Why does it hurt?

Run as hard as you can for a few minutes (or sprint at the end of a race) and you’ll experience a very specific sensation of burning, numbness and heaviness in your legs. For years, runners talked about “feeling the lactic acid” in their muscles, but more recent research has shown that lactate doesn’t, on its own, cause muscle pain – in fact, it actually helps fuel the muscles. So what does?

A recent st udy from the Universit y of Utah, published in t he jour nal E x pe r imental Physiolog y, ex plored this question by injecting three of the suspected c ulprit s – lact ate, protons (which determine acidit y level), and atp (the chemical fuel that powers muscles) – into the thumbs of 10 lucky volunteers. Sure enough, they were able to reproduce the unpleasant feeling of a hard inter val session. At low levels of t he three chemicals such as you’d produce naturally during moderate exercise, the reported sensations were mostly related to fatigue; when the concentrations were increased t o correspond t o vigorous and ext reme exercise, t he sensat ions switched to pain.

Which of the three chemicals is to blame? All of them, it turns out. The fatigue and pain sensations only occurred when all three were injected simultaneously; if any one of them was missing, nothing happened. The researchers conclude that muscles have two separate groups of receptors, one that triggers fatigue at low levels and another that triggers pain at high levels, both of which require lactate, protons and atp to be triggered. The research is part of a larger effort to understand what ultimately makes us slow down. Does muscle pain force us to back off, or is it just an unpleasant byproduct that we can ignore during races? Knowing which chemicals are responsible will help researchers answer that question.