg test but with no pre-fatigue. In that case, they didn’t reach exhaustion, even though leg muscle fatigue was the same as in the arm-leg test. This reinforces the conclusion that fatigue in the muscles isn’t what makes you stop.)

Instead, the best predictor of when the subjects would give up was their level of perceived effort, which reached the same level in both trials at the moment of exhaustion. Somehow, the effects of arm cycling lingered and influenced the overall perception of fatigue during the leg trial—which, of course, raises an even bigger question: what is effort? In this case, the study focused specifically on the sense of discomfort in the legs and breathing, but there’s plenty of debate about what contributes to the sensation of effort and how it should be defined.

The details of that debate get pretty technical, but I think there’s a worthwhile takeaway message that also flows from my blog post earlier this month about the relative merits of data versus feelings for monitoring training. Judging your training efforts based on how you’re feeling isn’t necessarily settling for vague or incorrect feedback. Studies like this reinforce the fact that, at the moment of truth, how you feel is really all that matters.

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