Cause: Metabolite accumulation in your muscles triggers signals to your brain.
Occurrence: Mile or 5K races, fast surges, or finishing sprints
Antidote: Short, fast interval workouts

Imagine the searing muscular discomfort of a hard interval workout—focused entirely in your thumb. That’s the odd sensation that 10 lucky volunteers in a University of Utah lab experienced in 2014, when a research team led by professors Alan Light and Markus Amann injected a cocktail of metabolites—the chemical byproducts that build up in your muscles during intense effort—into their thumbs. The results were profound: They created sensations of fatigue in subjects not moving a muscle.

For decades, scientists and athletes have talked about “lactic acid burn” triggered by intense exercise. When you run hard, you eventually reach a point where your aerobic energy system—the ultra-efficient fuel supply that relies on oxygen delivered by your heart and lungs—can’t supply energy to your muscles quickly enough. You turn instead to anaerobic (oxygen-free) energy sources, which provide much-needed fuel but also generate metabolites that build up in your muscles. One of those metabolites is, indeed, lactate (a molecule that’s closely related to lactic acid). But despite its nasty reputation, lactate, on its own, doesn’t make you tired.

Light and Amann tried injecting their volunteers with three different metabolites: lactate; protons, which make your muscle more acidic; and adenosine triphosphate, a form of cellular fuel. When the chemicals were injected alone or in pairs, nothing happened. But when they injected all three together—bingo! At first the subjects reported feelings like “fatigue” and “heavy” in their thumbs, even though they were sitting. Then, when researchers injected higher metabolite levels that would correspond to all-out exercise, the sensations shifted to “ache” and “hot”—the so-called lactic burn, created in a test tube.

The results show that, regardless of what it feels like, your muscles aren’t being dissolved by lactic acid. It’s only when special receptors in your leg muscles detect a particular combination of metabolites that they trigger a distress signal that travels up your spinal cord, which your brain interprets as a burning sensation.

One solution? Train the receptors to be a little less sensitive by repeatedly triggering them in training. “The first time you do intervals after the off-season, you think you’re dying,” notes Amann. But after just one or two workouts, “it already feels a bit better.”

This is one of the main reason we recommend our SlimAndFitClub members to attend our classes min. 2-3 times per week, where we mostly do interval training to strengthen your muscle and extending your muscle endurance.

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